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Reading The ‘83/’84 Wiring Diagrams (“For Dummies”)
I haven’t seen many threads on the topic of how to read the Porsche wiring diagrams, and none on my particular model year (1983) which I understand uses a different format from the later diagrams. I recently tracked down some electrical gremlins and spent some quality time with these diagrams. It turns out that the ’83 version is not available online, but the ’84 is, and the cars are virtually identical.
I thought I’d start this thread to share the basics on how to read the ‘83/’84 diagrams for anyone else interested. While everything I share below is in that context, perhaps at least some of this will apply to other years as well. And feel free to jump in, add notes, corrections, and comments for this and for any other year. I by no means represent that this is a full or complete tutorial, but it will be enough to get you most of the way there.
I’d really like to thank Dr. Bob for both leading me through the wiring diagrams in a recent thread, and also for reviewing the contents of this post for technical accuracy – thanks Dr. Bob!!
SOURCES
First off, there are two different wiring diagrams made by Porsche for '83/’84 that go hand-in-hand: the “wiring diagram” and the “current flow diagram.” I picked up the wiring diagram at ligeti (http://www.ligeti.com/928/) and the current flow diagram from manualslib (https://www.manualslib.com/brand/por...utomobile.html). There may be other online sources too. I believe that Roger and possibly other vendors also have this information, and complete workshop manuals, available on CD.
Not every manual for every year is shown on the above websites, and it appears the “current flow diagram” is only available at the above source for years 1980-1983. Later years, at least after ’84, appear to have done away with this document. I’ll explain it anyway, using the 1983 version, after the wiring diagram.
Also, for reference, here is Alan's thread with single-page central electric panel diagrams (fuse, relay, and lettered plug locations) for all years (keep scrolling down until you find your model year): https://rennlist.com/forums/928-foru...all-years.html
For my purposes, I am going divide up the wiring diagrams into THREE parts for ease of understanding:
1) The central electrical panel diagram, labeled “Central Electric,” at the beginning of the wiring diagram document. This is a one-pager that shows an abstract representation of all inputs to, outputs from, and interconnections inside of the central electric panel via tiny numbers and letters which seem like a secret indecipherable code at first. It also shows all fuses, all relays, and all the lettered/colored plugs attached to the central panel. Because so much depends on this one diagram, I break it out separately.
2) The rest of the wiring diagram document, which shows the connections in the rest of the car and how they are connected with each other and to the central panel.
3) And finally, the current flow diagram which repeats much of the same information but, importantly, gives us information about specific wire colors, ground point locations, and connector locations. The current flow diagram – unlike the wiring diagram, which is largely divided by location in the car and not function -- is also helpfully divided into specific circuits/functions (such as fuel injection, ignition, etc.) that focus only on those areas.
To reiterate, everything I say below is specific to the MY'83/’84 diagrams.
The central electrical panel diagram is reproduced below:
This diagram is fantastic: It shows in a very compact form all the fuses and relays of the central panel PLUS all of the inputs and outputs of the panel (the colored/lettered connectors at the bottom of the physical panel). It also shows what all the mess of wires behind the panel is actually doing (that is, all the interconnects within the panel itself between the connectors, fuses, and relays).
What makes this interesting is that it doesn’t actually show most of the lines representing the physical interconnects, but instead relies on a pretty simple and clever kind of numbering system. I’ll call them “references.” That system is the key to understanding this page, and the rest of the wiring diagram. It actually ends up being more simple and less cluttered than if they had actually attempted to draw all the dozens of physical interconnects on the panel.
So let’s get right into it. Here’s how to read the diagram:
Let’s work from the inside out. First, the fuses are shown stacked up vertically on the middle left (along with their names and amp ratings), and all the relays to the right.
Next, find the first box drawn around the fuses and relays. It has the letters A through Z going around the outside, along with a bunch of little notations. Each letter A through Z corresponds to a colored and correspondingly lettered plug at the base of the physical panel in the car (though some letters/plugs are nonexistent in the car and on the diagram for some reason, like the letter “I”). Likewise, each of the notations for each of the lettered connectors is a reference to tell you exactly where each pin in that connector leads. The pins are numbered in the diagram along the perimeter of the box for each lettered connector, usually from 1 to 8, but it varies depending on the actual number of pins in the physical connector. By the way, if you pull any of the connectors on the actual panel, you can see the pin numbers molded into the connector itself.
