Fuel Injection Computer Self Repair

Fuel Injection Computer Self Repair

On my 928S2 (with LH-jet) I recently experienced an ECU failure. The engine cut out during level cruising. It would not restart.

One tow later (with the trusty Jag XJ-S :-) and its back in the garage. A quick check indicated that the fuel pump wasn't going.

The relay checked out OK, but was not being turned on.

I traced this back to a fault in the ECU. The ECU turns on the fuel pump relay with a transistor (which was OK).

The transistor is driven by a TTL 7402 chip, which has a dead output.. I replaced it with a 74HC02, and it works again.

Cheapest thing I've ever fixed on any car : cost all of 50c !!!!

Moral of the story : Always carry a wire link around to replace the fuel pump relay if your ECU dies.

'84 928S2 A/T

Unless your '84 is a Euro, you don't have the LH-Jetronic injection, you have the L-Jetronic. The LH is digital, the L-Jetronic is analog computer-based.

'93 GTS (LH-Jet + EZK)
'83S L-Jet

Certainly is a euro spec car. Imported from the UK to NZ in '95. 100% definitely LH-jet.

The test is pretty easy. If your fuel pump doesn't run, but removing and jumpering the FP relay does make the pump (and the car) run, then its probably this ECU fault.

A number things in the ECU could cause this problem

1) broken solder joint on connector or elsewhere
2) blown-up output transistor
3) blown-up 7402 chip
4) blown-up microprocessor
5) other bad joint in the ECU.

I just worked from the relay all the way back....

'84 928S2 (UK spec).

>Quick question on jumpering the fuel pump relay - when jumpered, I assume
>it just makes the pump always run when the ignition key is turned,

Jumpering the fuel pump relay socket between terminals 30 and 87 causes the pump to run all the time. To get it to operate only when the key in on, try jumpering from 15 to 87. My silber 78 was jumpered this way when I bought it. Didn't notice it was jumpered until a month later. The 15 connection will only power the pump when the ignition is on. BTW the jumper had an 8 amp inline fuse.

Dennis Wilson
78 928 US 5 sp
78 928 Euro AT
80 924 NA 5 sp

Thank you for all the tips on my friend's 87 928s4. I pulled out the LH BRAIN opened it and brought to an electronic repair shop. I asked them to replace all the chips except the 3 biggest parts (according to them its CPU, ROM and CMOS memory are very seldom to die).

Parts cost are very cheap. Labor is bit expensive. Total come to less than $500.

I took it back today and plug it back in... WHALLAA,,, my friend almost hit his garage roof jumping for happiness, THAT BRAIN STARTS WORKING and is desperate of RACE TRACK excercise.



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