rockatansky

I have this problem since I bought the car. Fix this is my next task.

Right now, with key in position #2 (dash lights etc) fuel pump starts and never stops. Of course when key at pos. #3 (cranking and engine running) it keeps going. But obviously in case of accident the pump will keep feeding fuel to the system which is not correct.

I have ordered a new relay in case this one is not working right. While I wait (two weeks) I'm doing some tests with the multimeter that I would like to share with you to see if hidden in this table is there any clue about the reason of the pump not stopping after the first two seconds of feeding.

Also I have attached an schema I draw with information of the forum to check if I'm going in the right direction to understand how it works. Thanks for any comment.

Mrmerlin

check the back of the CE panel for shorted wires.

IIRC the LH system adds a ground to the pump someone may have added a jumper the pump ground.

dr bob

The original CIS fuel pump relay includes a sensor circuit that receives tachometer pulses to determine that the engine is spinning. When they fail, a too-common temporary fix is to substitute a standard relay that turns on when the key is turned on. I recommend that you remove the fuel pump relay, and check it to see if it is the correct part. Put the correct one in, and I suspect that your problem will be cured. The origional part has one very narrow pin on it for the tach signal, and IIRC has two output contacts. The graphic on the relay can shows a rectangle with a transistor symbol in it.

rockatansky

I think Dr Bob is in the right direction. I have checked the relay part number and the one that is installed seems pretty modern compared with the rest of the relays in the car. Attached I post a picture of it.

Pins seems with same configuracion, but I suppose that maybe this one doesn't have the internal sensor that receives the tachometer pulses that you mention.

This one is KAEHLER 3.300.100 (seems made for Audi and VW for what I have investigated) and the correct part is KAEHLER 3.308.200 (which correspondes with 928.615.113.01).

MrMerlin, if the relay is not the cause, I will check for shorts at the panel.

Mrmerlin

Thanks for the info Dr Bob,
i have also seen a few examples where there were add on grounds put to the fuel pump relay so it would turn on.
I didnt know about the special realy.

rockatansky

Solved!. It was the relay. Wrong part number installed by previous owner. This one did the stop after one second with the key on position #2. And I feel like the start of the car has been stronger, fast and smooth than before (maybe is the stockholm syndrome). Here is a video of the first start with the new relay.


Reference for future rennlisters; served incredible fast from California to Europe, part number KAEHLER 3.300.208 for the Porsche part number: 928 615 113 01. Ebay seller was: 'suppersellstore'. It was for a Europe 1980 Porsche 928 CIS K-jetronic.

Thanks everybody for your help. Task marked as done, going for a new one...

Jadz928

Thanks for sharing your experience and following thru with a resolution. Great info for everyone here with CIS cars.

rockatansky

A pleasure Jim to contribute a bit at rennlist. I need to keep checking but I must say that the two starts after changing the fuel pump relay for the correct one the car has started really nice. Before that it starts but with more cranking, and this two first tests has surprised me how good it goes.

There is one more electric gremlin that appear from time to time and I havent seen it with the new relay (fuel gauge needle jumping crazy) but I need to keep checking next days to see if it was related.

In any case I recommend that CIS owners check to see which relay part number you have installed...

jpitman2

FWIW, I already have the same relay, and my car DOES NOT run the pump at turn on, but not crank. Stuck my head under the rear bumper, had somebody turn the key on/off a couple of times, nothing except the sound of a relay closing up the front somewhere . Other than this, the car starts from cold and unused recently with maybe 5 secs of cranking, which is longer than I would like.
Any suggestions please?
jp 83 Euro S AT 55k, CIS

rockatansky

Hi Jpitman. sometime since I changed the relay. With the new one mine do the 2 seconds push of fuel at the pre-crank position. Then when cranking it starts in 1 or 2 seconds (of course if it haven't been stopped for long time, which could take then up to 5 seconds). The time of five seconds you mention on yours sounds like your car needs the 2 seconds push before crank to do a better start.

To give you more information I should mention that my pump not always the do push. It's curious but the second time you start the car, it's like it guesses that it doesn't need a second push because it has been done recently and no second sound comes from the pump (you need to wait some time, maybe hours, to hear again the pre-crank push). Dont know exactly how it works, but if you go to pre-crank position in the first start of the day, you hear the pump, and then turn off the key without cranking, and then turn to pre-crank again, it doesn't do the a second fuel push.

I'm sure someone around could point you in the right direction to check if the relay is receiving the right information from the panel contacts, before buying a new relay.

daveo90s4

Joaquin, that would suggest that there is an additional piece of logic controlling the fuel pump circuit that either:
1. Checks for fuel pressure and if fuel pressure already above x then do not power fuel pump before engine is actually cranking, or;
2. Checks time since fuel pump last ran (as a proxy for latent fuel pressure still in system) and if time less than x then do not power fuel pump until engine is actually cranking.

Does anyone know if the electrics contain such logic?

rockatansky

That's what I thought. But never confirmed... at least that's what's doing in my car, not sure if it's right or not.

It would be good to know. If someone could explains us the correct behavior of the pump.

Also, if someone can help jpitman, the reason of a pump not doing the 2 seconds push with the key at position 2.

dr bob

The relay sees the tach circuit pulses that float then get pulled low as the engine spins. On my non-Porsche CIS cars, the pump would run or not run at key on, seemed to be related to how long it had been since it had last run. It doesn't have any direct sensing of fuel pressure. More likely it's related to the charge time for a capacitor in the pulse detection circuit.

rockatansky

Thanks dr bob. That clears my doubts. Jpitman, maybe you have a bad relay or for some reason is not receiving the tach pulses. Should be a way to troubleshot those but have in mind that, because the time delay the second time you try, the pump should not turn at the pre-crank position.

Dave928S

JP ... do you know anyone who can loan you a known good ignition box you could swap out?