marknsf

After being stuck in a parking lot with a car that wouldn't start I wanted to have a better understanding of my '87 S4's electrical system. I wanted a simple table I could carry in the car that showed which fuses I should expect to be energized in various ignition switch positions without having to trace schematics. I’ve already learned that making assumptions about how things work on a 928 will lead you astray.

I also wanted to install my new GPS on an accessory circuit, preferably the unused telephone fuse.

Of course, these results apply only to my '87 S4 automatic, other cars and years may/will be different. Refer to Alan’s '87-'88 CE drawing for fuse names. With all switches off but the passenger door open (I don't know how anyone could get to the CE with that door closed) I found the following:

1. All positions (including key removed): Fuses 16-29 energized
2. Accessory position: Fuses 1-6, 14 and 15 energized (de-energized in start position)
3. Run position: Accessory fuses + fuses 7-13 + fuse 42 (fuel pump, oxy sensor) if the engine is being started or is running

Fuses 30 and above (except 42) de-energized. These are lighting fuses that depend on various switch and ignition positions I didn’t explore.

So the telephone fuse (9) is not energized in the accessory position, too bad.

While running these tests I noticed that sometimes the voltage showing on a plug in voltmeter I have in the cigarette socket show only 6-9 volts when the key was in the accessory position. Checking at the CE panel, fuses 1-5 also showed the low voltage but fuses 6, 14 and 15 showed a normal 12 volts. When the key was moved to run position all showed 12 volts.

First I swapped the X-bus relay (IV) but that did not fix the problem. So off to study the schematics using Alan's guide. It appears the X-bus relay does not power the X-bus despite its name. It seems to be energized by the X-bus and in turn sends power to other things like fuse 6 for the a/c system and fog light relay XIII. The X-bus, that powers fuses 1-5, is powered directly from the ignition switch by a trace that extends from the accessory to the run position inside the switch. Most likely a portion the trace in the accessory position has some corrosion after 27 years and the switch either needs to be cleaned or replaced.

The 928 is full of interesting design decisions, like why does the X-bus relay not power the X-bus or why does the radio have its own trace in the ignition switch in the accessory position.

I wanted to post what I have observed to add to the knowledge base. If anyone sees errors or has additions to the above please post, I still have a ton to learn about the 928 electrical system.

Mark

Alan

Yes these are things that are not always obvious.

Fuse Power:

My CE panel diagrams for each year have a section called "Input Bus" This is color coded & notated to indicate what feeds the fuse: (30) Red = Constant Battery Power, (15) Black = Ignition Switched Power, (X) Yellow = Accessory Switched Power. The ones not assigned a coding in this section are fuses with power fed from equipment or switches that are dependant on other configurations so are less predictable as to when they may be powered (check the diagrams to determine).

X-Bus:

The X-Bus (X) is always sourced by the ignition switch, on the earliest cars that's all there was. Later cars ('85-'89) added an X-Bus Relay as a repeater relay designed to reduce the load on the ignition switch portion. Functionally (X) or the 87 output of the X-Bus Relay are the same thing (unless the relay fails). Note that the X-Bus relay is the relay called* "X-Relay" not the relay in relay slot position X (a possible source of confusion).

* Porsche does a horrible job of labelling: individual sheets sometime have no relay name, just slot ID. On the CE panel Pages it is variously called "Relay X", "X-Relay",

Later cars ('90-'95) changed the functionality and items that previously used the repeated X-Bus Relay output (e.g. fogs etc) started using the output of the "Supply" relay instead. The supply relay output is a new power bus that is active when the ignition is active - except during starting, while the X-Bus is active when the accessory or ignition are active except during starting.

Radio (R) Power Bus:

The separate radio (R) power bus output of the ignition switch is interesting - at first glance it seems quite redundant since it is active during accessory, ignition but not starting. So the stock ignition switch actually has (R) switched the same as the (X) bus. It is perhaps for lower noise.

The other problem all along is that (R) feeds the radio and (R) is not fused - except on the radio itself. This really is a blunder of the first degree! Un-fused circuits in a place where most of the nasty amateur butchery takes place!

