Hello all,

I recently acquired an '86 F250 with a 460. The wiring is very poor on this truck. The wires to the distributor and coil have been laying on the motor for a good while, and the wiring harness connectors have seen better days. Last weekend, I decided to pull the wiring harness and inspect it for problems. After repairing a few bare spots in the wires and cleaning all the connections to the module, coil and distributor, I installed some wire loom over the harness, put some dielectric grease on all the connectors, and replaced it in the truck EXACTLY how I had removed it.

Guess What? It won't start. No spark from the coil. I thought that maybe I had missed something in the harness, so I took the loom off and checked everything...they look O.K....connections are tight and clean. I seperated all the wires just to make sure they weren't grounding out on anything.

Next, I pull out the multimeter and the Haynes Manual. According to them, some of the voltage on this truck is reporting is way off.

1. The book states that the "batt" side of the coil should have about 6 to 8 volts when the key is in "Run" and a full 12V when in "Start". My readings show 12V in "Run" and about 10.5V when you try to start it. Why is it doing this?? I suspect the ballast resistor is bad (it has not been removed), or the ignition switch has some play in it? I just replaced the ignition switch about a month ago in conjunction with a stalling problem.

2. I also checked the voltage on the ignition module. Red and White wires are within spec, but orange, purple, green, and black are all about 10.5 - 11V. It is my understanding that the black wire is supposed to be a ground that attaches in the distributor body. Why is it carrying positive voltage?? I have cleaned this connection inside the distributor. I suspect this is the problem that would keep it from starting?


I'm guessing that I stressed something when I started pulling on a harness that is 18 years old ....any ideas?

The coil, ignition module, and spark plugs are all about a week old.

I checked the coil today and it ohm'd out within spec for primary and secondary resistance tests.

 

What color is the wire grommit on the iginition module?

 

it has the blue grommet.

 

one problem, as you mentioned, is the ballast resistor. It could be burned out. Just to test, you can bypass it and try to start the engine. If it runs shut it off and replace the ballast wire.

The other problem is the voltage on the black ground wire. A ground wire should read Zero volts. You have short in the ignition harness.

You might look to trying to get another harness from a salvage yard - from your description of the harness - another one would be a good investiment.

 

I feel that your problem is an open ground wire. If the ingition module is like mine, it is bolted to the fender linner which is plastic and the wire is its only ground. Run a temporary wire between the engine block and the ground wire at the module. If the problem is an open ground wire, this will fix it. If this doesn't fix the problem, at least it did not cost much to test my theory.

 

thanks, i'm going to try that when i get home tonight. I'll let you know what happens.

 

it worked! i spliced into the ground wire and hooked it to the firewall...it cranked right up. guess the ignition module was not grounded. Thanks!

 

Ford150:

How did an open ground on the module impose voltage on the Black wire to the distributor?

oscar

 

Oscar; The lack of a ground caused the entire module to float at battery voltage. With no ground reference the entire circuit including the case of the ingition module itself would measure battery voltage. When he measured the ground wire and found voltage, he was measuring the battery voltage that was trying to get back to ground. This type of problem can be baffeling because finding the problem depends on where he was measuring from. What I mean by that is if he had the negitive probe of his voltmeter on a good ground, he would see battery voltage on everything including the module case and everything attached to it. If he happened to have the negitive probe on the module itself he would not have measured any voltage because both probes would be at battery voltage. What triggered me to think about the lack of ground was a problem I had with a HAM radio. It was dead and everything I measured in the radio was 12V, even the case. Of course the problem was a broken ground wire. I'm only glad that the radio problem turned out to be helpful to a fellow truck owner.

 

i'm glad too...just for the record I measured the ground wire right where it came out of the ignition module harness. After I got it to crank, I rewired the entire harness from the module to the distributor. I can't believe what a difference a good ground made. The truck had a slight miss...but that's all gone now. Thanks again!

 

Ford150:

Thanks for the explaination - I didn't know the case of the module would go to battery voltage. Applying what you learned on the radio ground showed a lot of insite to this problem - sometimes it's strange where you can pick up bits of experience and what else that info can be used to trouble shoot.

Oscar