Okay, so I've read through a number of related posts but as of yet I've not found a solution addressing this particular problem Here's the deal:

The truck is an 78 model year F-250, presumably with an 87 wiring harness. The wiring under the dash looks to be factory stock for the most part...save for an aftermarket stereo That's probably where the stock aspect of this vehicle ends. The engine has been replaced and the truck was modified (poorly) for full time plow duty. As I originally received it, it had a starter relay with only the "s" terminal and the two power poles....no "i" terminal. It started and ran fine for 3 years until about a week ago.

All of a sudden it would crank, but not start. I replaced the ignition module and as soon as the key is turned to the "on" position, the starter begins to crank. I guess the good news is the truck actually does start and run. Unfortunately the starter runs too. If I use the old ignition module, the starter cranks only when the key is turned to "start" but then it won't fire up and run.

Using a DVM I've been able to verify the starter relay is not welded closed. In fact, what's happening with the newer ignition module installed is that there's +12v on the red wire with a blue stripe connected to the s terminal on the relay any time the key is in the "on" position. If I disconnect the wire to the s terminal on the relay, turn the key to on, and then briefly touch the wire for the s terminal, it fires up and runs perfectly.

If I perform the same test with the old ignition module in place, the s terminal wire behaves correctly - only applying +12v when the key is moved to the "start position. Again, there is no wire in the harness for an "i" terminal on the starter relay.

I also did purchase a newer start relay with both the s and i terminals to test it. Using only the s terminal, the relay behaves exactly as the old one.

Any ideas out there??

Thanks!
Bob

 

Are both the replacement module and the original the same color?

There are 2 different types, the gray one is called a push start and the black one a CCD.

Most parts places will try to sell you the incorrect black one as a replacement for an '87.

There are quite a few posts about the differences in the two modules and about the parts houses having the wrong info if you want to learn more about those subjects.

If your harness and distributor are from an '87 you should have the gray one.

On my '88 there is a red/blue wire in the (gray) module's wiring harness that is part of the starting circuit. During a recent wiring issue I had involving the starting circuit I verified that if you feed 12 volts to that red/blue wire at the module connector it will activate the solenoid and spin the starter.

I don't think the later years that use the black module have this red/blue wire at the module connector, but I don't know this for a fact as I have recently found errors in the diagrams I have for these trucks and no longer trust them (Alldata and Mitchell)

However, my truck had the incorrect black module on the distributor and it was starting and running fine. I switched to the gray module and it made no difference I was able to detect thus far. So I would say its a fair possibility you have another wiring issue.

So, if your replacement module is a different color than the one you are replacing you may be able to fix the issue by just putting the same color one on.

If that doesn't work and/or you want to straighten out the wiring issue you will have to determine whether you have the harness for a black module, or a gray module.

I think the most obvious difference is whether or not you have the red/blue wire at the module connector as I mentioned before.

Another difference would be whether or not you have a remote mounted module or if its on the distributor, both black and gray modules can either be on the distributor or remote mounted on the fender - or wherever they put it on your truck in this case. For the purposes of our discussion I don't think it matters where the module is mounted, but I did want to mention the two options to avoid possible confusion.

The next question would be which computer they used when they rigged this up. One would assume they used the one that matches the harness, but you know what they say about "assume."

I guess how far you want to go with this depends on what you are doing with the truck. If its just going to continue to be an old plow truck just switching modules to the same color will hopefully make it start and run again. If you want to make it right, or as right as it can be in a '78 truck it will take a fair amount of work.

Another option, which I can't say I recommend, but would probably work is to find the red/blue wire at the module connector and cut it. I suppose if its a beater plow truck and your driveway is full of snow and you can't return the module you have this would probably get it running. I would cut it back a few inches from the connector so you could reconnect it if necessary - I would also tape both cut ends so it doesn't ground out.

 

Thanks very much for taking the time to provide that level of detail. Turns out the new ignition module was faulty. I went to the parts store and ordered another based on the OEM part number for the Duraspark blue grommet ignition module and they handed me the same one as the first. I tried it and it worked! I tried the first one they gave me once more just to make sure and yep, it's bad. Wish I could back charge someone for the 4 hours of my life I lost troubleshooting a bad part!

Thanks again!