Jabhill

Hi, my name's Josh. I'm trying to help a friend fix the non-running 1984 928 he bought that will not start due to no power to fuel pump.
First, the most important details. The previous owner parked it for 5-6 months under an overhang in his backyard. He says it ran fine when he parked it. The battery was dead when he tried to start it so he tried jumping it. He said he put the clamps on the wrong battery posts and jumped it. And that I imagine is the cause of the problem.

I replaced the battery because it had 0.9 volts in it and tried starting it. It turns over fine but the fuel pump doesn't come on. I did the relay jumper wire with no luck. In checking the wires, I found that the power (red/green wire) and ground (brown wire) to the pump had continuity. The power wire is shorted to ground. I traced it to the relay panel on the passenger floorboard (bottom right 4th harness in) and isolated it, and it was no longer grounded out. In checking ground to other wires, I found that a red/black wire (power?) on the same harness plug was grounded out. It looks like it runs to M249 solenoid valve and/or Starter Interlock Backup Light switch. On the relay panel side, it runs to Starter Relay XIV point 85. I checked the Starter relay and it's grounded out on 3 points. And that is where I'm stuck. The back side of that panel is such a PITA, I got frustrated and had to take a break for the evening. Any ideas of where to check next, or what could have gotten shorted out? Thanks
the engineer that designed that panel.

Jabhill

Morning bump. I'll entertain any and all ideas on what could be shorted. I know it is somewhere under the dash or running into the engine compartment.

GlenL

Do you have a set of manuals? You need the wiring diagrams. It's a torrent, if nothing else.

Wisconsin Joe

First off the "It ran fine when I parked it, all it needs is a fuel pump" line is about as common as it gets. Sort of like "Trust me, I'm with the government."

Jumping it wrong is potentially really bad. It can fry the computers.

Will the fuel pump run if you put 12 volts directly to it?
Have you checked for spark and injector pulse when cranking it?
Will it run on starting fluid?

And yes, the CE panel (Central Electrical) is a little bit confusing. You really need the manuals to figure it out. They are available as a free download - there's a link in the "New Visitor" sticky.

GlenL

"It just needs a tune-up" is so 20th century.

On the OP... It's an automatic?

Also, there's some interlock logic on when the pump runs. It doesn't just run when the key in ON. I don't know all the permutations but some run for a moment when the key is turned and some wait for the crank sensor or distributor pick-up to sense turning. Stopping the pump reduces fires.

soontobered84

Did you replace the FP relay with the correct relay?

It takes a special relay for US'84 model.

Jabhill

I know what "it ran when I parked it" usually means. I wouldn't have bought the car, but my friend thought he was getting a good deal.
So to run down the questions asked:
Yes, I bench checked the pump and it works.
Yes, it will fire using starting fluid.
No, I didn't check for spark because the pump wasn't coming on (even while cranking) and it fired on fluid.
Yes, it's an automatic.
I didn't replace the FP relay.

The power wire to the pump is shorted to ground due to a short somewhere else. I'm sure it's due to the previous owner jumping it wrong. I've traced the short into the CE panel to the starter relay XIV. I just couldn't figure out where it went from there because the wiring diagram that I found and tried to use wasn't very legible, and the wires on the back side of the panel are too tight and tangled to trace. I'll check the diagram Joe said is in the sticky thread.
Besides the ECM, what other computer modules are there that could have been fried to cause a short to the starter relay?

Wisconsin Joe

There are 2 computers. One for the ignition, one for the injection (L-jet on an 84 US IIRC).
If it runs on starting fluid, you have spark. You also have a good crank sensor and a good ignition computer (is it an EZF on an 84?). So 2 less potential problems.

If the short in the fuel pump circuit is in the CE panel, then you might be better off getting a replacement. 928 International has used ones for a reasonable price. Make sure you get the same one. 84s had some midyear changes in the electricals and the different ones are not compatible. Use the part number on the top of the actual panel to make sure you get the right one.
Or you could chase down the short. Good luck with that (not being sarcastic).

One more thought - Have you tried to start it with the fuel pump being powered from an outside source? That would tell you if the L-jet is working. Then you'd just have to get proper power to the pump.

Again, trying to eliminate potential problems. You may have already tried some of this.

Jabhill

Hey, I'm taking any info you guys give me whether I've tried it or not. I've never even driven a Porsche, much less tried to troubleshoot and run down wires on one 30 years old, which is exactly why I came here to ask for help from people clearly more knowledgeable on these cars than I. So thank you.
Chasing down the short and getting to the CE panel is what did me in. I didn't have a readable diagram, and the wires on the back side are so tight and tangled I was afraid of pulling out/breaking a wire. I found those diagrams on the ligeti website from the link in the newb thread. I'll put that to use the best I can.
Since I know the short is somewhere behind the starter relay, one question that may help narrow it down more. Of the 4 pins on the starter relay, how many should be grounded, and (bonus question) which ones? I've got continuity to ground on 3 points, 85, 86, and (I suddenly can't remember which) 87 or 30. Thanks again. I know how frustrating it is trying to help someone troubleshoot something you don't have hands on.

soontobered84

If you pull the starter relay, is there still a short? What I'm asking is: is the short inside the relay or have you isolated it into the wiring?

Jabhill

I did continuity checks on the starter relay, and one of the others to compare, and they ohm'd out the same. Continuity across 85 and 86, OL across the others. What I didn't do was replace the relay and see if the short still existed. But with the relay checking the same as the other one, I went on the assumption the short is not the relay, but wiring or components from the relay back to origin in dash or engine compartment.

dr bob

[ wrong diagnosis! ]

Rich9928p

A couple of clarifications to the discussion:

a) the L-Jetronic injection-based US/Japan 928s (1980 - 1984) only have an L-Jetronic injection module, there is no spark ECU (as installed in the LH injection-based cars). The same "transistor" ignition box is used from 1978 - 1984. The spark signal is triggered by a hall sensor in the distributor.

b) The fuel pump in the 1978 - 1984 transistor ignition equipped 928s is turned on by the fuel pump relay (special relay previously mentioned), which receives the RPM signal from the transistor ignition box and only turns on the fuel pump when the engine is spinning.

c) L-Jetronic AFC Relay XVI: To make matters even more complicated, the factory workshop wiring diagram for the L-Jet cars has an error, it is missing a circuit (only has the 87 circuit). There are two separate circuits, 87 and 87a that are powered from the 30 circuit. When making a jumper wire, it must have 3 prongs (Y shaped) and plug in the relay socket in all 3 positions or the engine won't start. The 87 circuit powers the fuel injectors, the 87a circuit powers the L-Jetronic injection module pins 10 and 11 via connector W4 and W5.

If anyone would like a corrected schematic, drop me a line at info@electronikrepair.com and I'll send you an updated PDF.

Rich
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