DME relay question

[wjdunham]

Originally Posted by wazzz

10.5V while cranking isn't that bad, considering the current drawn by the starter. Battery voltage always drops a little while cranking. Wherever you measure it, including upstream the fuel pump, you'll see the dropped voltage.

Seeing 10.5V at the pump fuse does not mean that the pump is drawing any current. It could be just dead and cut open (not shorted). If you don't hear any buzz, it is probably dead.

Good points, thanks!

I have a fuel pressure gauge, I'm gonna give that a try later today, but I think I need to find an O-Ring for it first...

[wjdunham]

hooked up the pressure gauge - 0 PSI while cranking the engine.

Any chance it is the fuel pressure regulator - do those hard fail and just shut off the fuel flow?

[dtwa]

Originally Posted by wjdunham

hooked up the pressure gauge - 0 PSI while cranking the engine.

Any chance it is the fuel pressure regulator - do those hard fail and just shut off the fuel flow?

The fuel pressure regulator in my 85 cut fuel pressure intermittently before failing completely giving zero pressure at the test port on the fuel rail. You might want to check pressure at the fuel filter exit. Careful, if your fuel pump is working you should get something like 100psi there.


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[wjdunham]

And the answer is, Fuel Pump for $150...

Not really that bad of a job to do, the magic tool is the crimper to pinch the short fuel line from the tank to the pump. About 2 hours of work total and it's buttoned back up and ready to rock.

Thanks everyone for the help/suggestions.

Bill

[BoxsterGT]

The 90mm M12 Supply Hose from the Tank to the Pump does not need a crimp.

Norma hose clamps are just fine as this is a suction or neg pressure into the Pump.

It's the Hi-Pressure Fuel Out from the Pump that requires the crimps.

Len

[wjdunham]

Originally Posted by BoxsterGT

The 90mm M12 Supply Hose from the Tank to the Pump does not need a crimp.

Norma hose clamps are just fine as this is a suction or neg pressure into the Pump.

It's the Hi-Pressure Fuel Out from the Pump that requires the crimps.

Len

Sorry Len, I just meant a tool to pinch the hose so the fuel tank didn't empty itself all over my garage floor. I did indeed just re-use the hose clamps on that short line from the tank to the pump.

The rest of my fuel lines look very good, suspect they were replaced at some point before I took ownership of the car.

Bill

[MAH77]

Anyone who looks this thread up for a culprit to a no-start issue. 1 other check is if when you are cranking the engine the tach does not move it is the reference sensor. I just dealt with this 3 weeks ago. Car died mid drive, tach fell like a stone. Replaced DME, same no start issue. Installed 2 new sensors (use the BMW sensor, BOSCH 0261210002, for half the Porsche price) in about 2-3 hours.

[wjdunham]

Originally Posted by MAH77

Anyone who looks this thread up for a culprit to a no-start issue. 1 other check is if when you are cranking the engine the tach does not move it is the reference sensor. I just dealt with this 3 weeks ago. Car died mid drive, tach fell like a stone. Replaced DME, same no start issue. Installed 2 new sensors (use the BMW sensor, BOSCH 0261210002, for half the Porsche price) in about 2-3 hours.

That's a very good suggestion. I could have saved myself a bit of headache had I checked that the tach was bouncing while cranking the engine.

I'm surprised it's only 2-3 hours to replace those sensors. I figured it would take me at least an half hour to take all the blower/heater components off so I could get to the wiring, and an hour to put it back together (always seems to take twice as long to re-assemble). Add another few hours to get the old sensors out and new ones in, at least a half hour of which would be fiddling with the wires through the grommet in the rear engine tin... I was thinking more like 4-5 hours from start to finish, I don't have a lift.

[hierogk]

Please help me understand the fuel pump wiring circuit. If the DME relay controls the fuel pump on the 84-89 Carrera cars, why do they also have the red fuel pump relay?

[FrankM_]

They might have a red relay, but it isn’t for the fuel pump. On my 1985 the red relay is for the a/c. All other relays are black.


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[hierogk]

Thanks Frank. I was looking at a 1984 fuse box label found on post #78 on the following link: https://forums.pelicanparts.com/porsche-911-technical-forum/753748-made-fuse-box-label-my-88-a-4.html.

It shows a fuel pump relay, but the label may actually be for an SC rather than a Carrera. Thanks for your help.

[FrankM_]

Originally Posted by hierogk

Thanks Frank. I was looking at a 1984 fuse box label found on post #78 on the following link: https://forums.pelicanparts.com/porsche-911-technical-forum/753748-made-fuse-box-label-my-88-a-4.html.

It shows a fuel pump relay, but the label may actually be for an SC rather than a Carrera. Thanks for your help.

Must be an SC than although I always believed all 84's were Carrera's ?

I do not see it on my car (1985) or in the Bentley manual and it is clearly the Motronic ECU that controls the second phase of the 'DME relay' that switches on the fuel pump, and in the case of US/Japan car also the oxygen sensor heater.

[hierogk]

Frank,

It was probably just a mistake by the person who made the label. I was looking for a printable label to put on my 1984 Carrera fuse box. I'll keep looking for one that is right for my car. Again, thanks for helping.