Also note that the entire page itself is framed by a larger box that forms a letter/number cross-reference grid like a map. In fact, every other page in the wiring diagram contains the same type of grid, and you’ll be using it to find particular references.
The key to understanding this chart (and what had me really confused for a while) is that there are TWO reference systems going on in this chart. German over-engineering at work, no doubt!
Every reference on the OUTSIDE of the box refers to a grid section in some later page of the wiring diagram document. You’ll need to scroll down through the various pages until you run across the corresponding “map grid” reference, which is kind of like finding the right block on a street but not the right house. As an example, let’s say you’re looking at connector T, pin 1, and on the outside of the box see a reference to “G22”. See below.
You’d then start scrolling down in the document, looking at the outside grid box on each page. (You’ll notice that the letters across the top generally stay the same on every page, but the numbers running down the side generally increase). Grid reference G22 lives on what the document calls “page 4”, titled “Passenger Compartment and Rear End.” Within grid G22, just like on a map, there may be multiple electrical connections going on and you’ll need to pinpoint the exact one originating from connector T pin 1. Here’s the area around G-22:
Finding the exact termination from connector T pin 1 is easy – it’s labeled as “T1”. It is connected via a 6-pin connector to the pulse sensor (and although this diagram doesn’t show it, you might happen to know that the pulse sensor is located in the differential area – more about locating components later). So now you know exactly where that pin’s wire leads to!
For every reference INSIDE the box, even though some of them have the exact same format (a capital letter followed by a number), this is now always a reference to something else inside this SAME box, within the central panel. However, the references now come in multiple forms. Here they are:
- A letter followed by a number (like “T1”): refers to a lettered connector and pin number. This is the case when one pin in a connector is directly connected to a second pin (usually in a different connector). So, for instance, we can see in the previous picture for connector T that pin 1 is labeled “H1” on the inside of the box. That means that T1 is connected over to connector H, pin 1, inside the central electrical panel. Likewise, connector H, pin 1, has a reference “T1” on the inside of the box to refer to the same connection back to T1.
- A Roman numeral followed by a number (like “XIV 85”): refers to the corresponding numbered relay and terminal number. This means the pin in question connects to a relay. For instance, a reference “XIV 85” means that the pin connects to relay XIV, Terminal 85. For another example, refer back to the picture above of connector T. Its pin 8 has this reference: “XIX-XX 31, XVII 31”. This means that pin 8 connects to both Terminal 31 of the combination relay XIX-XX (the lighting relay) and to Terminal 31 of relay XVII (the fuel pump relay).
- “Si. Zu.” or “Si. Ab.” followed by a number: refers to a fuse number. “Si. Zu.” refers to the fuse input, “Si. Ab.” refers to the fuse output. Going back to connector T again, we see that its pin 5 has this reference: “Q1 Si. Ab. 13”. This is another example of a shared connection – the pin connects both to connector Q pin 1, and also to the output of fuse 13 (which we can see on the page is labeled as the fuel pump relay). (By the way, the output side of T5 is labeled “B22”… that’s a reference “outside” the box and thus to the map grid… and if we look that up on the grid, we can see that T5 connects to the fuel pump – as expected!)
Oh, and by the way, you’ll also see that SOME wiring connections are actually drawn in as lines, as you’d normally expect. Most of these lines are related to the basic 12V connections from the battery to various relays and some of the inter-relay connections.
While there’s some other stuff happening on these diagrams, it’s all pretty self-explanatory (such as wiring harnesses and connectors). But one other thing I’ll mention is that certain numbers keep popping up, and they have standard meanings that ultimately derive from our DIN standard-based Bosch electronics. For instance, Terminal Circuit 30 is an always-on 12V “rail”. Terminal 50 engages the starter. There are standard DIN circuit numbers that sometimes share a designation on a device like a relay. The standard DIN circuits we look at in the 928 include circuit 30, which is unfused battery voltage and always has the standard red wire color. Circuit 31 is always chassis ground, and has a standard brown wire color. As mentioned, circuit 50 is always the trigger to the starter solenoid, and is always yellow. Circuit 15 is on when the ignition switch is in position II (engine run) or position III (Start). There are non-DIN standards including a circuit X, which is on when the ignition switch is in position II (run) only. Circuit X includes a circuit that’s connected to terminal X on the ignition switch, but may also include a circuit that’s through a relay triggered by the ignition switch contact X, or may be a relay that has a coil bridging the 15 and 50 contacts on the ignition switch to provide the same function. There are more standard circuits too (see “References” below). As you work through the diagrams you’ll see how it all works.
