Fuel pump not receiving power. DME relay?

pbs911 · pbs911's Garage

The 3.6 SC had to be flat bedded home this morning. Just stopped running. I pulled the fuel line, cranked the motor and was not getting fuel. I jumped the power wire at the DME relay to the fuel pump wire and the fuel starts flowing. I have no power at the fuel pump wire at the DME relay.

Can I just wire the fuel pump wire at the DME relay to the power wire at the DME? Here is a pic of the DME relay. It came from Timmins. Is this a standard Porsche part?

ZOA NOM · ZOA NOM's Garage

Motor would suddenly shut off. Pull over, turn key off, then restart and drive on. Replaced DME relay (~$25 through my mechanic), and it hasn't happened again. I wouldn't wire around the relay, because the fuel pump is not pumping all the time. The DME gives power to the pump under certain circumstances, and not under others. I think the pump is disabled when you decelerate from speed and the RPM's are above idle speed. If you push in the clutch and let the car coast, the pump will be given power to keep the motor at idle.

makaio · 04-04-2003, 12:09 PM

Welcome to DME relay world Paul. That's the same relay my 86 3.2 takes. Sounds like you need a new one, because that is what supplies power to the fuel pump, as you have found. Lukily you live in a Porsche rich environment, so finding a new one is no problem.

88911coupe · 04-04-2003, 02:41 PM

That is nearly an exact duplicate of the DME relay sitting here at my desk that I took out of my 88 except mine does not have the date on the bottom. I don't know if it was really bad or not but I replaced it anyway about a year ago.

pbs911 · pbs911's Garage

Well I just ordered a relay for a 89. I hope it is the same. I am still waiting for the Porsche guy with our host to give me a call back. BTW, I tried to bypass the relay and run the fuel pump directly but there are other problems with the relay apparently. Hopefully a new one will solve the problem. If it wasn't for the extra 100 ponies and gobs of extra torque with the 3.6 this would really piss me off.

Mike Feinstein · Mike Feinstein's Garage

My 6 month old DME relay gave up the ghost today also...must be something in the air. Car was idling in the garage, stumbled and quit. Cranked but wouldn't restart. Wiggled the connector at the relay and it started...wiggled again and it began to stumble. I keep a new one in the glove box all the time. Plugged it in and problem solved.

Good idea to keep a spare handy.

88911coupe · 04-05-2003, 08:51 AM

What is it about them that makes them so prone to failure? I have my old one apart and there does not seem to be much to them. Can old ones be tweaked to be more robust and reliable?

ischmitz · ischmitz's Garage

The DME relay is a very simple two-stage relay. Stage 1 switches on the ignition coils, injectors and the DME box itself when you turn the ignition to ON. It also supplies power (B+) to stage 2.

Stage 2 is the fuel pump relay. It powers the fuel pump and the Oxy sensor heating and a few other things on the engine. It gets switched on by the DME (Gnd) once the computer detects flywheel movement.

As to the failure my brother just got a tip for his VW T4 bus that a batch of DME relays from Hungary is known for 'cold' solder joints causing exactely this problem. Sure as hell, when we opened his you could see the issue. Now Paul, see what your relay reads...

I would be surprised if the fuel pump turns off when you lift off the throttle above idle.

Paul, your relay looks different from what the 89-97 relays look like. The have flatt spade-type connectors. I am surprised that your kit does not use the 89- relay socket and the correct relay.

ischmitz · ischmitz's Garage

Quote:

Originally posted by 88911coupe
What is it about them that makes them so prone to failure? I have my old one apart and there does not seem to be much to them. Can old ones be tweaked to be more robust and reliable?

The solder joints of the heavy components crack with time. You can use a good soldering iron with propper soldering tin to reinforce the joints. This worked fine for my brother in his DME relay.

Furthermore, my guess is that the heavy relay body gets glued to the PCB with newer versions to lessen the stress on the solder joints.