Hello all, I've learned a lot from this site with repairing/upgrading my 86 f250, but need help with a specific issue. First, background on the truck: 1986 F250 4x4 light duty with 302 EFI motor and T18 transmission with an unknown amount of miles. Best guess is over 200k. Over the past year I've been slowly rebuilding this truck and now its gotten me stumped to the point of possibly selling. The dang truck will not stay running. It randomly cuts power to the engine while driving like the key is being turned off. No warning or anything just dead. I can restart the truck instantly, but only if I turn the key completely to the off position then back to start. If I keep it in the run position and try to crank it over the coil delivers no spark to the distributor what so ever. I have replaced the distributor and replaced the internals of the new distributor with motorcraft sensors and a motorcraft ignition module ( the module tested good at the parts store). I've replaced the computer, EEC relay and coil with no luck and even tore apart the harness to look for broken/bare wires or grounded wires. Still have the same issue. Before I replaced the computer, coil and relay the truck would die after a few seconds unless I disconnected the SPOUT. With the SPOUT disconnected the truck would never shut off until I turned the key off. I never drove with the SPOUT out since it controls timing advance. After I replaced pretty much everything related to the ignition system I can make it 20ft or 5 days before it dies on me. Very random which makes pinpointing the issue difficult. I need all the ford wisdom I can get here! I really dont want to sell the truck because I've invested so much time and money but I have run out of ideas. Thanks in advance!

 

Huge clue right there. Have you replaced the ignition switch? The switch has multiple poles. Sounds like the pole feeding the ignition is leasing the farm with an option to buy. When you cycle the the switch to off and then back to run, the contacts clean themselves just enough to run for a little while.

I'm not one to troubleshoot via catapult (Pull!), but I'd be willing to gamble your time and money on a new ignition switch. Details here, click on the "Ignition switch" tab:

Ignition - Gary's Garagemahal (the Bullnose bible)

 

i thought about the ignition switch but in my head it didn’t make sense as to why i would have to return the key to the off position for the engine to fire and why turning the key to accessory then back to start would result in no spark condition. I’ll have to give that a shot. I’m at the point of blindly replacing parts!

 

 

As I was reading your first post I was thinking the same thing as Karl replace the ignition switch.

Yes the switch down low on the steering colum on the top of the tube.
Should have 2 screws holding it in place.
Dave - - - -

 

Yep, the electrical part, not the key cylinder.

No idea why the system fails consistently when going from Acc to Run, but is okay from Off to Run. But that in itself is another huge clue. You do something different with the suspect part and the symptoms change.

Is this a slam dunk? No, but moving parts are the most likely culprit in a failed system. Besides the innards of the distributor, there are no other moving parts in the ignition system.

 

I agree, ignition switch also. It could be arcing in there and finally fail, when you exercise the switch it rubs it clean and it runs again till it gets a bad burnt connection again. Just a theory.

 

Do you have a "tilt steering wheel"?
Jim

 

 

you could always remove the two 11mm nuts that hold ign switch to topside of lower steering column, lift it off the actuator rod, and push toward accelerator side of column. Then it will be exposed and hanging from under dash. You can clean, cycle it with phillips head or something (it's kinda tough). While you're monkeying around, you should insert a small phillips in divet under ignition tumbler and extract ignition tumbler and clean the heck out of it and it's hole. Its very quick and easy to remove and clean.

 

I'd suggest the wiggle test first with the key. After starting the engine, try wiggling the key just a little. If the linkage is a little stiff (old grease at the key cylinder, for example), it may need a little assistance for the switch itself to be fully seated in the Run position.

To test the switch itself, read this, but with one minor adjustment:

https://www.ford-trucks.com/forums/1...on-switch.html


In that epic tale, I was having trouble with a different pole in the switch, the second from the left in that diagram. I tested that pole and confirmed a failure by running a voltage drop test between fuses 1 and 9.

I suspect your fault is the first pole, the one at the far left in the same diagram. Splice S401, at the left of the diagram, is what feeds your ignition system and the EEC relay via wire 904. To test that pole, run the same voltage drop test as above, but put your meter leads on fuses 1 and 15 or 1 and 18. With the engine running, if you read anything more than 0.5VDC between those two points, that pole has failed.

Another way to test that same pole can be done under the hood. See page 29 here:

Start & Ignition - Gary's Garagemahal (the Bullnose bible)


Just to make things confusing, the switch is drawn a little differently. It only shows 3 of the 4 poles, and the suspect pole is shown second from the left, NOT the far left. Be careful of that. You can run the same voltage drop test of the suspect pole as follows:

1) Put your meter's red lead on the battery's (+) post.
2) Put your meter's black lead on wire 20 (White w/ light blue marks) at the coil. This should be the (+) terminal at the coil if labeled. If your coil has the horseshoe connector, there should be enough room to backprobe it.
3) With the engine running, any reading more than 0.5VDC indicates a failure somewhere between those two points. That one pole of the ignition switch is the most likely failure point.

 

 

I would say yes.
You posted it happens on & off and no set time or anything so with this when you tested and got the .5VDC it could have been at a good time and not the failed time.
The switch is cheap enough and some what a pain to R&R that I would just get a new one.
If that does not fix it and you know it should be good, new parts have been bad out of the box, you can move onto look else where.

I had 2 used switches and both were hard to move with a screw driver on the bench. I cleaned & lubed them and took the easier of the 2 to use in my project.
I followed how to adjust it and it is not right. To turn the truck off I have to push in the safety button that I would have to push so I could turn the key a little more to remove.
Because of this and it being adjusted by the book I think the switch is not 100% good but will use it till it craps out.
Dave ----

 

I noticed a major problem with your response, quoted above. You will need to go back and edit the post. It should read more like the following:

"Karl, you are absolutely amazing. I got a reading over 2 volts. The switch is definitely bad. I never could have found it without your stellar expertise. Gosh, you sure are swell."

Use your own words, of course, as long as you cover all the main points.

All seriousness aside, I'm not sure what to tell you. The switch still could be intermittent, but behaved when you tested it. Who knows. Intermittent faults like this can be a nightmare to diagnose. But for lack of any better options, I'd still lean towards a new switch.

Dave mentioned his switch was a pain to replace. When I replaced mine, it went very smoothly. Remove the one piece of trim and unbolt the bracket, and then the whole column tilted down for decent access. I think mine took about 45 minutes, including adjustments.

 

The pain part twisting myself to get in to unbolt & bolt it back up.
That is 1 reason why I have not made the adjustment so when I turn it off I don't need to push in the button.
I know it needs it because if I give a little more turn it will turn off.
Dave ----