The good news on the current flow diagram is that it provides several more pieces of information in addition to the wiring diagram to help give us the complete picture. First, it breaks down the electrical system into specific inter-related operational subsystems, like the starter/ignition/fuel pump, the ignition switch, and so on, so that it’s often much easier to understand and trace each specific system. This is in contrast to the wiring diagram which is based more on physical location (e.g. front end, passenger compartment, etc. where all systems are intertwined). In addition, it provides a couple of pieces of critical information that are mostly or completely absent from the wiring diagram: the actual wire sizes and colors, and the physical location and type of the various wiring loom connectors. This is great if you're trying to trace down where a particular wire runs -- you'll not only be able to find out what it looks like, but what connectors it runs through, and even if the wire changes colors along the way.
The bad news on the current flow diagram is that it uses its own, different system of references totally different than the wiring diagrams. Having said that, it DOES refer to the same terminal numbers as the wiring diagram.
As such, you'll sometimes need to refer to BOTH the wiring diagram AND the current flow diagram to fully understand any given circuit, both in the conceptual sense of how it works and also how it is implemented in the actual car.
There is a lot happening in the current flow diagram, and to be honest, everything you need to know is on the second page of the document where there is a legend that specifies what all the symbols mean.
However, let's work through an example. I'm using the '83 version of the current flow diagram I found online.
EXAMPLE
I recently worked on diagnosing an intermittent no-start condition on my ’83. I knew from other tests that one possible culprit was an intermittent or high resistance ground from the starter relay, number XIV. Let’s trace that out in the current flow diagram, on “Part I”, which covers “Power Supply, Starter”, “Ignition”, and “Fuel Pump”:
Find the starter relay, which is relay XIV. Here’s a close up picture from the above figure with the relay outlined:
There is a lot of useful information here:
Pin 85 goes towards ground (shown on the figure, but cut out above).
Pin 86 starts the process when circuit 50, the horizontal line depicted above the relay, is energized. This is a case where we need the wiring diagram to tell us what circuit 50 does. We can look this up back on the wiring diagram and work backwards to see that pin 86 is connected out through connector A pin 6… which goes to the ignition switch… which in turn is energized from a line from connector A pin 2… which in turn is connected to rail 30, which is always 12V. We can also see that there is a terminal 50 labeled on the ignition switch in the wiring diagram. So, circuit 50 (coming into the relay via its terminal 86) is +12V when the key is turned to the “start” position. This will energize the relay and close the contacts from terminals 30 to 87.
Terminal 30 is connected to rail 30, always +12V (we know this from the wiring diagram).
Terminal 87 is what actually leads to the starter’s terminal 50 (again shown in the diagram but clipped above) – in fact, it’s the famous yellow line in the 14-pin terminal under the hood – but I’m getting ahead of myself!
We can also see that Terminal 87 leaves the central electric panel via Connector O, pin 5, and that the ground line Terminal 85 leaves via Connector U, pin 7.
OK, so let’s start debugging! Where does line 50, the starter line, go, and where does that pesky ground line go? Let's look further down in the same diagram and see:
By convention, the bottom rail on the diagram is chassis ground.
So, we can see that pin 87 goes out through Connector O pin 5 as above, goes through a box labeled “T1414”, and is connected to Terminal 50 at the starter. I've colored it in yellow, above.
The box labeled “T1414” is explained back on the legend page at the front of the document. The subscript, 14, means it's a 14-pin connector. The superscript, here by coincidence also 14, refers to the pin number within that connector. And finally, the legend page ALSO tells us the location of every connector in the document (and hopefully in the car): there is only one such connector, "in engine compartment, right." The famous 14-pin connector by the jump post!
OK, let’s trace out the ground wire leading from pin 85 which I’ve colored in brown, it’s a little more interesting: It first goes through a brown and yellow line from Connector U pin 7, to a connector labeled “T41a”, then to a second connector labeled “T42a” where, by the way, it switches color to plain brown. It then grounds out at a location labeled "2" (the number 2 being inside a circle -- explained below).
