rbjones17

jfrahm

No tach signal means no crank sensor or no ECU to read the crank sensor. Thankfully one is more likely, and cheaper. Maybe there is an ECU relay that is dying also, I do not know. If VCDS can talk to the ECU but there is no tach signal that would be a clue.

BrianC72gt

To elaborate on jfrahm's spot-on observations: When it doesn't start, does your CEL come one. It should be on in RUN and START positions until the car thinks the engine is running. No CEL likely means no power to ECU, so multifunction relay in the passenger's footwell relay box under the carpet or ignition switch (never hear of one go bad on these cars). If you get a Check Engine Light, but no tach, then crank sensor.

Zumkopf

I don’t recall seeing a CEL on cranking or otherwise, but to be frank seeing the CEL while cranking is so ordinary I doubt I’d have noticed. I’ll have to make it fail again and check. In that theory’s favor, the area under the passenger side carpet was wet (blocked sunroof drain). I’ll try to find a replacement relay and swap it out and see if that does the trick. Still doesn’t explain why it starts when cold but not when warm, though. That would point towards something attached to or near the motor, no?

audinutt

It's a faulty crank position sensor.

hillpc

I've had intermittent no start and suddenly die issues due to the sensor wires insulation melting against the exhaust due to careless positioning of the cable by me. Also had the senor crap out at a later time.

audinutt

my crank sensor would cause the car to fail to start after the vehicle was hot...

Zumkopf

To finish the story: after working on the car for several hours, the shop was unable to diagnose the problem, though they did get it down to finding no ignition signal. Had it towed to my place, Based on advice on the board here, took a shot and ordered an OE (Bosch) Crankshaft Position Sensor for $26. And that solved the problem.

The job was relatively easy. Pull the left front wheel and the CPS is easily visible (though a long reach). Old CPS bolt came right out. Rather than try routing the wire by hand and worrying about proximity to the exhaust manifold, I just attached a wire to the old CPS connector, pulled it down (fussy), attached the wire end to the new CPS connector, and pulled it up (easier). That way it automatically followed the same routing. One more hint: the electrical connector location is deceptively tight. You can only get one hand in there, and due to the angle it pretty much has to be your left hand.

Z
’98 A8 D2 144k

jfrahm

Thanks for the follow-up. Wow I hope that shop did not charge you anything, hard to imagine they took more than 30 minutes to determine there was no ignition signal. This is a pretty common problem across all makes and models.