1978 Dodge Power Wagon Ignition System Testing Flow Chart
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Thank you. Doug.
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Thanks,
Bill
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This video couldn't have come at a better time, and I've followed your flow chart. I'm not sure if this counts as a duplicate post. I don't mean for it to, but haven't had much reply in the repair forum where I posted previously: www.scannerdanner.com/forum/post-your-re...tart-weak-spark.html
" I drove the truck, hauled a load of dirt with it. Got home, went to move it. It started fine, but stalled under load. Would idle fine and stall when put in gear. This quickly deteriorated into failing to start. I initially thought this was a no-spark issue (I did the old school, pull the plug, hold it to the exhaust manifold and look for spark. There was no spark at the plug).
I followed the video for troubleshooting no-spark on the 78 power wagon, and the troubleshooting diagram found here: www.scannerdanner.com/forum/attachment/11814?download=1
This was superbly helpful to identify that it's actually a weak spark, and not a no-spark. The incandescent test light on positive and negative sides of the coil all behaved as expected with key in the run position as well as cranking the engine. At this point, based on the video, I replaced the coil because I didn't pay attention to the conversation about identifying weak spark. New coil in place, exact same issue. After this, I realized that the video discusses weak spark. While I don't have a spark tester, I tried to measure the gap the spark can jump at the end of the coil wire that leads into the distributor cap. While the spark does exist, it's incredibly weak, jumping no more than 1/16th, maybe an 1/8th of an inch.
The troubleshooting diagram linked above does say "if the crank circuit is not functioning, spark will be there but too weak to start the engine." Does this mean I need to look more closely at the starter relay? In your video you simulate a weak spark by pulling the connector off of one prong here (admittedly, your spark was still jumping an actual gap. Mine is almost none existent, but it is there). How would I validate this is the component that is the issue? or maybe it's not and I'm chasing my tail?
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From memory, you should have about 5v on coil positive with the key on and then 9+ volts cranking. Ideally the cranking voltage should be very near battery voltage during cranking. With a good battery that would be at least 10v
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Coil positive in run position measures 4.95. Cranking measures 8.75. This is a brand new battery, but has been doing a lot of dead cranking as we've been troubleshooting.
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This is where a lab scope comes into play. A current ramp test would tell us a lot about that ignition module and if it is controlling that coil properly.
This now sounds like an ICM problem, as you've already changed the coil and it is the same problem and you've already confirmed coil positive supply voltage is correct and that your bypass circuit is working.
So we are left with an ICM that MAY have a poor ground, a coil negative wire with excessive resistance or just a bad ICM itself.
No ground on the ICM is easy to recognize but a poor ground may not be with basic test equipment.
I hate to tell you to put an ICM (ignition control module) in it without verifying coil negative and ICM ground first
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I truly cannot tell you how to be able to recognize this without a scope.
But check out this image!
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Paul Townsend
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