C531XHO

I am sure this has come up many times but haven't found definitive guidance on the site. Had problem starting my 81s so jumped the fuel pump relay which gets it going. replaced the relay but still no start. This suggest to me a problem with the LH ECU? Any thoughts??

hr928

I'm going thru the same process....I feel your pain. It's quite a common thread. You'll find a lot about it. As to definitive; check fuel pressure even if your pump is working. Then check for spark. If you have both then compression. Not sure about a 1981 model but start with the basics; clean all your grounds then start checking every relay and fuse having to do with fuel spark and compression. You'll find the culprit. Good luck.

Barry Chan

You need to give more info.....car sat for a while? It ran now it doesn't. Does it crank?
Start with all electrical clean and clean.....grounds and CE panel, check 3 relays...injection, starter and fuel pump relay, change fuel pump fuse and make sure all are correct amps.
Then there's ignition switch, fuel pump, fuel regulaor/damper, temp II sensor.....plug wires, coil...
These guys are good but they need more info to give you answer!

GlenL

An 81 has L-jetronic fuel injection and not LH-Jetronic. Big difference in parts.

If it starts when jumping the relay then it's likely the new relay. Was it new? Was it a Porsche part or aftermarket? Do you hear the pump run when turning the engine ON without turning to START?

C531XHO

Hi all, thanks for your comments. Ok so the car has been working beautifully all summer, but refused to start outside the house when I stopped to pick something up. Suspected the relay but didn't have facilities to inspect/fix so I jumped the 30/87 connections. Car started as per usual. Relay was reworked by one of several electronics engineers who work for me, they tested function on the bench and confirmed repair. On fitting the relay to the car it's still a no start, my question is should I be looking naturally to the ECU as the fuel pump signal is not being made by it?

928GT8108

Did you check the flywheel sensor socket?

dr bob

Fuel pump relay is 'triggered' by the tach signal pulses "generated' by the ignition control unit. Have your engineers verify the tach pulses to the relay. You can find the tach pulse at CE connector O7 (letter "O", wire 7, Green color) and at 31b in the FP relay socket. Easiest to see with an oscilloscope. When you had the relay jumpered and car running, did the tachometer work? The tach and FP relay see the same signal from ignition. Not definitive, but a good clue.

Verify that you truly have 12V out of the 'tested' relay with FP load on it. Relay contacts wear/burn, to the point where the pump may be running but not fast enough due to voltage drop through the relay.

Fuel pump relay needs a solid "15" signal from the ignition switch (on with key in "run" or "start" position.

Ground connections at the CE panel (right above the panel) are critical to proper function of lots of things including this relay. Cleaning them carefully (and all the others on the car) is part of what should be an annual ritual for most non-desert climate owners.

Wiring diagrams and workshop manual diagnostics are your friends. There are downloadable sets listed in the "new visitors" thread at the top, or better, you can buy a CD set with manuals plus all the support manuals and tech pubs from Roger at 928srus. (ROG100 here on Rennlist).


HTH!

Barry Chan

You may have a hot start issue if it was running then you go to restart and it won't start yes?

curtisr

As mentioned earlier, I would swap the fuel injection relay out for a new one in your case.

I would also buy extra of the fuel pump, fuel injection and starter relays and keep them in the glove compartment. Yesterday, my '82 decided to start and quit at the nearby strip mall. Cost me $50 for a tow as I did not have any extra relays in the car. This time it turned out to be the fuel pump relay. The last time? It was the fuel injection one.

Barry Chan

Or keep some jumper wire in glove box

curtisr

+1

C531XHO

Thanks for all the suggestions. Managed to spend some time one the car this weekend, the fuel pump relay jumper didn't work when I tried it but in trying to make contact I pressed the adjacent relay which got the pump going. Removed the relay/fuse panel to have a look at wiring behind and noticed that the adjacent relay pushes on the power wire from terminal 30? From memory to the fuel pump fuse. On close inspection the top fuse holder was dirty and a bit distorted. Cleaned it and the car started fine on both jumper and relay - lesson learned, start with the basics! Fingers crossed that's it..
David