I have an 84 f150 5.0 302 auto that I can’t figure out. It ran fine until a week ago and now it won’t start. It has new fuel pump, fuel line, fuel tank, sending unit, carburetor, coil, plugs, wires, and ignition module. It has spark and is getting fuel but won’t stay running. It will start but shuts off as soon as I let go of the key. I’m lost please help!!
Faulty electrical part of the ignition switch? You need to check for power at the coil with the key in both the RUN and the START positions.
You say it’s getting spark but that does not mean it’s getting a spark when the key is released.
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The coil’s low voltage + power comes via two different paths. With the key in “start”, directly from the battery. With the key in “run” or “on”, from the battery, then through a ballast resistor, which reduces electrical power to coil when engine is running to prevent coil overheating. I expect the first path is ok, but something wrong w/second path. I have similar Ford truck to yours, only 10 years older. On mine there’s a big starter relay bolted to firewall which is part of this circuitry. I’ve never had that exact problem tho, at least not yet …knocking on wood …lol … I expect it’s an easily repaired problem. My guess, ballast resistor has failed.
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Can you start it and then put it in drive or release the clutch to disengage the starter while still holding it in start? Then turn the ignition key between run and start back and forth to see if the position of the ignition is what is causing it to shut off.
Finally tracked down the issue. After hours of testing I found the ignition control module was bad.
Good for you for figuring it out. Thanks for posting the update, good to know. Ideas/clues you could provide on how to make this diagnosis would be helpful to others here searching on similar symptoms. Engine still needs hotter spark to start, but it may be that the ballast resistor path was eliminated & replaced by the ignition module circuitry. Fewer wires, but harder to diagnose.
We just started testing the wires in the harness at the ignition control module and found power coming in but not going out. Had it running today and it died. 5 month old coil quit on me. I can’t win with this truck.
hmmm … Makes me a little suspicious the problem hasn’t been completely solved. The ignition system’s job (presumably done by the module) is to produce a hotter spark (high voltage) during cranking, then back off to a less-hot spark (lower voltage) after the engine starts. If that spark voltage reduction doesn’t happen, and the cranking-spark is used while driving down the road, could easily lead to an early demise of the coil. You’ll know for sure if replacement coil also fails early on. Too dangerous to try to measure spark voltage, but if you have access to another similarly equipped Ford truck you could compare the temperature of your truck’s coil to the other truck after a short ride. If yours is much hotter, a problem likely remains.