gronkX

I'm trying to figure out what's wrong with my '86.5 5 speed, and I suspect it's either the wiring harness or the computer - at this point, the 30 year old wiring harness is probably the leading suspect.

Last year, my mechanic found that if one of the fuel injector connectors was shorted out, the car wouldn't start. (The short was inside the boot). That particular connector was replace, but there are 7 more that are probably suspect.

Is it possible to tell if I have a short? Are each pair of wires run directly into the computer (via the 25 pin connector) or somewhere else where it would easy to get to?

Rich9928p

All the fuel injectors are wired together in parallel, so if one circuit is shorted all are shorted.

For the 1985 - 1986 25 pin LH, pin 13 is the connection to the fuel injectors. The 12V power is provided via the fuel pump relay. The fuel injectors are turned on by the LH pin 13 pulling the circuit to ground via an internal transistor.

You can check the resistance between the "ground" side of the fuel wiring harness by unplugging the fuel injector connector and checking for continuity (near zero Ohms) to LH pin 13.

The 12V side of the fuel injector socket should have 12V when the fuel pump relay is jumpered between relay socket positions 30 and 87.

Rich
'93 GTS Cover Girl
'87 S4
'79 5-speed rescue
'79 Euro 5-liter track beast

SeanR

Pull the boots on all of them and either plan to replace them or just mickey mouse them. The rest of the harness is most likely ok in the short term. The wires all terminate about 6 inches back from the fuel rails in to one big bundle each side. Depending on the year there are other wires tied in to them.

gronkX

So if I'm understanding, I need to check from the fuel injector connector - there's nothing I can do from the 25 ping connector.

With the fuel pump relay jumped, I should have 12v across the FI terminals, and there should be no resistance between the ground side of the FI connector & pin 13.

mark kibort

this might be the 6 liter engine issue as well for my friend..... Ill do some testing along these lines... thanks!

gronkX

OK, so I pulled the connector off one of the injectors.

With the fuel pump relay (XX) jumped, I was NOT getting any voltage across the two terminals.

With both 25 pin connectors unplugged from their respective computers, I have about 2 ohms resistance between the two pins of the FI connector. That would indicate that there's a short somewhere in the harness, correct?

mark kibort

I was thinking you need to get voltage reading from one side of the connector to the physical ground. if there was a voltage measured from the connector, the injector would be on with the fuel pump jumped. the injector turns on when the computer makes a ground for the injector. (pin 13..) right??

gronkX

I did, and both sides of the connector seem to be grounded. Which would explain why there's no voltage.

Rich9928p

The "+" (positive) side of the fuel injector connector is connected to the "30" circuit (12 V always on) via the fuel pump relay socket positions 30 to 87 (the relay provides the switching). So if the fuel pump relay socket is jumpered 30 to 87, all 8 of the "+" fuel injector connectors should be at 12 V (or what ever the battery voltage is). If the relay is not jumpered or the fuel pump relay is not turned on by the LH fuel injection module, the "+" side of the fuel injector circuit is floating, i.e. not connected to anything.

The "-" (negative) side of the fuel injector connector is connected to LH pin 13. It is floating (not connected to anything) until the LH fuel injection module provides the ground ... this is what determines the period that the fuel injectors are opened.

If the fuel pump relay 30 to 87 are jumpered, you should observe around 12V at LH connector pin 13, as there is no voltage drop across the parallel circuit if no current is flowing. The parallel resistance of the fuel injector circuit is around 1.75 Ohms if all the fuel injectors are 14 Ohms.

NOTE: the fuel injectors for the 1984 Euro and 1985 - 1986 928s are powered by the fuel pump circuit. From 1987 to 1995, it is the LH relay that powers the fuel injectors.

Rich
'93 GTS Cover Girl
'87 S4
'79 5-speed rescue
'79 Euro 5-liter track beast

gronkX

So I have +12v at LH pin 13 with relay XX jumped, but I have 12v at BOTH sides of the fuel injector connector. So it seems like this would confirm that somewhere in the fuel injector wiring, there's a short.

Now, the next question will be how to track that down

hb4

I think that would indicate a short only if all the injectors were disconnected.

Otherwise, with the FP relay jumpered and pin 13 floating I think you'd read 12V on both sides of a disconnected connector. 12V via the FP relay on one side and then passing through to the other side via any remaining parallel connected injector coils.

gronkX

If that's right, it sounds like I'm not going to be able to tell with just a simple meter.

I've seen a couple of references to a "noid light" for testing fuel injectors - if there's no spark would there still be pulses to the fuel injectors?

hb4

To answer a question in the original post; no, each pair of wires not only do not run all the way to the 25 pin connector, they don't run separately to any connector - at least the way I'm reading the WD.

The '+' wires for cylinders 1-4 appear to the spliced together inside the wiring harness; similarly for the '-' wires. Cylinders 4-8 are also wired this way, but separately.

Reading the '86 WD, if you disconnect all of the injectors, plug 'W' on the CE panel, and the Oxygen Sensor (which is connected to the FI '+' circuit at plug 'W') - then the '+ 'set of wires and the '-' set of wires are isolated from each other and cannot be connected through the resistance of a load.

Then, if you measure ohms or a direct short between the + and - terminals of any connector you have a short in the wiring harness somewhere. If there's only one short, then you should see the same value on the rest of the connectors of that set, and the other set should measure open.

Assuming the rest of your wiring harness from the tails back is OK, if you replace the injector tails for that set of cylinders, the short should go away. As you mentioned, the other 7 are suspect so you might as well replace them all.

dr bob

Note that [only] the '89+ cars have the injector wiring separated into two separate segments. From the "ignition protection relay" socket on these cars, you can see the resistance of the two sections separately. The injectors are segregated based on which of the two ignition systems fires the related plugs; if one of the ignition systems fails, the related injection loop is disabled by that relay. For diagnostic purposes, measurements on these later cars should be made from that relay socket.

The resistance through the injectors is pretty low, to the point where having 8 injectors connected presents about 2 Ohms when you measure with a meter. Some meters and test lead combos will show a small resistance number (not a true zero) when test leads are touched in resistance mode. When you test with injectors still connected, get a baseline resistance measurement by touching the two leads together, then test the loop. The loop measurement should be about 2 Ohms higher if there are no shorts in the loop.

hb4

Thanks, I see that now in the '86 WD. The two banks are tied together in the 'W' plug and somewhere near the LH. So all you can tell is that there is a short 'somewhere'. With a decent ohmmeter.