Vorsicht

More data:

1. Checked the fuse for continuity: Good
2. Checked O2 wire: all good there.
3. Key on, jumpered (30 & 87b) & (30 & 87). Multimeter neg attached to grounding point, positive to each of the three terminals in succession. All get 12V+
4. At the fuel pump, multimeter neg to fuel pump neg, pos to pos: Zero V.
5. Neg to frame, pos to each terminal on fuel pump: both read between 4 and 15 millivolts. Don't know what this means.

Questions: Which grounding point does the fuel pump use? Where does the positive (green/black) wire go from the fuel pump? I am assuming it goes up to the fuse box?

Any suggestions?

IainM

Check volts across pump terminals

Check pressure on fuel rail

Did you replace with Porsche pump?

Did it come with new back flow valve?

Some good detective work is needed

JB 911

Sounds like you are getting closer! Do you have access to the workshop manual? It might have wiring diagrams. And break down which systems use which grounding points. It could be a grounding point I guess.

In the meantime that fuse panel where the relay goes comes apart with just a couple of screws and you can look under it to see if indeed the black/green (quoting you) wire goes to it. I can't imagine it would go anywhere else. Good opp too to check the connection and look for any signs of previous heat under that panel where the wire connects the DME relay terminals/receptacles.

You did say if you ran power directly to the fuel pump it ran and started the car? If that's the case it seems you are narrowing it down. It would have to be the wiring between the panel and the pump wouldn't it? Or that ground point. Or,,,, a hidden fuel pump kill switch a previous owner installed and you bumped it trying to get to the $20 you dropped

techman1

Hmmm, I am wondering if the electrical side of the ignition switch is having problems. Just to confirm with the wiser folks out there, key on to run, the relay should give power to the fuel pump, right? I would check the leg that comes to the relay, from the switch. Maybe it is not getting the signal to energize and give power to the fuel pump.

JB 911

I thought that too, but Peter wrote

"3. Key on, jumpered (30 & 87b) & (30 & 87). Multimeter neg attached to grounding point, positive to each of the three terminals in succession. All get 12V+"

I read that to mean he has power at the relay with key on so that should eliminate anything key, or immobilizer/driveblock, no? Plus the car cranks, but doesn't start, that eliminates any driveblock/immobilzer/alarm, no?

He has power at the relay, no power at the pump. Has to be somewhere inbetween, no? Maybe some wise ones will chip in. I remember a post like this a couple years ago. Can't remember what the end result was.

Vorsicht

I discovered I was using the wrong terminal at the fuse box (wrong 87b) hence the prior problems. Sorry to those helping me for my idiocy. Moving on.

Test 1.

1. 30 & 87b jumpered. Pump works.
2. 30 & 87 jumpered. Car tries to start, coughs, burps, pops & dies. 5 seconds. Tried 5 times to eliminate air in fuel line from pump change.

Separately, Test 2.

Using both DME relays in succession (un-jumpered), key on. Pump does not start. I'm guessing I have 2 bad DME relay's. Or perhaps, 'key on' signal not coming thru which would nullify the pump as well as the start (juice to elec/injectors). See test 3.

Test 3.

Plug inToreB/Scantool to my laptop. Key On. Scantool will not connect.

Hmm. Does this mean a faulty ignition switch? Is there a way to test if 'key on' state is reaching DME without installing a new Ig. Switch?

MB965

ToreB/Scan tool may need the immobilizer off to connect, but I don't have the Tore tool.
My Durametrics tool needs the immobilizer off to connect to the car.

Vorsicht

I'm leaning in this direction now. Not an elec. genius but understand basics. I need to figure out how to test 'key on' at fuse box. Given this diagram how to test if injectors are getting juice? I would think that jumpering 85 & 86 and measuring voltage across the two would tell me, no? I'm assuming the 944 relay schem. is the same for 993.

Vorsicht

No Immobilizer 1995 Car.

MB965

Mine is also a 95 but with immobilizer, less joy then your happy face!

Using your diagram you should be able to measure nominal 12V between 3 (86) and a body ground with the key on. But you already confirmed 12V power to 3 (86) with your Test 1 when the pump ran.
Might be worth a try to cycle and giggle the ignition key and see if the 12V stays constant at 3 (86).

When you jumpered 2 (30) to 8 (87) you did not have 12V available to the fuel pump, which may be why the engine cranked but did not start. I am assuming the relay is not installed when you are doing the jumpers.

ToreB

The Porsche service manual suggest to start with checking the power supply at the Motronic engine control (DME) unit connector. Terminal 30 and 15 must be checked, pin 18 and 27.
Cheers,
Tore

Vorsicht

I am doing all this without the fuse in the slot.

The diagram on my dme relay seems backward & confusing so I will refer to the absolute position of the pin slots. So looking straight down from the right fender at the dme socket the top three spots are small vertical pin (left), large vertical pin(30-middle) and small vertical pin (right).


The one on the left is getting nothing and the one on the right is getting 12+ volts using a body ground. I assume the right side is 86?

Also, in test one I did not test 86 directly, only 87b & 30 which did make the pump work.

Vorsicht

ToreB wins the prize. Thank You!

Several weeks ago I installed some new Coco mats. During that install I must have jiggled something loose.

When I went to to check the DME connector as Tore suggested I found that it had come loose, just barely. The metal retaining latch was jutting out.

I pushed it back in, popped in the DME relay and VROOM~!

Thanks to all who pitched in. Now relieved to have P-car back!

Regards, Peter

IainM

When you turn on the ignition, the fuel pump runs for a short while (~3-5secs) then stops until the DME sees the crank spinning. I primed my new pump with the 30-87b jumper trick and verified 53lbs on the fuel rail.

In the relay diagram you show, the box marked "fuel pump" is the energizer coil for the relay, so the fuel pump cannot be turned on until the "main relay" is energized. The the DMW can pull pin 85b to ground to turn on the pump.

Jumping pin 85 to 86 is a bad idea because you are shorting out the coil and could damage your DME (or worse)

What fuel pressure do you get when you jump pins 30 and 87b?

Vorsicht

IainM "Jumping pin 85 to 86 is a bad idea because you are shorting out the coil and could damage your DME (or worse)"

Damn glad I didn't do that!