What is “T41a” and “T42a”? Looking back at the legend again, we see this is a 4-pin connector, referring to pins 1 and 2, and it's labeled as connector "a". (Unlike the 14-pin connector where there is only one such connector, here there are multiple 4-pin connectors and Porsche gives each its own lower-case letter). So, we have established that the ground actually goes through the same connector, out through pin 1 and back through pin 2 -- a loopback, we can deduce. And finally, the legend tells us that the 4-pin connector "a" is located "in spare wheel well."
After going back through pin 2, the wire changes colors to plain brown and goes to ground point 2 (in a circle). Again, the legend has a location for every numbered ground point -- here, it is "at rear of wheel arch underneath right rear trim panel."
So there we have it – wires, locations, and an understandable circuit diagram. And, in conjunction with the wiring diagram, we can literally trace the entire electron flow from the battery to ground. In my case, understanding that the ground point for the start relay is actually located in the REAR of the car, I was able to start focusing on locating/cleaning that ground AND the associated 4-pin loopback connector that the ground passes through.
Note that the referenced circuit and flow path is specific to 5-speed cars, where the jumper/loopback shown between T41a and T42a actually passes through a switch on the automatic gearbox (on the auto cars) that's often referred to as the "neutral safety switch." That switch closes the loop from the relay to that rear ground point only when the gear selector is in the Neutral or Park position.
References between Sheets in the Current Flow Diagram
That just about wraps it up on the current flow diagram, but there’s one more thing to mention. And, of course, it involves yet ANOTHER system of references – and also of course, it’s a different reference system than on the wiring diagram!!
Every once in a while you’ll see something like this: A circuit which seems to suddenly stop, labeled with a Roman numeral. This is an example from Sheet 1:
This may seem like a reference to a relay, but it’s not. (Relays are also shown with Roman numerals, but they’re always depicted as a relay). Instead, this is how Porsche ties together multiple pages in the current flow diagram. Most of the time, each page in the current flow diagram is largely if not entirely self-contained. But sometimes, there are connections to other systems or circuits depicted on separate currently flow sheets, and that’s where this system comes in.
The reference refers to Sheet III (in the current flow diagram document), “track” 1. “Tracks” are the small numerals displayed across the bottom of every sheet. Frankly, most of the time I think they can be ignored, except for this one use case. Here, the above picture is shown on Sheet I, and it’s in track 27. (I guess this is why almost all the wires in the circuits are displayed as vertical lines). If you flip to Sheet III and start looking around in Track 1, you’ll eventually find a reference to “I 27” – that is, Sheet I, track 27. That’s a match! And it means that it’s the same line crossing through from one sheet to another.
Below, I’ve attached copies of both the wiring diagram and the circuit flow diagram. Plus, I’ve attached a couple of other general resources which may be helpful. The first is an article entitled “Understanding European DIN Wiring” from 2003, which explains the underlying automotive standards (and also walks us through common European wiring diagram conventions). The second is a table of terminal designations according to DIN 72552, which details the many standardized terminal and circuit designations used on European cars (such as the designation of the starter circuit as “50”).
EDIT: I'm also adding in the "928S Electrical Troubleshooting & Repair" document which I received from Wayne Strutt (thanks Wayne!!) This document is packed with information: The DIN terminal numbers used in our cars (again); an overview of the ground locations; the central electrical panel layouts 80-85; and information about the central warning system. Cool stuff! Wayne also sent me a PDF of the "Electric 928 Training" manual which has some very detailed information on the operation of most (all?) of the electrical subsystems on the car (and, amazingly, a how-to tutorial on reading the wiring diagrams! LOL!) Unfortunately it's 13MB and it exceeds the 5MB limit on attachments here, but PM me with your email address if interested and we can see if that works.
As an example of how to use the wiring diagrams in action, let's use them to run through a very typical scenario. Let’s say your ‘83/’84 US car doesn’t crank, meaning that the starter and solenoid don’t move at all when you turn the key. Here are the steps to follow to isolate the cause. With some minor differences with relays and pin locations, the general procedure applies to other years.
Before you start, the regular advice always holds. First, ensure that your battery is fully charged. From Dr. Bob: “Check the battery condition. My annual maintenance protocol includes sampling the electrolyte for specific gravity. A cheap (~$3) tester from Wal-Mart or your parts store is fine. It has an eye-dropper with colored ***** inside. Each ball is a different density, so a low specific gravity of fluid in the tube may not float the heavier *****. The colors give you an idea of cell condition based on the heaviest ball still floating when you draw a sample from a cell. If any sample has a green ball falling, you are casually comparing battery price and availability. When any yellow ***** are sinking, you are seriously shopping. If any red ***** start falling, immediately take a car other than the 928 to the battery store. The specific gravity test offers you a little warning before you get stranded. The test must be done with a fully-charged battery, and the battery must be allowed to stabilize for at least 30 mins after charging ceases.”
