Early 944 Reference Sensor Problem
 

Hi there,
After putting the engine back in my car it won't start. I knew the reference sensor was old and replaced it but that didn't do the trick. I followed stuff from Clarks-Garage and have validated that I'm getting the proper Ohm level on both the Speed and Reference sensors. However I am NOT getting a AC volt reading on the Reference Sensor but am on the Speed Sensor.
According to the site you can adjust the bracket to set the correct clearance to .8mm (I think that is the size). That sets the depth off the Speed Sensor though and that is getting me a volt reading. Does that seem like the likely culprit or are there other things I should be trying?

Thanks,
Dustin

The clearance shouldn't change unless you take the bracket off. I'd suspect a problem in the connector or the harness back to the DME. If you have the pin-out of the DME connector (I'm pretty sure it's in the factory shop manual), you should check your reference sensor resistance there.

Note - my early car did run without the reference sensor. It was very hard to start (hold the throttle wide open) and ran very poorly, (un-drivable) but it would run if you kept into the throttle. Once it stumbled up to about 3000 rpm, it ran OK.

Unfortunately I did remove the bracket. It's a long story, but I knew well enough to measure the distance from the teeth of the starter ring to the sensor when putting it back together so I figured the problem wouldn't be caused by an incorrect gap. Having said that it was running perfectly before I pulled the engine so I'm not sure what would cause the connector to suddenly stop working.
I did remove what I think was a 3rd party alarm system when the engine was out of the car. At first I thought it might be caused by that, but it wouldn't explain why I'm not getting any AC volt on the #25 and #26 Pin when cranking the engine.

The reference sensor signal is fleeting - only one pulse per revolution. You might just be missing it or your meter can't pick it up on an AC scale. I think the only way you can really read it is with an oscilloscope.

The problem in my early car turned out to be the harness side connectors where the sensors plug in. There was enough corrosion and resistance in the wire that the signal wasn't getting through to the DME. These are inductive sensors which put out a very low power signal, so they are very sensitive to voltage drop in the wires.

Ok, I can try cleaning those. Here is an image of what I removed from the car. Doesn't look factory to me and was attached to a horn that I suspect was for the security system. Wonder if removing that has caused the engine to not run.
Does anyone recognize this part as factory or not?

http://forums.pelicanparts.com/uploa...1401384968.jpg

Thanks

That doesn't look like anything I've ever seen on my early car. Porsche definitely did not ever use card edge connectors like that box has. I think you might have opened a can of worms by removing it.
It's probably interrupting power to the ignition or fuel pump or the DME. You might be lucky enough to be able to trace it's harness back to where it attaches to the factory wires; you might see a factory wire that is cut and both its ends go into the foreign harness.
If you don't already have it, the factory service manual has the complete wiring diagrams. You should be able to find a PDF. I personally like the early wiring diagrams, but you can also refer to the 87 924s diagram; it's mostly the same but in the newer layout.

definitely not factory.