Dme power question

[isocyn8]

Can anyone tell me which wire from the ignition switch powers the dme or dme relay?

[Outback Porsche]

The black wire. There should be two black wires coming from the same point on the switch connector.

Cheers,

[isocyn8]

Thanks.

[isocyn8]

Hey outback, is there any chance you can tell me where the red wire goes. Trying to fix a no injection pulse problem and the wiring behind the ignition switch is all hacked up from an alarm. Thanks.

[TexasBlake]

Originally Posted by isocyn8

Hey outback, is there any chance you can tell me where the red wire goes. Trying to fix a no injection pulse problem and the wiring behind the ignition switch is all hacked up from an alarm. Thanks.

Download the electrival volumes of the factory service manuals from my site: Documents - Porsche 951 - texasblake.com-

They will tell you what every wire in the car is. They're magical.

[Outback Porsche]

Or here for the older/NA cars;

Manuals

Cheers,

[isocyn8]

Hey guys, Im trying to figure out why my fuel pump will only run if jumped out at the dme relay. I have put a new relay in along with a known good one, but still the same problem. Does the dme signal the relay to close the circuit for the fuel pump, making the pump come online? If so it would make me think I need a new dme.

[Outback Porsche]

Yep it does. Look here for dme relay explained;
85(early) 944 starting issues...

There's a good chance that you have a bad solder joint within the dme. Look specifically at the connections for the large transistors.

Cheers,

[isocyn8]

I just pulled the dme out of the housing and I am checking it out. It has areas that look like dried varnish. It flakes right off, but wondering if this is something that would short out the dme or not.

[Outback Porsche]

Originally Posted by isocyn8

is something that would short out the dme or not.

No, but it will contaminate the solder during rework. Use plenty of flux the help the solder wet to all surfaces if you have to rework it.

Another thing to look for is water/moisture ingress - that'll stuff up the DMEs day.

Cheers,

[Outback Porsche]

Originally Posted by isocyn8

Hey guys, Im trying to figure out why my fuel pump will only run if jumped out at the dme relay. I have put a new relay in along with a known good one, but still the same problem. Does the dme signal the relay to close the circuit for the fuel pump, making the pump come online? If so it would make me think I need a new dme.

I gather the engine ran okay with the relay jumper in?

[isocyn8]

Nope with the relay jumped the fuel pump ran but still no pulse to my noid light. Does it sound like im heading in the right direction? I have chased every wire. replaced the reference and speed sensors. I do have spark and tach bounce. I checked the bottom of the fuse/relay board and it had some burned wires that I fixed, but turned out to be wires for the blower motor. I also replaced the ignition switch. Have I missed something? Thanks again for all of your help.

[Outback Porsche]

I know your pain. These things can be frustrating. It does sound more and more like it's a faulty DME but before you go get another one, have you checked the injector impedance? They are nominally 2.4 Ohms (the service manual says 2-3 Ohms is good). Because they are all effectively in parallel, one bad one can bugger them all.

Cheers,
Jeff

[isocyn8]

Hey Outback! I did the resolder thing and gave it a shot. The damn thing is alive!!!! haha. Don't know how long the dme will hold out, but it looks like it is the problem. Thank you for all of your help.

[Outback Porsche]

Good news. If your happy with the solder job it should hold out for a long time.

Curiosity - was it one of the large transistors on the side of the case or did you just do them all?

Glad to have helped.

Cheers

[isocyn8]

I did them all. nothing really jumped out. Was really surprised when I heard it fire off.

[Outback Porsche]

Cool. It's a good win.