Second, have you cleaned ALL your grounds and checked your battery ground strap and engine ground strap? See the “New Visitor” thread for information on how to do that. I’ll also share some advice at the end about the importance of cleaning contacts and using DeOxit any time you’re working on the electrical system.
Third, have you replaced or swapped your starter relay, XIV? Replace it with a spare, or try swapping the starter relay with the horn relay (X). By the way, it’s a good idea to always have a new, spare starter relay and fuel injection relay.Try starting the engine again and see if that works.
Fourth, remove the top cover of the 14-pin connector by the hot post. (Don’t actually pull the connector apart because the little connectors can simply fall out of the plastic housing). Pin 14 in the connector will be a yellow wire. You can do a hack test on the starter here: Turn the key to the “run” position, and make sure you’re in neutral or park, your parking brake is set, and that it’s safe to start the engine. Using some alligator clips or a wire, touch one end of the wire to the hot post, and very carefully touch the other end to the yellow pin for a moment. If the starter engages and starts the engine, that’s great news, as it means the yellow wire leading from there down to the starter (via the front-of-engine harness) – and the starter itself – are OK. If this test does not crank the starter, then you either have bad connectivity in the engine front harness, or there’s a starter problem.If you suspect a continuity issue, then carefully unplug the 14-pin connector, making sure that the pins stay in, and use an ohm meter to test continuity between the yellow pin there and the yellow wire at the starter itself. This tests continuity of the yellow wire in the front-of-engine harness. If continuity appears good, you may have a bad starter.
The next series of posts will focus on ensuring you receive a 12V signal at the yellow pin when the key is turned to Start.
As an overview, here’s how the system is supposed to work: when the ignition key is moved to the “start” position, the ignition switch connects 12v to the starter relay (a very low current draw). The starter relay then engages, bridging its internal contacts to connect 12v to a wire (the “yellow wire”) which is capable of carrying the current draw of the solenoid. This energizes the solenoid, moving the starter to engage the teeth of the flywheel ring. The starter itself then draws current directly from the battery positive cable to crank the engine.
Possible, or at least common, failures include: a faulty ignition switch (which doesn’t send 12V to the starter relay); a faulty ground on the low current side of the starter relay so that the relay never engages (possibly caused by a bad ground or a dirty connector in the path to ground, and on automatic cars, possibly a bad transmission interlock switch which lifts ground to prevent starting except when the gear selector is in P or N), a faulty XIV starter relay, dirty connectors to or from the central panel, a dirty 14-pin connector near the jump post under the hood (the yellow wire passes through here on the way to the starter), or faulty wiring in the harnesses between the central panel and the starter. The below tutorial should be able to help you isolate any one of these causes.
Let’s follow the current path, starting with the connections around the ignition switch and central panel.
Turning the key causes a 12V connection to be made to the starter relay. Let’s look into what’s going on there. Here is a closeup from the wiring diagram, page 2 (central panel):
Unswitched battery voltage arrives at the central panel through three connectors, seen at the lower right of the diagram. Those connectors are, on earlier cars, three individual red wires which plug into the central panel in the middle of the board, in between connectors “N” and “O”. I believe later cars have a different connection point. For what it’s worth, battery voltage travels from the battery, through the battery cable underneath the car, to the starter. At the starter, the battery cable meets the front-of-engine harness, and from there to the alternator and to the hot post under the hood near the 14-pin connector. This is the large red wire that feeds the hot post. At the hot post, the large red wire is connected to three smaller red wires, which travel through the main underhood harness back to the central panel forming the above three connections.
The three unfused connections together form “Rail Term 30,” an always-on 12V circuit. Rail Term 30 is connected inside the central panel to connector A, pins 1 and 2 (we see that over on the left). Those pins lead out to grid references J11, C11, and E11. For our purposes, we only need to focus on grid reference J11 – the ignition switch:
When the key is turned to the “Start” position, we can see that the wires from connector pins A1 and A2 get connected to connector pin A6, and is labeled “50”. As discussed previously, this is the starter circuit.
