DME Relay or something else?

[Skip1]

So over the last couple of days I've had more and more trouble starting the car ('84 911 3.2 Li) RoW with Canadian motor.

Today, I was working on the car in the garage and had the engine idling, and the idle dropped to around 500 rpm from 800 (as normal), and then just stalled. Couldn't start it after that for some time. Tried several different things (including spare DME Relay that had worked in the past, but not new), and still no start. I also confirmed that the fuel pump worked over the last few days and still today, by shorting it to direct power.

Final attempt tonight, was that I shorted the fuel pump as stated in other threads, and tried to start it while shorted. Fired up and ran smoothly for 10+ minutes, until I shut it off, so as not to overload another circuit.

Looking at the wiring diagram, it seems pretty simple, but what I'm trying to figure out is, could there be a break in the wire from the ignition switch somehow, or could it be two bad DME relays?

Thanks

[HawgRyder]

If the replacement DME is not new...I would suspect it has been subject to the same vibrations the old one experienced.
The problem as I understand it...inside the DME package is a circuit board with components soldered on...vibration breaks solder joints!
Either get a known good DME...or a new one to try.
Don't throw the old ones out!
Have a tech open them up and look them over....they are repairable.
Also...keeping a replacement DME in the glove box may not be the best idea unless it is vibration protected.
Bob

[JJ 911SC]

Bob

After reading this, I'm sure you mean the "DME relay" as oppose to "DME" in your posting. Just in case someone...

Pat

I should have give you mine that I now longer use.

P.S.: so schnell kann Sie vergessen haben, Sie sind aus Ottawa (your location)

[Skip1]

Yes, it is the DME Relays I replaced, not the DME. The engine runs fine, once I short the fuse box for the pump to another fuse (live), the engine started and ran smoothly. So am not sure if this also bypasses the DME Relay (I think it would)?

[Chuck.H]

Yes you can bypass the DME relay with a wire jumper on the fuse block to the fuel pump, so your problem could well be the DME relay. The DME computer provides the signal to the DME relay to enable the fuel pump once the engine starts spinning, so it's remotely possible that the speed sensor is faulty, but I'd certainly start by replacing the relay.

Chuck.H
'89 TurboLookTarga, 360k miles

[Skip1]

Thanks for the comment. I've just ordered two new DME Relays from the local Porsche Dealership just south of here. Will arrive tomorrow (gotta love that kind of delivery system - helps to be in the same country as the manufacturer).

I did replace both speed and reference sensors over the last few months, so hopefully it's not those.

Hope it's as simple as the DME Relay - would rather not have to "go diggin" further, as I have a great round trip through the region planned this weekend.

[Skip1]

OK. So, here's the ground truth. Can't believe this even happened. Got home this afternoon and decided to print out the cicuit diagram for the fuel pump while I awaited the delivery of new DME Relays tomorrow. Checked all the point to point from DME to DME Relay, to ground to fuse panel, to pump and finally to the ignition switch. Always the last thing isn't it. The electrical plug (round in shape with a dozen wires coming off it) which connects to the ignition switch had moved about 1/4 of an inch off fully home (I guess through vibration? Seems really strange). Pushed it back on, reconnected the fuse panel to pump connection, reinserted the fuse, a few cranks and off she went, humming like it hasn't in a while. No surging, no sputter just smooth.

Chalk it up to, "back to basics" and checking the wiring, although this is a first that I've seen the harness plug "loosening off" the ignition switch. Has anyone ever seen this before? If so, how do I avoid it in the future (besides the obvious of looking under the dash every so often)?

Thanks for all the comments, on the road again - next, transmission fluid change tomorrow...

[JJ 911SC]

Hopefully your were out during the hail storm...

[Skip1]

No hail. Last night I adjusted the clutch cable (was way too tight, causing problems in shifting when hot). All seems smooth now, including going into 1st gear without grinding, that's just sweet.

Had it out for a nice run yesterday after fixing all the little things I noted, and all is working well together. Next....looking at adjusting throttle linkage (seems a little wonky and less than responsive to me).