Blue44

I'm wondering if anyone has had this problem and what the fix is.

My 89 200 Q (196k miles)with an MC motor dies out when it gets hot enough for the radiator cooling fan to come on. I believe it has to do with the flywheel flexing when it gets hot. I am losing ignition spark. I'll open the hood and let some extra air in and five minutes later I'll get spark back. Then five-ten minutes later I lose spark and have to do it all over again.
It runs perfect until it gets to normal operating temp.

Just purchased the car for $1000
Today- replaced the injectors with new OE Bosch.
oil changed semi synthetic.

I am told the distributor was just replaced so I'm looking at the flywheel timing and hall sensor.

Does anyone have any advise? The timing dowels on the flywheel look perfect.

MarkPan

Hall sender is in the distributor, so it's already changed then - wiring could still suck though.

flaky crank sensors have been known to fail when hot (eg, pouring water on one can bring it back from the dead temporarily), so I'd look there.

The crank and tach sensors are the same (with a different PN and colour plug), you can try swapping them and see if that fixes it.

Blue44

I think I'm going to try swapping the sensors(crank X tach). Sounds like a great idea, Thank you. Before I do this though I'm going to try to read the amperage coming off the crank sensor with my handy thousands of an amp to 100amps digital gauge. I'd like to know what it's putting out when it dies and to see if it goes down when as it gets hot.

Thanks for the idea Mark!

Blue44

I just swapped the crank and tach sensor but I still have no spark when it gets to normal operating temp. I also, because I have a new one laying around, swapped the 4 pin coolant temp sensor. I still have the problem. I do not have a wiring diagram for this vehicle. Does anyone know where I can get one displaying the ignition system? I know Hayes has them for cheap. Does anyone know of any online? It looks like a day on the motorcycle

Amps check out at about .01 to .o5 depending on temp for both crank and tach.

Thank you

audi_marv

could it possibly be your cooling fan going bad, requiring a ton of juice to kick it on. when the fan kicks on, it draws power from everything else, and engine dies. unplug fan and see if car dies when it reaches op. temp.
just a thought...

Blue44

Thank you Marv. I should have made my description more clear. The cooling fan cycles on and off. Sometimes twice and then the car dies when it is on or off. The car dies when it has gotten just past cooling fan cycling and the car is in normal operating temp mode. I have an old Audi 5000 turbo Haynes manual and I'm going to look now to see if there is a direct link to other components which may be effecting the ignition during the closed loop operation. My hope now is that the ECU, or some other electronic gizmo sees something right at normal operating temp that does not "jive" and in turn shuts down the ignition spark.

audiqv8

When the no start occurs, crank the engine and see if the tach bounce. The tach should bounce. Otherwise you are losing the rpm signal and without this signal the ecu will not send a trigger signal to the coil. Check or replace the rpm sensor located next to the ckp (crank position sensor).

If the tach does bounce then it could be the ckp (crank position sensor) and you will need a scope to verify that the signal is present.

You need the following signals crank, rpm and cam (aka hall signal) the crank and cam signal must be in sync before the ecu will send a trigger signal to the coil.

audi_marv

Right-0. Well in that case I think audiqv8 is steering you in the right direction. I believe if your rpm sensor is going out, the car will not restart when hot...but if your crank sensor is going out then the car will die whenever the sensor gets hot and the signal is interrupted (or vice versa . You can also put an ohm meter on the sensors. they each should read 1.0 resistance. 0.95 is not acceptable.

Blue44

Thank you audiqv8 and audi marv for the advice/info. I now have no spark at all. I'm glad the car is in my garage. I must have screwed something up when I switched the tach sensor with the ignition crank sensor. I am looking at the wiring diagram for a 88 5000 MC motor and see that everything goes to the ICU. I'll be visiting a bone yard tomorrow since I do not have loads of cash to just dump on new parts. My hope are that I will pick up some good parts and possibly some spare ignition stuff for a fair price.

I will ohm out the sensors now and report audi marv. Thank you very much for helping and standing by!

Alex, Blue44

Blue44

Also a little tid bit if you're interested in this topic.

When the car got hot last and died I poured some cold water on the crank and tach sensors - no change - car remained without ignition spark.