So, electricity goes from the battery, to the central panel, out connector A pins 1 and 2 to the ignition switch, and back to the central panel via pin A6. If we go back up to the closeup of the central panel, above, we can see that pin A6 is connected to “I-II 50, XIV 86”. Because we’re focusing on the starting system, we can ignore relay I-II (the rear defrost relay) and look only at XIV (the start relay). So, turning the key energizes the start relay, terminal 86.
Let’s take a look at the start relay again:
12V comes in to terminal 86 from the ignition switch (via A6), and current flows through the coil in the relay and to ground through terminal 85. The ground wire leaves the central panel through connector U, pin 7. As we saw in an earlier post in this thread, we used the current flow diagram to determine that the ground itself is located under the right rear quarter panel, and on its way there goes through a connector in the spare wheel well and, for auto cars, the transmission interlock switch.
Likewise, when the ignition switch energizes terminal 86 and current is allowed to flow to ground, then terminal 87 is bridged by the relay to terminal 30 (the always-on circuit). Terminal 87 is the “yellow wire” destined for the starter. The wire leaves the central panel via Connector O pin 5, and we already know that it travels in the main engine harness to Pin 14 of the 14-pin connector by the hot post, and from there through the front-of-engine harness to the starter.
By the way, the debug procedure outlined below will work on virtually any year 928, but you’ll need to read the wiring diagrams for your year car to determine the actual connectors and pin numbers. I believe that some later 928s did away with the starter relay and instead use a bridge in its place to send 12V (from the ignition switch) directly down the yellow wire, in which case you won’t need to debug the starter relay, but the rest will still be relevant.
Let’s check that the ignition switch is receiving, and sending back, 12V as intended.
First, unplug the 14-pin connector at the hot post (leave its cap on so that none of the pins can fall out). This way, we can turn the key to the “start” position and measure voltages without actually starting the car.
Move the key to the “Start” position. (The car won’t start because the 14-pin is unplugged). This engages some of the car’s electrical systems and places a load on the battery. On my car, it takes a few seconds for the battery to reach a steady state voltage in this condition. It’s useful to do this so that you have a known and relatively steady baseline voltage.
With the key still in Start, measure the voltage at any of the three red wires entering the central panel, forming Rail Term 30. Hint: There’s often enough space at the back of every connector to insert your DMM probe from the back to make contact with the brass female terminal, so you don’t usually have to unplug any of the connectors on the lower part of the central panel to perform measurements. On my car, that measurement was 12.03 V.This is your reference voltage.
You can now let go of the key. However, for each of the following tests, move the key to “Start” each time and wait a few seconds before measuring – again, to ensure that you are measuring from a known reference point.
Now, move over to Connector A, Pin 2. This sends 12V to the ignition switch. It should be the same voltage as the input to the central panel. In my case, I measured just slightly less, 11.9V at A2. If it’s zero (or at least significantly less than 12V), you may have a wiring issue in the central panel itself, or perhaps more likely you have dirty connectors. Remove, clean, and re-insert Connector A with some DeOxit.
Next, let’s see if the ignition switch is actually doing its job. Measure the voltage at pin A6 – this is the voltage coming back from the switch when turned to “Start”. Again, it should be the same (or nearly the same) as what you measured on pin A2. In my case, I measured 11.75V coming back from the ignition switch. If the ignition switch or related wiring is faulty, this would be zero (or at least a lot less than 12).
Finally, let’s see if the 12V coming back from the ignition switch is reaching the start relay. Unplug the start relay (XIV) and measure the voltage at the socket for terminal 86 with the switch in “Start”. It should be the same voltage as you measured at Pin A6.
Relay and Ground
Assuming that you have 12V coming into terminal 86, and that the car still isn’t starting, then let’s check the other three terminals on the start relay.
First, check terminal 30 with the relay removed – that’s the Rail Term and should always be 12V.
Next, again assuming that terminal 86 is energized in Start, it could be that you don’t have proper ground on terminal 85. If there’s a bad ground, no current will flow, the relay won’t close, and the car won’t start. With the relay removed, test continuity between terminal 85 and ground measured from a convenient place (such as the ground point above the central panel, or I even used the exposed bolts on the passenger door hinge – but always measure to the same place for consistency). Oh, and by the way, as a test just touch the leads of your DMM together to measure that resistance by itself – on my DMM that alone is about one ohm, and subtract that from all other measurements you take.In my case, I was initially measuring six ohms from terminal 86 to ground, or even more – that’s too high. (If you have an automatic, make sure you’re in P or N).These little relays don’t need a lot of current to energize, and by the same token, even a small amount of resistance in the ground can render them inoperative. Using the current flow diagram, I traced the ground wire from U7 to the ground point at the right rear, under the rear quarter panel.
