Mach 5

Car = '86 951. I need help from someone who may have installed one of these before. I installed the instructions per 944online and Lindsey explicitly. Now the car won't start. It started fine before I started this project, so I know everything else is in order. Not to mention that as I was tracing down the problem in the first place I checked off all the usual items (DME relay, fuel press, spark, etc.). The only problem was the bad reference sensor wiring.

I have confirmed that each sensor is connected to the right plug, all the wires/pins are in the correct place, etc. I even used a test light to confirm that each wire had been labeled correctly and verified that each wire was in the correct position from the pin location to the pug ends. If you've done this, you will recall that the instructions change the sensor grounding from two points (pin 5 and 23) to a common ground point on pin 5. I did this as instructed, i.e., both sensor grounds are joined to the brown ground on pin 5. No luck.

What am I missing???

Roger

931guru

Use an oscilloscope (as described in the factory service manual) and check the waveform at the DME (ECU) for proper shape and amplitude. Anything else and you are wasting your time.

Mach 5

What will this tell me? Thanks.

Roger

Mach 5

FWIW, I rewired the sensor ground wires at the plug the way they had been from the factory, with the DG sensor grounded at 23 and BG at 5. Versus the way the instructions tell you to wire it, with both sensors grounded at 5. In this configuration, it is wired EXACTLY as it was a couple hours ago when it started and ran, with the old wires. The only change now is new wires. No start. Baffling.

Roger

JacRyann

1. It will tell you if the sensors are spaced properly from the flywheel teeth.
2. Will tell you if the signal has the correct peak-to-peak voltages (amplitude).
3. And will tell you if the ref-signal starts properly on a rising-slope instead of descending-slope.

Did you have the spacer-washer under one of the sensors originally?

Foyley

Be careful here as both plugs are not wired the same! ill dig in and find my drawings and post for you!

Foyley

Refer the attached drawing and you will note the pin out changes for each plug! pins 1 & 2 are reverse position on each plug

Cyberpunky

have you tried swapping the leads around ?

Mach 5

I'll back up a bit. The car ran great for at least 6 mos. Two weeks ago we were at an autocross and the car stalled while waiting in queue and would not restart. After swapping relays, checking fuel pressure (gauge on rail), checking spark, still no start. Then I wiggled the reference sensor wire near the plug. It started and we drove it home. Ran for a couple days, then would not start. Wiggled sensor wire/plug, started. While it was running, wiggled it again, stalled. Removed tape that previous owner had put on the wire just aft of plug and saw that the covering had cracked off the wires and the strands were exposed. Seemed like a pretty safe bet that this was my problem. Ordered a new ref sensor harness from 944online and installed it. Now it won't start.

I guess in my mind, if I was having this problem, and had not yet identified the offending sensor wire, it would make sense to check the operation and gap of the sensors. Is it possible that the sensors that had previously functioned properly with the old wires (when the wires were't shorting out) would somehow not function the same with the new wires?

By all means, I appreciate the suggestions and feedback.

Thanks.
Roger

Mach 5

Yes I did. Thanks.

Roger

Mach 5

Yes, I have these installed correctly. This is how the wiring harness came from 944online. In addition, I used a test light to verify that "25" at the plug corresponded to "25" in the DME plug, etc. They were all labeled, and therefore installed, correctly.

Thanks.
Roger

Mach 5

Does the physical mounting of the DME provide any sort of chassis grounding for the DME itself? I have been trying to start it with the DME laying on the passenger floor, not attached to the chassis. I assumed the DME was grounded through the plug at pin 28, but I thought I'd ask anyway.

Thanks.
Roger

Mach 5

Another question: When I check conductivity at the DME plug with a test light, I get a closed circuit (light goes on) when I touch ground at 28 and at 5, but not at 23. I'm not sure if this matters, since when I originally hooked up the harness, I grounded both sensors at pin 5, per the instructions, and it wouldn't start. Then I changed this back to how it was originally wired with one sensor grounded at 5 and the other at 23 (23 also carries a ground for the O2 sensor).

Any thoughts on this?

Roger

Cyberpunky

I read somewhere that you shouldn't try and start car unless DME is mounted as it does get ground through chassis when mounted. Maybe others can confirm ?

Mach 5

Well, that would explain it...and suck if I fried the DME.

Anyone else on this grounding of the DME via mounting?

Roger