I have a 1979 F-250 with the 400. The truck has '79 chassis and electrical, and a '73 body. I have driven it for a year and a half since the body switch. Recently the truck quits while driving. The truck idles fine and doesn't quit when idling, and when it quits, it's just like turning the key off. It doesn't spit and sputter like its a fuel issue, seems like something in the ignition. To attempt to solve this problem without thinking about it too much, I replaced the coil, ignition module, starter solenoid, and ignition switch. The plugs are 6 months old, the distributor cap was replaced during the body switch so about 2 years ago. I tried a trick that a good mechanic friend of mine suggested which is hooking a hot wire right to the coil. If it runs fine its an electrical connection, if it keeps quitting its an electrical component. Fried the wire going from key switch to starter solenoid, replaced that with all new wire as well. But when the coil is wired up in this fashion (with wires removed to prevent smoke), it runs absolutely fine. As soon as I disconnect this wire, it starts to quit again. The only thing I have planned to do is replace the coil resistor wire. Has anybody else had a problem like this, or does anybody else have any ideas or insight on what I can do. I need to get the truck running right by next week since I need it in time to use it working on the farm. Any input is appreciated thanks guys.

 

With all due respect new electrical components don't mean they function properly, they can be faulty right from the supplier, especially since most of this stuff is from overseas i.e. china, korea, taiwan, etc. unless you're buying an authentic made in the USA Ford, Motorcract or Autolite product (even though Autolite has recently farmed out the production of their sparkplugs to Mexico and China) you need to be suspect of its origin and buy wisely, but given your description i would suspect either your module or coil is at fault, only testing will tell.

 

I did use 3 different ignition modules, and did have them tested at autozone, the guy tested it several times, and it came back ok, but I still did buy a new one, and the truck did the same thing with all 3 modules, I even hooked up an extra ground to the case of the module (another tip that another mechanic friend suggested). I also tested the resistance between the low voltage points on the coil, and both the old coil and the new coil test fine (I can't remember the exact resistance off the top of my head).

 

Possibly it could be a loose ground in the ignition switch itself, or a short maybe??

 

When does the truck quit? after you drive it awhile? or right away? a module can test "good" at the parts store but fail under real world conditions, heat is their enemy and the test they do at the parts store is basically meaningless.

 

Had this or something like it happen to me last fall. Tried most of what you have, then broke down and bought a new distributor and have had no troubles since.

 

The switch itself I think would be good, One since its new, and 2 since it acts the same as the old one did. Not to say it might not be defective, but I'm reluctant to buy and try another one. The switch and the coil are both tru-tech brand, I've heard they are a decent part, and they are half the price of motorcraft. I feel the same way about the ignition module test at the parts store, especially since the problem was sort of intermittent, that's why I've had 3 different all new modules plugged into the truck to test it, and the problem was the same with all 3. The truck doesn't quit while idling, and the idle doesn't ever change, or seem abnormal. It starts to quit when going down the road, so basically conditions above idle while under some kind of load is when it would quit. Recently it does it while putting around the yard at the farm where it's at. It does it when the engine is still cold, and when it is at operating temperature. Their doesn't seem to be anything in particular that triggers it either, I've hit bumps and it will quit, and it'll quit on a perfectly smooth blacktop road as well. It feels exactly like turning off the key in your truck when it quits. I have a 4 speed so when it does quit, the engine will still rollover and try and fire since it's in gear. Sometimes it will refire, other times it takes holding the clutch for a bit and then dropping it. Sometimes you have to just throw it in neutral and start it with the key. Then after that it may run fine for a mile or 3, and then it will do it again. It isn't consistent that's for sure.

 

Thought about the distributor, but I have no experience changing a distributor, so didn't want to change that until I had exhausted all the other alternatives. Plus I have factory air conditioning, so the hold down bolt is hard to get to, even with a distributor wrench, since the a/c brackets are in the way. Side note: Is it complicated to change the distributor, or is it just setting a new one in, and making sure all the wires are connected correctly?

 

First off, don't assume anything about new components. As you've already learned, throwing parts and money at the truck only causes more frustration. When the truck dies, are you able to start it up immediately afterwards, or does it have to cool off for a while?

 

The truck will start right back up while driving, either by putting it in gear and dropping the clutch, or actually starting it with the key. Yeah, I know new components aren't always trustworthy, but so far they all seem to be working correctly, and the problem before and after changing them is the same, the only component I suspect and have yet to change is the coil resistor wire, which I hope to change later today.

 

Have you checked the resistance of the coil wire? Disconnect the negative battery cable, then disconnect the coil. Turn the key to RUN, and measure the resistance between the BATT terminal of the coil horseshoe terminal, and the positive battery cable. This will catch everything upstream of the coil.

 

Is there any particular ohm range I should be looking for when it comes to the resistance, or am I just looking for a low level of resistance?

 

The resistance wire itself should be about 1.3 ohms, give or take, but because you're looking at the entire circuit, you can expect to see a little more. Ideally you'll see 1.3, but because of the ignition switch contacts and dash harness, it could get as high as 2.0.

A better test you can do, if the truck will stay running long enough, is to measure the voltage from the BATT terminal of the coil to engine ground (unpainted metal on the block) while the motor idles. Here you should see 7-9 volts. If that's the case, then you'd know the resistive wire is in the acceptable range. Its purpose is to divide the voltage between itself and the ignition coil primary winding, because these coils are not rated for 12 volts.

 

Another thing I haven't seen any mention of is the pickup module. Disconnect the distributor electrical connector, and measure the resistance between the ORANGE and PURPLE wires on the distributor-end of the connector. The resistance should be between 400 and 700 ohms.

 

I finally got a chance to get back out to the truck today and heres what I got. By doing the resistance test mentioned to test the coil wire, I have 1.7 Ohms of resistance from the Batt Terminal to the positive battery terminal. I tested the voltage from the Batt terminal with the truck running and to a good engine ground and I get 10.8 Volts. The range was 7-9 volts, and I'm almost 2 volts higher, so does that say anything about the resistance wire being bad, or is it still fine since the resistance was in a good range. Here is the part that is most interesting to me. The pickup module at the distributor. I measured the resistance between the orange and purple wires, and I come up with 479 Ohms, which is right in the acceptable range of 400-700. Now another test I got from a repair guide was to test resistance from the orange wire and ground, and then the purple wire and ground, and in each case the resistance was supposed to be over 70,000. I tested this and a reading wouldn't come up. This is not a zero reading, but the meter just couldn't test it, like hooking up to 2 unrelated circuits. This test said that if any of those 2 readings differ, then my stator is defective, and that it should be replaced as a unit. Does anyone have an idea on this? Is this test even a good test? Just seems to me that I should've gotten some kind of resistance in the wires since they both run through the distributor, and they had measureable resistance between each other.