I asked about this before, but no one seemed to have the answer, and the truck fixed itself. So, now that it is doing it again, I will ask again. I have a 69 f100 with a 390. Everytime I turn the key the engine will fire right up, but as soon as I release the key and it goes back into the run position from the start position, it will shut off. I can even rev the engine with it in the start position. It is a spark/electrical problem, NOT a fuel problem. I have checked all of the distributer parts, the coil and some of the wires and they are all fine. I also resently replaced the igniltion switch. If I crank the truck over and over and over while smashing on the gas it will eventually decide to start and run. From what I have seen, there are no loose connections. So, what should I look for?
thanks for any help
brad

 

All I can think of is:

Ballast Resistor from relay to coil.

Wrong Ignition switch

 

Turn the key to the on position and check voltage at the coil. It sounds like you won't have 12 volts (actually more live 8) there. Back track along the line until you find 12/8 volts and replace the faulty part or wire. The resistor would be a logical culprit but ya never know until you get the volt meter out. It could easily be a break in a wire. Since the 'truck fixed itself' I'd be suspicious of a bad wire.

 

I just took out the ignition switch and started the truck with it hanging from below the dash. I then moved the switch around and the engine shut off. I kept moving the switch around and the engine would either stumble, shut off, or shut off and turn back on. So, there is obviously a loose wire or a bad connection. All the wires are clean and tight in the back of the ignition switch and the fuse box. Do you guys have any suggestions as to where to start checking connections before I decide to cut off the wire loom. I don't know where all the wires lead to make a mess of things by cutting up the wire loom (aka lots of electrical tape). Does anyone have a color coded wiring diagram?
thanks for all the help
brad

 

The only wire that would make the engine cut out by moving the switch around is the ignition wire going to the coil.

What is it Pink ?

EDIT: Doesn't this wire to the coil also hook-up to a terminal at the starter?

 

I have more like 4 volts across my coil terminals, but the truck runs great. Does this mean I need to go through the wiring and look for problems?

 

Bigric, Don't measure across the coil. Measure positve lead to a good ground. The negative lead go to the distributor and that adds a whole bunch of weird stuff to your reading. Also, if it ain't broke don't fix it.

Seif, Sounds like a bad wire or connector at the switch. You're gonna have to open things up and fix what needs fixing. You might try running an ohm meter from Bat to Ign on the switch and see what happens. If it opens and closes then the switch is the problem. Based on what you said I would focus on the switch/connector as the culprit NOT THE FUSE BOX Good luck

 

I just spent some time wiggling the wires coming from the ignition switch as the truck was running. I narrowed the problem down to either the resistor wire (pink--the only reason I know that it is the resistor wire is because it says "resistor wire" right on it) or a red wire with a yellow stripe that goes to the same bullet as the resistor wire. My first question is: where is the resistor? I looked under the hood to try and find the red w/ a yellow stripe wire and the pink wire. I could not find either of them. The wire for the coil that comes out of the firewall is red w/ a SILVER stripe. I did find another pink wire that looks and feels and is the same gauge as the resistor wire, but it has a stripe on it and it goes to my oil pressure gauge. The resistor wire has no stripe on it. So, where can I find this resistor wire at without cutting up my wire loom? Witch wire in the back of the ign. switch is the ign. wire? I think that the problem is actually just in the section of wire within a 6" distance from the ignition switch because sometimes when I was wiggling the wires, I could hear a spark. I think I will just go ahead and replace the whole wire though. So, does anyone know the exact gauge this wire is? It feels kind of funny because it is soft and more rubbery than the other wires.
thanks for all the help
brad

 

The wire, aka resistor wire, is actually red with a green stripe but due to the age and the fact it has been dropping voltage by producing heat for the past 30 odd years it is now pink with a black stripe and mushy
I cut it off at the igntion switch and added a new 16 guage wire from the switch to a ballast resistor, from a Chrysler product value was around 1.1 ohms or so, that I installed under the hood.
I did this mod several years ago but I remember that the brown wire from the "I" on the starter relay met this resistor wire in a splice under the dash by the drivers knee area, then another red/green striped wire went from the splice to the the coil +.
Seemed to me like another idea from Mr Ford that wasnt all that bright
The resistor wire, which gives off heat all the time the truck is running, was hidden in a wire bundle and the wire from the starter relay to the coil went inside the cab instead off direct to the coil.
Anyway I changed mine out and never had a problem with it afterwards.
Good Luck

ps
The resistor wire in my truck also said "Resistor Wire Do Not Cut" all over it......I kind of ignored that though

 

Run a piece of wire (like the ones with the aligators clips on the ends) from the switch ig connector to the positive side of the coil and see if it stays running. You'll be running the coil at 12 volts but thats ok for a short period. If it runs then you know that the problem is in the coil wire/circuit . Do what sparky said and you're problem should be solved.

 

I wondered how long it would take Sparky to find this He is the wiring God of Ford trucks (he helped me with a problem on my old 89.....)

 

Just to make sure the resistor wire was the problem I completely disconnected the resistor wire and the related wires on the coil and the starter relay. I then ran a new wire from the ign. switch (where the resistor wire originally was) to the coil. I then ran a wire from the coil (same post as resistor wire) to the starter relay. I did this based on a generic wiring diagram that obviously doesn't parallel my truck's wiring scheme. The engine started right up but still shut off whenever I moved the ignition switch around. So the resistor wire is not the problem. Sparky, After the resistor wire was spliced by the brown wire and the red wire, where did the resistor wire go? Is there a resistor that the resistor wire goes to or is that just a term for the wire? Also, I am interested in the mod you said you did with the chrysler resistor, but I don't understand exactly what you did. Did you just run a wire from the original ignition terminal on the ignition switch to the chrysler ballast resistor, and then from the resistor to the coil? If so, did you then run a wire from the coil to the starter relay? If I go to Autozone to get a ballast resistor, what car should I ask for one off of and how many ohms of resistance do you recommend?
This has become a very confusing situation since even after replacing the resistor wire the engine still shuts of. There has to be something else under the dash that affects the ignition circuit in the "run" position. What do all of these other ignition switch wires do?
thanks again for the help
brad

 

Not Sparky but will try to help.

The resistor wire is just that, a special length of resistance wire. It's not copper but some other material that has a certain ohms per foot value. Going from memory, that wire runs from the ignition switch to a connector on the inside of the firewall by the radio. The bypass wire comes in thru the firewall and connects to it. Another wire comes out where the bypass wire came in and goes back thru the firewall and on it's way to the coil. So basically you have the resistance wire connected to the bypass wire and off that joint is another wire that goes to the coil. The power flow when running is thru the resistance wire to the joint and then to the coil. When starting, the power flow is from the solenoid to the joint and then to the coil. Hope this makes sense.

 

I understand now. I noticed that the resistor wire was some other type of metal when I cut it. If I run a ballast resistor instead, then I don't have to worry about using a special wire with a certain resistance, right?
Any suggestions on what type of ballast resistor to use? What gauge wires should I use? When I hook up the ballast resistor, do I put it in between the splice and the coil, or the ignition switch and the splice?
thanks for the help
brad

 

Keep the wiring stock unless you thoroughly understand automotive wiring systems. If you modify it you will be on your own for repairs etc.