Alan

marknsf

Thanks Alan. I am understanding better all of the useful information you've been able to fit into your CE panel diagrams. I wanted to supplement that with a table that establish a baseline for my car's CE panel that I could use for troubleshooting when things go wrong. For example the sunroof fuse (#14) is not fed directly from the X-bus but is it powered when the ignition switch is in the accessory position, yes it is.

That a car has 45 fuses and there is no fuse for the radio is truly incomprehensible, especially considering all the damage a short in that circuit could cause. For circuit tracing I have a "Supplement to Repair Manual 928 (XVIII - USA) Contents: 928 S circuit diagrams, '88 models". It's close enough to my '87 for most purposes. In it the ignition switch internal diagram shows the trace that connects to the radio bus as only covering the accessory position. I thought there might be a separate circuit for other positions but I didn't find one. Obviously the radio works in other positions so that drawing must be wrong.

Mark

Alan

On an '88 (and basically all years) the R terminal is shown connecting in both Accessory and Ignition positions (take another look).

Fuses #14 and #15 only seem to be powered by the X-Bus - They actually aren't. They depend on the window regulator relay. If you close the doors (or tape down the pin switches so the car thinks the doors are closed) -> turn the ignition to accessory - fuses #14 and #15 (windows) will be powered - turn the ignition off and remove the key and they will still remain powered until a door is 'opened'.

Its important to understand the details - else you can get surprised.

There is a radio fuse - however it is just in the wrong place and therefore doesn't protect the wiring.

Alan

drwhosc

Hey Alan, where can I get a copy of your diagrams. I have to tackle my electronics in the very near future, and I have lots of systems that need attention. I hope I can bring them back, and a good diagram will certainly help...

Tony

Handy info!

Alan

These are not wiring diagrams/schematics - they relate to the Central Electric (CE) Panel only (fuses & relays).

https://rennlist.com/forums/showthre...eferrerid=6055

Alan

marknsf

Alan, I get what you saying, which is why I tell my wife not to open the door until I roll up the windows. So I understand that many circuits like the window regulator are more complex. In no way does my little table replace reading the schematics and understanding how the various system work but I don't carry the schematics in the car - maybe I should.

For the radio, I see that fuse 22 connects the radio (and amp) to the 30 bus. So the radio bus through the ignition switch is just a turn-on signal. The radio bus isn't fused so if shorted it could cause damage. Also if someone used it to directly power a replacement radio the the excessive current could damage the ignition switch.

Does the '87 S4 model with the booster amp option have a antenna amplifier? My '86 944T also uses a windshield antenna and it has very poor reception. I'm thinking of adding a roof antenna to the S4 like later models.

Mark

drwhosc

thanks Alan. Handy chart. Missed it the first time around

Alan

Yes you have an antenna amp (white wire), it may not be powered which will give you much worse reception than simply not having an amp - check that.

A roof antenna is a lot of work to add... I'd test the stock config first.

There is always a permanent supply and a switched (R) supply for the head unit - both should really be fused.

Alan

Wisconsin Joe

On my 85, the antenna amp is a black wire. What year did this change? Or, what years are what color?

Would you suggest an inline fuse for the power supplies (switched & constant) going into an aftermarket head unit?

On my 85, the constant power comes off fuse 24 (interior lights), so it's fused - made it confusing and difficult to locate a permanent power wire when installing, I had pulled 24 to keep from running the battery down and then forgot it was out
My switched power from the ignition had disappeared, so I ended up using the cigarette lighter power instead (that's already got a fuse).

These are more curiosity questions than anything.

Alan

The antenna amp may have either a black or white wire - OR depending on where you intercept it - it could be both black & white. On an '87 the segment nearest the amp should be black - but there is a single pin connector where it changes to white before it gets to the amp.

So its possible it could be either color.

I think an inline fuse near the suppressor on the R line is not a bad idea - though I generally dislike having fuses sprinkled around randomly. Porsche should have originally routed the (R) supply through a fuse on the CE panel - and this should also be fairly easily to do as a retrofit utilizing a spare fuse location.

Alan

Wisconsin Joe

Thanks.