I replaced my 86 F150 6cyl Duraspark III distributor w/ attached ignition module with a Duraspark II distributor and fender-mounted ignition module. Twice now, the ignition module has failed. After replacement, the truck starts right up. The first time, the idle was too low and the truck stopped -- it wouldn't start again. I replaced the ignition module and it started right up again. I left it running for about 15 minutes, then went inside. When I came back out, the truck had stopped and wouldn't start.
I have a manual and think it is wired right. Does it matter that the coil is from the earlier Duraspark III setup? Is there something about a ballast resistor that I may have eliminated?
What is happening?

 

On your '86 I think you changed the TFI ignition to a DSII.

 

The TFI coil has a lower primary resistance than the DuraSpark II coil. The TFI coil primary resistance should be .3 to 1.0 OHM. The DuraSpark II coil’s primary resistance should be .8 to 1.6 OHMS. Of course, the lower the resistance, the more the current and therefor more heat in the ignition module. It may be a good idea to install a DuraSpark II coil and see if the new coil fixes the problem.

Your problem could also be caused by the lack of a ballast resistor. The TFI module does not require a ballast resistor and the DuraSpark II does. If you have an accurate OHM meter, you can test the ballast resistor by the following procedure:

Disconnect the two wire connector on the ignition module (the one with the red & white wires).

Disconnect the wire from the coil’s “batt” terminal

Measure the resistance between the red wire (of the two wire connector of the wiring harness) and the wire that was on the coil’s “batt” terminal. You will be reading the resistance of the ballast resistor. You should read no less than .8 OHMS and no more than 1.6 OHMS. If you read basically no resistance, that indicates that you don’t have a ballast resistor. The factory ballast resistor is a length of resistance wire in the wiring harness that plugs into the ignition switch. It is difficult to tell if your harness has a ballast resistor just by looking at the wiring harness. My '84 had a ballast resistor in the wiring harnsss in spite of the fact that it was equipped with a TFI system. I guess Ford was allowing for either type of system in '84. I just don't know if you '86 has a ballast resistor or not.

If you determine that you don’t have a ballast resistor, you can add one in series with the coil’s “batt” wire.

Good Luck, and let us know how you fix the problem.

 

Ok. Resistance between red (start) wire that connects to ignition module and white (run) wire that connects to batt on the coil is 7.4 ohms. (Not what I expected.) The coil's primary resistance is .3 ohms.

What should I do to fix the 7.4 ohm resistance?