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Hey all, I just recently got this 1970 Ford F100 and am having a tough time chasing down an issue. I was wondering if anyone has had anything similar happen or could give any insight? The truck is not wanting to fully start. When I turn the key it cranks, fires, but when I let go of the key it shuts off. I checked under the dash and didn’t see any bad wires, didn’t see anything concerning under the hood, I “hot wired” the coil to the battery and it did start and run doing that, so I wouldn’t think the coil is bad! I did put a voltmeter on the coil with the ignition on and it’s not reading 12V when I test so i would imagine the issue is somewhere from the ignition switch to the coil… I already replaced the ignition switch with no luck. I wouldn’t believe it’s the points and condenser or distributor since the coil isn’t getting juice. Any help would be much appreciated in case I’m missing something! Thank you and sorry for the long post!
Welcome to FTE!
Do you still have the stock points distributor? If so, you should not have 12V to the coil with the switch on. It should be 4.5 to 6.6 volts in the run position. You only have 12V to the coil when the engine is cranking. A full 12V to the coil on a points distributor will burn up the points before you can drive to the parts house to buy new ones. Let us know what you have.
Welcome to FTE!
Do you still have the stock points distributor? If so, you should not have 12V to the coil with the switch on. It should be 4.5 to 6.6 volts in the run position. You only have 12V to the coil when the engine is cranking. A full 12V to the coil on a points distributor will burn up the points before you can drive to the parts house to buy new ones. Let us know what you have.
it does appear to have the stock points. It’s definitely not reading 12 to the coil, more like 0.1-0.4 V.
it does appear to have the stock points. It’s definitely not reading 12 to the coil, more like 0.1-0.4 V.
That isn't enough voltage by a long shot. Should be 4.5 to 6.6 volts. On your ignition switch there should be a pink resistor wire, it usually has a fire sleeve over it. They usually have a male plug close to the switch. Unplug it and check to see if you have 12V coming out of the ignition switch in the run position. If you do that resistor wire is probably no good. Or the plug at the cowl is corroded. You can get new resistor wires form NAPA and most parts stores. You replace it by cutting it close to the cowl plug and use a crimp connector to splice in a new one. Standard Ignition 2 Terminal Ballast Resistor RW-34 | O'Reilly Auto Pa (oreillyauto.com)
I need to add this. I just assume folks have done this before.
There is one thing about checking the voltage to the coil. The way it is done is to disconnect the wire going to the distributor and run a wire from the distributor side of the coil to a good ground. Disconnect the wire on the + side of the coil. Connect your voltmeter between the + side of the coil and that wire and turn on the ignition that is the voltage reading that counts. If you just check voltage from the + side of the coil to ground, you won't get a valid reading.
I need to add this. I just assume folks have done this before.
There is one thing about checking the voltage to the coil. The way it is done is to disconnect the wire going to the distributor and run a wire from the distributor side of the coil to a good ground. Disconnect the wire on the + side of the coil. Connect your voltmeter between the + side of the coil and that wire and turn on the ignition that is the voltage reading that counts. If you just check voltage from the + side of the coil to ground, you won't get a valid reading.
thank you, I will give these a go when I get back to the house. It looks like a lot of this stuff is original, so I’m considering going get a new coil, coil wire, and resistor just to rule those out. I figure it can’t hurt to swap them, especially considering they’re not very high cost items!
Crop Duster, I'm assuming this is the plug you're referring to coming off of the ignition and connection with a bullet connector? I am getting a good reading from this plug with the ignition on, so that would lead me to believe it would be the wire/plug itself (which doesn't look fantastic inside. You're advising to cut this plug and crimp a female bullet end on there and replace it with the wire in the link you posted in your reply, correct?
That pink wire in the link should be in there somewhere and it should stand out from the rest because they have a fire sleeve over them. That picture doesn't show much. Don't cut anything until you are certain you have identified the resistor wire. The only wire you may have to cut is the pink resistor wire and there is a chance it may already have a plug on both ends and may not need to be cut at all. The resistor wire is around 3 feet long and is usually coiled and taped to the harness under the dash. Here is a wiring diagram for a 70 model. It is best to print these, so you don't get lost following wires. wiring-diagram-70_master.jpg (2559×1200) (fordification.com) Follow the wire back from the coil until you are certain you have identified the correct one.
I personally don't care for these online freebee diagrams, but I don't have a 1970 factory diagram. This is what a factory wiring diagram looks like. The F100-F350 section for 1980 trucks is 5 double sided pages that fold out 4 feet long. You can't really compress all that into 1 page without leaving stuff out.
Thank you guys, I actually found the source of this issue on the ignition wire leading to the coil deep in the dash. Took a minute, but I found it. She started and ran fine at first. When I took it for a test drive it was fine at first, but when it got to temp/hot it wanted to die when I came to a stop or was idling. I’m wondering if the new coil I put on it is bad or something along those lines. Will be putting the old coil back in tomorrow since that was the original issue and see what happens. One thing down and onto another I suppose hah!
Yes, most of the time anymore it seems!
Definitely good idea to keep the old parts until things are settled. With the crap-for-parts we get these days you just can't tell when you're going to get a good one, or just add to the list of possible bad things to check. Ignition switches and starter relay/solenoids are way up on the list as well. Just like points and condensers...
Glad you got it narrowed down though.
