Hi there. Over the summer, I was given a free Ford truck. Its in pretty good shape except one problem... I'm having quite the issue with it's ignition system. Maybe someone here has seen this issue before?

First off, about the truck. Its a 1985 F-250 4x2 with a 351HO and AT. It has the Duraspark 2 ignition and an unmodified engine.

The truck runs quite well when I can get it to start. But I'm having a starting issue. The truck has never had an issue turning over. But it frequently wont start with the key in the "start" position. If the conditions are right in the carburetor, after the engine rotates and the key returns to the "on" position, only then will it attempt to start on the final engine rotation.

I've done a spark check at the coil and it confirms as long as the key is on the "start" position, there is no spark at all. But once the keys let go of, I'll get spark on the last rotation of the engine. I replaced the ignition switch thinking that may have been the issue. This seemed to correct the problem for a day. But it returned back to the no starting problem the next day. I replaced the ignition switch again, and it did not fix it. I've since readjusted the switch around 6 times with no change.

I've performed a coil test and it passed with marginal, but passing results. I've checked the ignition lock cylinder and switch actuator rod, and they seem fine.

I've been able to get the truck to start normal on occasion with random results by jiggling the ignition module harnesses, jiggling the ignition switch wire loom, tapping on the voltage regulator, slamming the key in and out of the ignition, banging on the column shifter, etc. Each has worked once, but none works consistently. I'm certain its coincidence when it starts with each thing I tap or jiggle. There have been times where don't tap or jiggle anything, and the truck starts normally.

Any suggestions are much appreciated and as I progress through this, I'll keep you updated.

Thank you!
Jeremy aka 85BigW


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Can you confirm you are getting 12V to your Duraspark module at the white wire while cranking?
Do you have a spare module you can try? No need to attach it just connect the 2 plugs.

 

Can you confirm you are getting 12V to your Duraspark module at the white wire while cranking?
Do you have a spare module you can try? No need to attach it just connect the 2 plugs.

 

My neighbor should be home to help me test this in a few minutes. I'll have an answer for you shortly. And I dont have a spare module on hand, but I can get one probably tomorrow.

 

Should the plug with the white wire be connected or disconnected from the module while testing?

 

OK, I checked the voltage with the plug connected. Im getting a fluctuating result of around 8.5 volts from the white wire while cranking the engine. This was done on the plug with one red wire and one white wire leading to the control module.

 

This diagram shows the white wire controls the modules ignition retard circuitry.
The red wire should be hot in start and run.
It does not need to be plugged in. You just want to see where the fault lies.(harness or module)
If it tests good in start and run I suggest the box is bad.
I keep a spare, so thought it would be simpler to just try it, than get a friend and tester.

 

Sorry, double post...

 

The reason it starts when you release the key to RUN is the voltage jumps up to a useable level for the coil to provide spark.
Try a jumper wire from battery + to coil + to see if this helps.
The battery voltage should not go below 10 volts while cranking. Might need a new battery.

 

I'd considered the other battery I had (which seemed to be functioning fine) as weak. So the battery thats in it is now brand new and supplies plenty of voltage to the system.. An unnecessary purchase.

My ultimate hope is to not continue to throw parts at it. Rather to actually find the problem and repair it. I will definitely check the voltage at the coil + while cranking again. Another solid suggestion
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For a cheap fix, check the plug gap.
The smaller the gap, the more likey it will start.

The 8.5 volts you got on the white wire seem to low, try the test again but pull the small wire on the starter solenoid ( "S" connection). This will remove the high current draw of the starter.
If you are good with a voltmeter you can try to find the source of the voltage drop while the starter is cranking the engine over,... or you can just clean the battery connections, ground cable connection, starter solenoid posts... until you find the weak connection.

Jim

 

I agree Jim. 8v is way too low. According to the book, its supposed to be within 1 volt of the total system voltage, 12v. Also, the plugs are brand new and properly gapped.

So, for giggles I readjusted the ignition switch again. This did not help. I did however find that jumping the starter relay with a screwdriver provides immediate spark while starting. The truck started up beautifully. The ignition module is not the culprit I am going to get another ignition switch again. Last time I installed a new one, the truck started fine for about 2 days before returning to the same old shenanigans.

Somewhere in the main wire harness, I have a bad wire. I'm not sure where yet. This could get interesting.

So my thoughts on a shorted wire. I'm inclined to presume a 12v wire carrying that type of load would certainly fry something in the electrical system if it was touching ground. So my options are; A frayed wire with low continuity, OR; A bad ground.

Another symptom of this (which coincides with the poor electrical connection theory) is slow cranking while using the key. Its really labored trying to turn that engine over. Yet, when jumped at the starter relay, the engine rotates quite rapidly and the engine starts nearly immediately. I'm not sure if that's because, when jumped at the relay, there's actually spark/combustion assisting the engine rotation.

Keep the thoughts coming. I'm getting closer!. And I appreciate every suggestion so far.
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I would install a push button start switch from the dash to the starter solinoid until you can find the problem. At least you wont be stranded.

 

The large forward connection on the starter solenoid is the pick up point for power to the electrical system, so start here. You should see a few fuseable links attaching here.

This is also the same point where you mash a large screw driver to jump the solenoid.... and then it starts fine....look for a weak or loose connection and clean everything until it is shinny new looking.

Jim