[urq] intermittent ignition cutout

[Craig Cook]

Hi Ed,
 
Sorry to hear about your luck.
 
Going from memory  I think the third wire you're referring to is the shield wire for the 
rpm  sensor. I believe it goes to the ecm and it taken to ground internally by the ecm. As for the tdc sensor I think that's the one in 
the distributor. If that's the case once the engine starts the computer doesn't look at that
signal again. The car will still run perfectly with the distributor unplugged. The rpm sensors are 
known to fail seeing as they are of the variable reluctance design. The windings inside the sensor  can and do go open circuit based on temperature variances due to expansion of the sensor windings. Once the sensor cools and contracts, the sensor will function as intended.
I would suggest that you get an ohm meter and measure the resistance of the sensor to get a baseline # when the engine is cold. Then immediately after the engine has stalled take a second reading and compare the two readings.
Keep on mind that the readings will vary slightly even with a good sensor because temperature has a slight effect on the windings. you are going to be looking for a large number change usually in the thousands of ohms. Keep in mind that the sensor has to be unplugged to take the readings.
One last thing. Although this kind of failure is extremely rare, I've seen a few vehicles (not urq's) where the tack itself will short internally and take the primary side of the coil to ground. When this happens the tack will read 0 rpm even when  the engine is still in a coast down condition. If it's not too much trouble disconnect the tach and try a road test again.

 
I hope this helps.
 
Cheers,
 
Craig Cook.
Ed Kellock <ekellock at adelphia.net> wrote:
Had hoped that my IC and burnt wire adventures would yield some change to the cutout.
Still chasing the wire, so won't know for sure until its resolved.

But, I've gotten a pretty clear fix on the behavior of the cutout, purely subjectively of
course. The cutout does appear to be temperature sensitive to some degree, though not
summer heat temp type sensitivity.

The car starts up beautifully and runs nicely as it warms, but as it does, the cutout
manifests. It's almost always momentary and as long as it's not a idle (even sometimes if
it is), the cutout is short enough that the engine will catch and continue running.

I bought some resistors to put in place of a couple of sensors so as to fake out the ecu
and either eliminate or isolate the issue. In looking at the tdc and rpm sensors, there
is a 3rd wire for ground. I haven't quite sorted out how to rig that to fake out the ecu.

I have swapped in a spare ignition module (thanks Dennis!), however the behavior was
unaffected.

The cutout is definitely electrical in nature. The tach drops to zero and I think even
the fuel pump quits, though I'm not positive about that. I replaced the ignition switch
and the fuel pump relay about a year or 2 ago while diagnosing a no-start situation.

I've been wondering about the load reduction relay. Mine appears to be original. I do
hear a click when the ignition cuts out. Haven't been able to isolate it yet though.

Just a recap and thinking out loud. I'm open to your thoughts.

Ed
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