931 advice

[Smart931]

While preparing my 1982 931 for its smog test i noticed a fuel leak.
After changing the offending rubber hose. The one from the in tank pump to the adjacent pump, i found that the car wouldn't start.
I checked that fuel was present, checked for air locks and blockages. Nothing.
After paying a mechanic to look at it with no success i found that the car is totally without spark. I've tested the coil and it's working. The spark box behind the drivers headlight works. Next on the list is the crank sensor. How do i test it and why on earth would changing a fuel pipe at the rear of the car affect the electrics.
I have restored cars for twenty years and i've never come across this problem before. Also, i can't find any listings for crank sensors for my car, does anyone know where i can get one?

[924RACR]

Sounds like you'd do best to come over to the 924board.org forum... lots more Series II 931 knowledge there...

There's a couple of possibilities related to your crank sensor. First off, no, they are completely NLA. You'd have to find a good one from a scrapped car.

HOWEVER. The fuel pump relay is operated using a signal from the crank sensor; if that wire is interrupted etc, this can create a problem...

Was your car running previously? Sounds like it, but I need to be sure.

Hard to say now what the mechanic could've screwed up; the possibilities are endless. So we'll have to start from scratch.

[ideola]

+1^

It's important to start from scratch. You should assume nothing at this point, but rather use a highly systematic and methodical approach to rule out possible culprits. Here is a good place to start:
http://www.924board.org/viewtopic.php?t=28790

[Smart931]

Thanks for the advice guys.
Tested the crank sensor with a multi meter. The reading were way low. 027 ohms. From research it seems 300 - 350 is normal.

A mechanic "sorted" my emission problem by re attaching the vacuum pipes connected to the servo and emission pipework. I wonder if that's where the damage was done. I hate having "others" work on my car, but this all looks very complicated. I'm not used to cars with emission control systems.
Ill take a look through the 924 board.

Thanks again.

[Smart931]

Also, yes the car was running.
Here's a brief history.

Bought the car 2009. Smogged it for registration, passed fine.
Had a strange problem where it suddenly backfired and ran really rough, then stopped completely. Couldn't find the problem, changed spark plugs, checked the coil all the usuals. Then suddenly the car was running ok again. With the annoying habit of all of a sudden developing a surging tick over. It'd go from normal to 2000 rpm to stuttering then back to 2000. This wasn't all the time. I have used the car with this quirk for some time.

When time to smog came around, i took it in and it failed big time.
Took it to a mechanic who checked the timing and wound the emissions back down. He also sorted the tick over problem out. He re connected the small vacuum hose from the servo pipe. It dives behind the engine. I think the previous owner couldn't find its location. On the test drive the car was running great and then it had a huge stutter. The fuel light was on and the mechanic said it was most likely lack of fuel. ??

Anyway, the car was running really well. I was having fun in it. Just had to smog it. I knew that the rear fuel pipe had a small leak and was advised to change it. Which i did. Simple. I tried to start the car and no luck. I checked for an air lock in the pipe. I checked there was fuel all the way up the system. All good. I talked to my local garage and they suggested a few ways to check for fuel. All ok. So i have fuel but the car wont fire. Sprayed carb cleaner into the inlet to see if it would fire.. No.
I checked the spark from the leads and nothing. Same with the coil. There's power to the coil when you put the ignition on but the test bulb goes out after 4 secs.
Next mechanic does all of these checks also and there are strange results. Getting the odd spark from the coil. Bought a new spark box (behind drivers headlight) and a new coil and still nothing.

That's as far as i've got.
Is there any way of by passing the sensor to get a spark?
Does the 931 share a crank sensor with the audi 100?

[ideola]

No, the 931 DITC crank sensor is a unique beast. If yours is shot, the only option at present is to source one from a donor car. I am actively pursuing trying to find some an alternative for making a suitable replacement with commonly available off the shelf components, but that could be months or more away from fruition.

Your symptoms sound like the infamous loose connection at the DITC box. The systematic guide I linked to above covers this in detail with cross references to other threads on the topic.

To summarize, there is a silver box on the hump behind the radio / center console, under the heater core box. There is a black multi-pin connector that inexplicably only attaches with an interference fit (i.e. no clips or bands or anything to hold it in place). This connector is notorious for backing off, getting corrosion on the pins, and otherwise misbehaving. If it is loose in the slightest, you have the backfiring / surging problems you described above. At a minimum, you should reseat this connector. Better yet, read the diagnostic thread and do a proper R&R of the connector and see if that resolves your issues.

However, if the crank sensor is dead, the DITC plug is a moot point.

[Smart931]

924racr and ideola, thankyou thankyou thankyou.

You guys certainly know your stuff. I read your advice. Looked at 924.org, read the article on resurecting a 931 then checked my car.
What i found was indeed a loose connection onto the control box. The connector was attached with electrical tape and when i wiggled it the car burst into life.
I'm so relieved. Now i just have to get through smog.
Please turbo, don't weep oil at the wrong moment.
One more 931 on its way back to the open road.

[ideola]

Makes me happy This is exactly why so many of these cars have been abandoned, because mechanics without the necessary domain-specific knowledge of the DITC have screwed them up! Just glad we were able to get to yours before the mechanics screwed it up beyond fixing!!! Best of luck with the smog check, keep us updated on your progress!

[924RACR]

Yeah, that was my thought when reading your long post about the car's recent history... wouldn't hurt to check for corrosion on the pins for that connector on the DITC - mine has a regular problem with that.

Good luck!