1978 Dodge Power Wagon Ignition System Testing Flow Chart

This file is the one mentioned in my video. Hope it helps someone out there! Please feel free to ask any questions you may have and I will do my best to answer.

Link to the video

Thanks for the detailed tests, I went thru this 20 yrs ago on my 78 Plymouth, intermittant no start, it turned out to be a narrow airgap in the distributor pickup, after setting air gap to 0.008 inches the problem was fixed.

Thanks! For sure air gap issues on VR sensors (pick-up coils) can cause all kinds of problems!

Is there a link to the actual flow chart and diagrams you show in the video ?
Thank you. Doug.

yes sir, in my first post in this thread there is a PDF attachment. You have to hover your mouse pointer over a box to see it. Something I need to fix!

I'm looking at a for sale listing for a 1983 Dodge D250 that is not running due to no spark. The seller has already replaced the coil, spark plugs, plug wires and ignition control module. Would that just leave the distributor and/or wiring as likely suspects? Also, could the vehicle be "hot wired" just to get it home?

Thanks,
Bill

Could be a bad new part that was installed too. Can you hot wire it? Depends on where the failure is. Unfortunately you'll need to do some testing for me to give you any guidance.

Hey ScannerDanner,

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?

Yes! Let's now focus on coil positive voltage with a voltmeter with the key in the run position and then cranking. Get me those numbers, then we go from there.
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

Thanks for the reply!

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.

Okay, that should be good enough. You saw my coil firing with only 5v during cranking and it was weaker, but not like what you're describing.
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

I've tried to test the ground for the ICM. I'll just pull it off the firewall and make sure it's grounded properly in an effort to eliminate one potential. Then I'll have to figure out where to go from there.

An ignition module can fail in that they will not properly control an ignition coil.

I truly cannot tell you how to be able to recognize this without a scope.
But check out this image!

That is really cool. I certainly don't have the tools to do any more than I have, but I did give the module a good grounding and still had a weak spark. I'll probably throw that part at it and see what results. Thank you for the help!

Keep me posted. I hate to tell you to swap that part without knowing for sure. But I'll help you further if it doesn't fix it.

That was it! I replaced the ICM late yesterday and the spark from the coil wire to the tester was strong and jumping at least an inch. I plugged the coil wire back into the distributor cap and the truck fired right up. Thank you for your help!

Heck yeah man! I bet if we had a scope, it would have looked very similar to what I shared with you. Awesome! Nice job on your part for not being satisfied with the parts swapping mentality that exists especially within the Tin Grille community! Glad you are here. If you ever need anything, reach out to me

Cant seem to find the flow chart?? on you dodge.

It's a pdf attached to my first post in this thread