DME relay/fuel pump issue

GTS 22 · 01-31-2024, 06:10 PM

We are trying to get a 964 back on the road and are having an issue with the DME relay turning on the fuel pump. If we turn the key the engine will crank but not fire up. We traced the problem down to no fuel. We pulled the DME relay and found if we use a three wire jumper you can hear the fuel pump run and the car starts right up and drives no problem. So thinking the relay was bad we replaced the DME relay but no change the car will still not start. Is it a DME problem? or is there some type of immobilization through the central locking system or another system which will cut off the fuel? Has anyone run across this issue?

Note this car was purchased as a non running project so the history is unknown.

p911dad · p911dad's Garage

Over on Rennlist on the 993 forum someone has a similar sounding problem with a 993. One of the members opined it could be a flaky crank sensor (defective or out of gap tolerance) not sending a good signal to the ECU to run the fuel pump.

mysocal911 · 02-01-2024, 09:53 AM

Quote:

Originally Posted by GTS 22

We are trying to get a 964 back on the road and are having an issue with the DME relay turning on the fuel pump. If we turn the key the engine will crank but not fire up. We traced the problem down to no fuel. We pulled the DME relay and found if we use a three wire jumper you can hear the fuel pump run and the car starts right up and drives no problem. So thinking the relay was bad we replaced the DME relay but no change the car will still not start. Is it a DME problem? or is there some type of immobilization through the central locking system or another system which will cut off the fuel? Has anyone run across this issue?

Note this car was purchased as a non running project so the history is unknown.

You are correct!

LM964 · 02-02-2024, 01:59 AM

Yes, I think if the car is alarmed (active) then there is no signal to the relay for the fuel pump to switch on. You have an old aftermarket alarm installed? Think too if an airbag has been activated then a signal is sent to the relay to cut the fuel pump signal. Assuming also that fuse #34 is good(?)

I've been caught out with swapping relays in the past that also turned out to be partly defective too. Only sure way for me to check was to swap relays with another 964 and check for the same symptoms (or not).