I have a 1973 F-100. It has a 1972 Cleveland 351 2 barrel.


My engine is sputtering/ losing power in third gear at low RPM. I want to make sure my coil set-up is correct.


I have a 12 Volt stock coil. At the ignition switch I have 12 volts. I have 12 volts going into the ballast resistor, and 9.05 volts coming out of the resistor and going into the positive side of the coil.


Out of the car, the coil from positive to negative measures 2.4 ohms.


The coil from positive to the center measures 9,200 out of 20,000 ohms.


Does that all look right to you?

 

Yes.

Coils will fail with temperature, meaning work when cold, don't when hot. Or else they open-circuit or spew the insulating oil all over.

The Ford TFI solid coil like was used in a 1989 Taurus is a good coil.

R.

 

Those are the readings when the coil is cold. Do you think I should just replace it?

 

You could replace it but might might still have the same problem and be a little poorer.

It tests OK at least cold. Is the coil leaking or showing other signs of distress? If not, the coil what is causing the problem? If not recently done, I would suggest a basic tuneup (plugs/wires/cap/rotor/adjust timing/adjust idle) first. Also make sure the vacuum advance in the distributor is working smoothly. A shot of WD40 plus work it by hand can help.

 

That's a little over twice the resistance that it should read. Max should be about 1.3 Ohms. Sounds like it's time for a new coil.

.....and if it looks like this, it definitely needs replacing (still don't know what caused this to happen to my coil).

 

I put a new coil on there and its primary was 2.1 ohms and the secondary was 8,600 ohms and it didn't fix the problem. I don't think it is an ignition problem.


I put a vacuum gauge in the truck and it idles at 20 pounds and when I rev it goes toward zero.


When I'm driving down the road at 1,500 rpm, what should the gauge read? Also the vacuum drops below 10 pounds when the truck is under a load.

 

I had a GM truck that I had swapped out points distributor to HEI. Had symptoms similar to what you describe. Finally figured out the reman HEI distributor was somehow shorting internally when the vac advance would move. Ran fine at idle or heavy/full throttle. Just missed like you described under part throttle/light load conditions. Maybe not your issue, just an idea.

 

Well somebody said you'd only likely be a little poorer! They was right.

Vacuum will be about the same as idle, while at steady cruise. It's always described as inches, not pounds fwiw. It is load dependent ONLY as you discovered.

Actually low RPM in high gear, is a super torture test for the ignition system - coil, plugs, wires, everything. If you can putt along in third say at about 5 mph without any bucking or stumbling etc, that means the ignition system is tight. It's a legitimate diagnosis in the old school manuals.

One thing I'd look at is, if you are running an aftermarket coil that is 2 ohms say, you would likely want to bypass the ballast resistor that is already part of the wiring harness on Fords.

That is, aftermarket coils will say something like "must be used with resistor" right on the coil. If it doesn't, then don't. I'm running a 1.5 ohm coil in my 64 but with no ballast and have for years. Don't use both a higher resistance coil AND a ballast, if it's not designed for it.

Next make sure plugs are correct for application and gapped correctly. .035 more than likely. Make sure all the grounds and cables and straps are new. I even like to run a medium file over the boss where the distributor mates with the block before installation for good electrical contact, we want bombproof electrical connections for maximum current transfer. Some people even solder or braze a connection.

Use new spark plug wires, dielectric grease in boots and make sure that wires don't run parallel with each other for long runs, particularly cylinders that fire consecutively. New cap and rotor, etc. Let us know what you find.