For at least the ‘83/’84 models and possibly most others, there is a connector in the spare tire well (discussed in post 3 above). Locate the connector housing the ground wire. For my ’83 manual, that is a 4-pin connector with a loopback in two of the pins, with two other unused pins. For automatic cars, that connector leads to the transmission interlock. With that connector unplugged, measure the resistance from U7 to the connector pin 1 (should be two ohms or less), and from the connector pin 2 to ground (should be infinitesimal because the ground is only about a foot or two away, under the right rear quarter panel). If you have an auto, put it in P or N and measure the resistance across the connector half leading to the transmission – this should also be as low as possible, perhaps an ohm.
Re-assemble the connector in the spare tire well, again cleaning it and using a spray of DeOxit. In my case, I’ve had an issue with oxidation in this connector, causing a high resistance to ground which caused a no-start problem. Cleaning this connector solved the problem!
Finally, insert the start relay, engage the key to Start again, and measure the voltage at terminal 87 or simply at Connector O, pin 5. Assuming that you have 12V on terminal 86 (from the ignition switch), that you have 12V on terminal 30, that you have a low resistance ground (~2 ohms) and that the relay works, you should get 12V coming out on the “yellow wire” at O5.
As an extra (albeit largely extraneous) test of the ground, you can also measure the voltage on terminal 85 of the start relay when the key is in "start". Because this goes to ground through a long wire with a small amount of resistance, the voltage here should be in the millivolts when the relay is operating correctly. If it's absolutely zero, it indicates that no current is going through the coil in the relay, which (assuming you have 12V at terminal 86 and a proper low resistance ground path from terminal 85) means that the relay is bad.
Circuit 50/Yellow Wire Connectivity
If you are getting 12V out on Connector O, pin 5, when the key is turned to “Start”, then your ignition switch and relay are working fine. In this case, it’s either a continuity issue, or a starter issue.
I recently replaced the main engine compartment wiring harness in my car, which comprises central panel connectors M, N, O, P, Q, plus the three red wire central panel connectors, plus a bunch of associated grounds. This harness carries, among many other things, the “yellow wire” from the central panel, through Connector O, to the 14-pin connector at the hot post. I also recently replaced the front-of-engine harness, which mates to the 14-pin connector and routes the yellow wire the rest of the way to the starter.
If you still have original wiring harnesses, and especially if your car has been exposed to the elements, it’s very possible that the wires on the harness have corroded, or even that the insulation around individual wires has broken down and may even be allowing wires to short out.
You can test this by unplugging the 14-pin connector by the hot post, carefully removing its cap (careful that the barrel connectors inside do not fall out), and then checking the continuity from Connector O, pin 5, to the yellow wire in the 14-pin connector (it’s in pin 14). That will test the continuity from the central panel to the 14-pin connector. You can then also check continuity from the other half of the 14-pin connector (part of the front-of-engine harness) down to Terminal 50 on the starter itself.While you’re there, be sure to clean all the barrel connectors and give everything a shot of DeOxit.
Once you’re debugging around the central panel area, now is a good time to remove, clean, and re-insert all the fuses, relays, and all the colored connectors too. They are all susceptible to oxide buildup, but merely disassembling and reassembling them will help clean off any oxide barrier. Use a quick shot of DeOxit or other good contact cleaner as you go. Put down a bunch of towels around and under the central panel first to avoid making a big mess!!
wow....you put serious effort into that....thanks!
(the 83 flow diagrams will be a huge help....i had tried to cross reference 85 but there's quite a few different colors vs 84)
Thank you! I learned a LOT about the car in my recent renovation, and it's led me to appreciate the car that much more (and this comes from someone who's owned the car for over thirty years and has always done my own work!)
PS I also just updated the "Resources" post with a link to Alan Moore's central panel diagrams, and also with a very helpful Porsche electrical system document provided by Wayne Strutt (thank you Wayne!) I also updated the "Hints" post with some links to some of the electrical system aspects of my renovation project.
08-23-2019, 04:53 PM
Reading The ‘83/’84 Wiring Diagrams (“For Dummies”)
