What I see the most often is a stalled engine with a loss of spark when driving. Once the engine cools off it will sometimes start and run fine for a short while then do the same thing again. Often, but not always, the pickup up coil or ignition module [both are under the distributor cap] have to heat up quite a bit before they will act up. I replace about 10 ignition modules for every one pickup coil but either one can cause the same identical stalling problem. I keep a few known good modules around as test units so I can just replace a suspected bad module and road test it to see if it works. Another thing that can go wrong in these distributors are the magnets built into the pole piece. A weak or cracked magnet can cause all kinds of odd problems such as stalling when placed in reverse, misfires over 1,000 rpm, etc. Worn bushings in the distributor can also cause problems. As far as a specific test you can do to see which part is failing? I wish there was a relaible one. I've tried using a lab scope and module testers to diganose the culprit but you have to catch the problem as it's happening for those tests to be accurate.
1988 to 1993 or so GM TBI electronic control distributor. Rough Idle problems once it warms up, but when cold idles high with no miss! disconnect the brown timing control wire and if the engine smooths out than it is the pickup coil. Its money and time ahead just to replace the whole distributor, with a rebuilt one as when the pickup coil is replaced you should also install a new ignition module!
The pick-up coil/stator located inside of the distributor is what generates the dwell signal that fires the ignition coil. Symptoms of a faulty pick-up coil/stator are as follows;
Random mis-firing.
A rough engine idle.
Engine stalls out un-expectedly while running.
Engine stalls out after the engine warms up.
Engine will not start.
However, these symptoms are also the same for the ignition module located inside of the distributor except that a faulty module besides causing all of the above symptoms can also cause these additional symptoms to occur;
Engine will stall out after warming up for the same length of time, and almost on time like clockwork.
Engine will stall with climate or altitude change.
Engine spark but no fuel pressure.
Engine spark with fuel pressure but no fuel injector pulse.
Intermittent mis-firing.
If the ignition pick-up coil/stator is found to be faulty, then the ignition module should also be replaced, and the ignition module can be removed and most auto part stores will test the ignition module for free.
Also,
If you do purchase a new ignition module be sure that it does come with a silicone grease or a die-electric compound because it is a heat sink and the ignition module will burn up without it.
To install the new ignition module first clean out the mounting surface inside of the distributor. Then completely coat the metal contact surface under the ignition module with a thick coat the silicone grease or die-electric compound and do not leave any of the metal contact surface of the ignition module un-coated with the silicone grease or die-electric compound, and be very careful not to over-tighten the ignition module or it will be damaged.
You can also test the pick-up coil/stator by dis-connecting the wire connector from the pick-up coil/stator at the ignition module inside of the distributor, and using an ohm meter test the two wires in the ignition pick-up coil/stator wiring connector, and most pick-up coil/stator assemblies will ohm out between 500-1500 ohms of resistance (your vehicle may differ), however if the magnet is weak or the reluctor air gap is too great, then the resistance can test alright but still have a low voltage output.
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