3.2 No Start but sensors check okay. Advice Requested

[Quasimoto]

Last Fall, I was having some issues with an intermittent no start. I ended up finding a badly cracked/epoxied reference sensor connector. Since I was pulling the engine for a transmission rebuild anyway, I replaced the crank and CHT sensors, and repaired the harness side by installing a new JPT connector.

With the engine now back in the car, I still have a "cranks, but no start" condition with a known good DME relay.

Car starts and runs fine with a 30/87/87b DME relay jumper.

Here are the checks I have done thus far.

Checked at the DME Connector
1. CHT Sensor - 2.57K ohms - OK
2. Speed Sensor - 993 ohms / 2.8VAC - OK
3. Reference Sensor - 985 ohm / 0.5VAC - OK
4. Pins 5/16/17 are all good grounds - OK
5. Pin 1 - 12V - OK
*This seems to suggest that my repairs and sensor replacement were successful.

Checked at the DME Relay Connector
1. Pin 30 - 12V - OK
2. Pin 86 - only 4-6 volts with key in Run - Seems sketchy.

I do have the factory alarm, but it was jumpered out long ago using the information here on Pelican. (I'm beginning to wonder if possibly the contacts in the ignition switch are failing)

What have I missed, and where would you suggest I look next?

Thanks!

[cabmandone]

Wow! If it runs with the jumper it should run with the good DME relay installed unless for some reason you're not getting power to the coil of the DME relay. If I'm not mistaken you're just closing the circuit with the jumper. If the relay isn't getting power to the coil, it will not start.

[Quasimoto]

I believe Pin 86 is power to the DME 1st stage relay coil from the key/alarm, and I don't have full voltage there.

Question is, where is the first place to look next?

[cabmandone]

I don't know. I'd need to look at a wiring diagram and trace it. I think I have one on CD. If I get a chance I'll fire up one of my old laptops that have a CD drive and take a look. Hopefully someone will chime in. You might PM Steve Wong or Sal. Both can probably tell you off the top of their heads.

[cabmandone]

Here ya go.
no 12v power to dme relay socket pin 86

And Here
https://dorkiphus.net/porsche/showthread.php?t=16859

[Quasimoto]

I’d love to test that circuit after the ignition switch so I could be certain that’s the problem.

The schematic in the Bentley says that goes as Red/Blk to fuse 7 in the frunk, and from there a Blk/Red wire runs back to the alarm module, but I can’t find a fuse that matches the colors shown.

[cabmandone]

Don't know how long the leads are on your meter but you could disconnect the batter, plug one into the ignition switch harness, and start hitting wires till ya get continuity.

[mysocal911]

Originally Posted by Quasimoto

I’d love to test that circuit after the ignition switch so I could be certain that’s the problem.

The schematic in the Bentley says that goes as Red/Blk to fuse 7 in the frunk, and from there a Blk/Red wire runs back to the alarm module, but I can’t find a fuse that matches the colors shown.

The Porsche factory diagram indicates that pin 86 of the DME relay has a black wire from fuse 8.
The DME relay then provides 12V power to pins 1, 18, & 35 of the DME ECM. For additional troubleshooting info, check here;
911 3.2 No-Start Troubleshooting

[Quasimoto]

Thanks @cabmando

Originally Posted by mysocal911

The Porsche factory diagram indicates that pin 86 of the DME relay has a black wire from fuse 8.
The DME relay then provides 12V power to pins 1, 18, & 35 of the DME ECM. For additional troubleshooting info, check here;
911 3.2 No-Start Troubleshooting

Thanks, I was hoping you might weigh in on this. Yes, I used that troubleshooting list to get this far, very helpful.

On my car, the DME Relay pin 86 is fed from the factory Alarm module that is connected to the keyswitch. (It was jumpered out a couple of years ago)

Since I have low voltage at the DME relay pin 86 and it fluctuates with the key operation a bit, I suspect that it's the contact in the ignition switch. I would like to confirm it by measuring it after the keyswitch but cannot seem to locate that wire on the fuse panel as it's shown on the diagram.

Is it possible that my car being an 84 is different than the schematic I have for an 85, and this junction is made somewhere else?

[cabmandone]

At this point you know it's either fuse 7 or 8. I'm not following the difficulty at this point?? You could answer this question with a multimeter in about 10 minutes from here.

There are 6 25a fuses in the fuse box if I'm not mistaken. Even if you're not 100% sure which fuse is 7 or 8 this shouldn't be hard to figure out with a multimeter at this point.

[mysocal911]

Originally Posted by Quasimoto

On my car, the DME Relay pin 86 is fed from the factory Alarm module that is connected to the keyswitch. (It was jumpered out a couple of years ago)

Your wiring diagram is correct per the factory wiring diagram, i.e. fuse 7/8 goes to pin 86 of the DME relay via pins 15 & 87a of the alarm module.

Originally Posted by Quasimoto

Since I have low voltage at the DME relay pin 86 and it fluctuates with the key operation a bit, I suspect that it's the contact in the ignition switch. I would like to confirm it by measuring it after the keyswitch but cannot seem to locate that wire on the fuse panel as it's shown on the diagram.

You need to start measuring voltages from fuse 7, which comes directly from the ignition switch.

Originally Posted by Quasimoto

Is it possible that my car being an 84 is different than the schematic I have for an 85, and this junction is made somewhere else?

No!

[Quasimoto]

Originally Posted by mysocal911

You need to start measuring voltages from fuse 7, which comes directly from the ignition switch.

Thank you. It took me a minute to figure out which Fuse was 7. And of course, since I've fiddled with the ignition switch a bit, I am getting a full 12V there now. At least I caught it once yesterday.

Originally Posted by mysocal911

No!

That's good!

[cabmandone]

FWIW, my B.I.L and I fought a no start on a 914 with a Subie powerplant for probably 5 or 6 hours. I was convinced he had a crank position sensor problem. Ended up being his key switch which was only 2 years old. They fail.

Based on your other thread it looks like you're on the right path.

[Quasimoto]

I hope so. Decided to do the Hazard switch while I’m at it as it has a broken ear....yadda yadda yadda $250 bucks later, I have them coming Wednesday.

[cabmandone]

Originally Posted by Quasimoto

I hope so. Decided to do the Hazard switch while I’m at it as it has a broken ear....yadda yadda yadda $250 bucks later, I have them coming Wednesday.

While yer in there strikes again! LOL

[Quasimoto]

And there was much rejoicing!!! Thanks to all that offered advice and encouragement!

We are back to freakishly fast starts, so seats are going back in today just in time for some really nice weekend weather.

Interestingly, my troubleshooting was made a little more difficult because I could often read full voltage at all of my test points. I ended up opening a mechanical DME relay so I could see the coil movement, and using alligator clips to test the circuit. Even though I could read full voltage through the ignition switch contact with a meter, it would not pull in the stage 1 coil ...just wiggle it for a millisecond or so.

Replacing the ignition contacts and the hazard switch were a little fiddly, but it’s done and I’m thankful that Pelican had them in stock and shipped them quickly for a reasonable fee...

Edit:
I think my desire for a Singer has evolved from mechanical lust to just wanting to be done with the dodgy under dash electrics on those cars...lol