Help, 930 starts then immediately dies.

[jwakil]

initially I had what looked to be bad contact at the fuel pump #6 fuse so the car would crank without starting. Replaced fuse but now it will start then immediately die. Replaced both relays but didn't solve. What could it be? Waiting for tow truck now

[T77911S]

out of gas
unplug connector on top rear of AFM to run pumps with key on and engine off to verify pumps.
you can then remove one relay at a time to listen for each pump.
(it may run with only one pump).

jumper both relays to run pumps.

[jwakil]

Originally Posted by T77911S

out of gas
unplug connector on top rear of AFM to run pumps with key on and engine off to verify pumps.
you can then remove one relay at a time to listen for each pump.
(it may run with only one pump).

jumper both relays to run pumps.

Ok, I unplugged the connector on rear of AFM and the car actually started and ran fine. Re plugged and problem came back. With connector plugged I don't hear the pumps when key is turned to on. When connector is unplugged I hear the pumps, is this backwards of how it should be? I assume it's not good to have the connector unplugged. In normal operation does the AFM require an open and if grounded then it cuts off the fuel pumps? What should I replace/fix?

[flightlead404]

The airflow sensor relay grounds through the airflow sensor contact in the AFM. The airflow sensor relay in turn provides the power to energize the two fuel pump relays.

[jwakil]

Originally Posted by flightlead404

The airflow sensor relay grounds through the airflow sensor contact in the AFM. The airflow sensor relay in turn provides the power to energize the two fuel pump relays.

Ok, so I would expect that when I unplug the connector, the AFM relay would lose ground and the car would not start. However, it's actually the opposite. So does this relay energize the fuel pump relays when it is NOT grounded and cuts them off when it IS grounded?

[T77911S]

year of car?

the fuel system is kind of complicated if you don't read diagrams,

I think of it as 2 circuits.
one circuit controls the circuit that actually runs the pumps.

see the sticky at the top and look for my post near the end for ore details.

the connector on the AFM controls the yellow relay on the relay panel in the engine bay.
the yellow relay controls the 2 fuel pump relays.

if you unplugged the connector and it ran fine then more than likely that wire from the yel rly is grounded keeping the yel rly closed or energized, probably a rare problem.

the good thing Porsche did with this design is they used the normally closed contacts on the yel rly. that way if you have your problem or more common if the rly fails, it fails de-energized thus the car will run.

[T77911S]

I don't know your year. if you have an early car you don't have the speed relay.
if pin 85 of rly G is grounded the pumps do NOT run.

[jwakil]

I have an early car, 81'. I had another long thread with a fuel pump power issue a couple of months back so from that I concluded I don't have any yellow relay, no speed relay. So basically the overboost switch feeds directly to the G sensor relay (which I haven't really identified on my car but assume it is there). So ignore B, C, M, E, D on the diagram.

So my main question, which I believe you answered, is that when F is plugged in and working properly, it de-energizes G and the pump relays supply power to the pumps. Somehow my sensor is not working so it is energizing G and killing the power to the fuel pumps. When I unplug F, then I de-energize G. Does that sound right?

[356911930]

T77,

Looks like your diagram may need a revision. Is location "J" missing from the text portion of the diagram and then "K and L" may be mislabeled with respect to diagram labeling of "J" and "K"?

Rahl

[T77911S]

with F plugged in and the engine OFF, that sw closes putting ground to rly G energizing it thus turning off the pumps. (rly shown de-energized).

I think I remember your post about you may or may not have rly G.
don't remember the final result



never looked at the legend at the bottom, just use it for ease of wiring.

[T77911S]

[flightlead404]

Originally Posted by jwakil

So my main question, which I believe you answered, is that when F is plugged in and working properly, it de-energizes G and the pump relays supply power to the pumps. Somehow my sensor is not working so it is energizing G and killing the power to the fuel pumps. When I unplug F, then I de-energize G. Does that sound right?

yes. seems backwards but yes

[jwakil]

Ok, I'm at a loss now. After driving fine for a week or so with the airflow sensor contact unplugged, the car now seems to have the same symptoms even with the AFM sensor unplugged. It basically starts for a second then immediately dies. I also unplugged and grounded the overboost switch with no improvement. The only way I can keep the car running now is to directly apply power to pins 87 of the fuel pump relays (basically bypassing all the secondary safety stuff). So the next thing I need to test is my relay G in diagram of T77911S's posting. However, I have never been able to find where this relay is on my car. Is it the round type in the engine bay? If so, I only have one back there and I've already replaced without any affect. As stated previously, my car has no yellow speed relay. Where is the normal location for the airflow sensor relay?

[RarlyL8]

Have you thoroughly cleaned the fuel pump fuse housing and put in a new fuse?
If you jump the fuse will the pumps run?
Relay G is in the engine compartment on the fuse/electrical panel that is covered by the large black plastic piece that is held in place with a wing nut. The relay is closest to you when looking at it from the rear of the car. I posted a few pictures of that area recently in my restoration thread. Turns out my relay is dead, if I remove it nothing changes. Car runs fine, the air flow sensor contact being rendered dead as well.

[jwakil]

Originally Posted by RarlyL8

Have you thoroughly cleaned the fuel pump fuse housing and put in a new fuse?
If you jump the fuse will the pumps run?
Relay G is in the engine compartment on the fuse/electrical panel that is covered by the large black plastic piece that is held in place with a wing nut. The relay is closest to you when looking at it from the rear of the car. I posted a few pictures of that area recently in my restoration thread. Turns out my relay is dead, if I remove it nothing changes. Car runs fine, the air flow sensor contact being rendered dead as well.

There is one round relay in my engine bay panel in that position and I replaced with no effect. Then I decided just to ground pins 85 of the two fuel pump relays, effectively eliminating all the safety stuff. The started and ran fine. So something in the secondary circuit is not working and it's not the relay G itself, but in the wiring between the relay G and the AFM and overboost switch. Obviously I'm getting power to pins 30 and the ignition power is also reaching fine.

[T77911S]

so you think the round relay is relay G?

get a test light.
clip it to the fuses in the engine bay then ground the light to make sure it works.
the probe pin 87a of the relay socket.
if you have another socket, check it too.
this is the ground thru the over boost switch..

connect the TL to ground.
probe pin 30 of the socket with the key on.
this is the ground path from the FP relays. should have 12v on it.

[jwakil]

Originally Posted by T77911S

so you think the round relay is relay G?

get a test light.
clip it to the fuses in the engine bay then ground the light to make sure it works.
the probe pin 87a of the relay socket.
if you have another socket, check it too.
this is the ground thru the over boost switch..

connect the TL to ground.
probe pin 30 of the socket with the key on.
this is the ground path from the FP relays. should have 12v on it.

As I view the diagram, Pin 30 of relay G socket should have 12V if key is on but the overboost switch is disconnected (or relay removed), but should be ground if the overboost is connected and working properly, right?

[T77911S]

pin 30 should have 12v
87a should be ground
with relay removed.