I posted this on the end of my old thread and didn't get any replies so I thought I would start a new thread.

"Well I am back and it seems that I jumped to a false conclusion. Changing the plugs, distributor cap and rotor did not solve my bucking/stumbling problem; it only moved it to a higher RPM level, approximately 3000 RPM.

Remember this is a Frankenstein truck, carb'ed 1984 351 W in a 1989 F250. The ignition is from a description/diagram that Franklin2 posted to help another poster to convert a 1989 to carb./DS2. It includes a ballast resistor (Chrysler type) to drop voltage to the coil.

The only ideas that I had were a defective coil, or a DS2 module problem. I broke out the multi-meter and decided to check some voltages. Battery voltage was 12.89 v., voltage before the resistor was 11.78 v. (-1.0 v.), and voltage after the resistor (and at coil+) was 5.23 v (-6.5 v.) That seems very low for coil voltage, which I understand should be 7+.

It looks like I should replace the resistor, and I will, but would that low coil voltage cause the problem that I am having. It seems to my electrically challenged mind that it could. Any opinions welcome!and hope for the best."

Also I did these voltage checks with the ignition on but engine not running. Am I off in the weeds with how I conducted the teat?

 

Yes, that low of voltage could cause problems. The faster you spin the engine the less time there is to create a magnetic field in the coil, and low voltage hurts that by reducing the current.

And, you've checked the voltages correctly. But another test would be to jumper across the resistor and make a quick run. That shouldn't hurt the coil and it will prove if that is the problem.

 

A few more pieces of information. Checked resistance across the resistor, it reads 0.8 ohms but my meter reads 0.3 ohms when the leads are touching so that means 0.5 ohms, I think.

Coil primary resistance is 1.4 ohms - 0.3 ohms for meter = 1.1 ohms (in spec.). Coil secondary resistance = 8.2k ohms ( also in spec.). So it seems the coil is OK, but the resistor is out of spec. 6.5 volt drop across the resistor with 0.5 ohms resistance would calculate to 13 amp. current draw. Does this seem right?

I made a jumper for the resistor but haven't used it yet; probably try it Saturday.

Does anything jump out at anyone from these figures? All help appreciated!

 

E=IR, so I=E/R. Therefore I = 6.5/.5 = 13 amp. But, not so fast as the voltage drop across the coil and the resistor = 12 volts. (Less the loss across the DS-II module, but let's ignore that for the mo.) So, I = E/R = 12/(1.1+.5) = 7.5 amps. So, you cannot be pulling 13 amps through the resistor that is in series with the coil.

If the voltage drop was really 6.5 volts and if the current was really 7.5 amps then the resistance (R) = E/I = 6.5/7.5 = .867 ohms. But, that means the overall resistance was really 1.1 + .867 = 1.967 ohms. Therefore the current was 12/1.967 = ~6 amps. And then the resistance is 6.5v/6a = 1.08 ohms. But now the overall resistance is 2.18 ohms and the current is ........................

Bottom line: I don't trust your DVM's ohm readings. Volt readings are much easier and, therefore, more accurate. In any event, I still think you have too little voltage. Put the jumper on and test it.

 

I think the .5 ohms reading across the ballast resistor is not accurate.
Does it glow red hot, which might raise the resistance with current flow?

If you measure the voltage drop across the coil ... I would guess this to be 4 volts in your case.

So 4 / 1.1 = 3.6 amps current flow

6.5v / 3.6 amps = 1.78 ohms (calculated) for the resistor

The resistor you bought should have an ohms value, did it?

The correct resistor value should be about 1.1 ohms.

JEGS Performance Products 40103: Ballast Resistor 1 Ohm Use with: | JEGS


Jim

 

Yea, Gary, the DVM is a cheapie. The jumper is already made and ready for Saturday which is supposed to be a warmer day. I will post the results.

Jim: I have not seen any indication of excess heat like a red glow. The resistor was one AutoZone gave me when I asked for one for a Chrysler application. I should know better than to trust them.
I have an Echlin ICR11 (rated at 1.35 ohms) reserved at NAPA to be picked up on Friday. I picked that one because Ford's stock resistance wire is rated at 1.3-1.4 ohms. Do you think the Echlin ICR11 should work?

 

Check the "horse shoe" connector that snaps onto the coil. Or better yet, replace it altogether. They're certainly cheap enough.

I had an intermittent miss that finally went away after I replaced my horse shoe connector. The contacts on mine were mighty dark and the plastic was cracked, and when I tried to run resistance and volt checks I got weird and inconsistent readings at the coil.

 

That's good advice.

 

I pulled the connector yesterday when I checked the coil resistance and noticed that the posts on top of the coil where not exactly shiny so I cleaned the posts and the horse-shoe connector contacts with a wire brush. The connector itself seemed to be in good shape and snapped on tightly.
I haven't driven the truck since then but will test it, before I install the resistor jumper for that test.
Thanks for the tip.

 

My coil connector failed also.

The resistance value is shown in the 1986 EVTM as 1.1 ohm
Parts book has it at 1.3-1.4 ohm
https://www.ford-trucks.com/forums/1...-resistor.html

I would try the 1.35 ohm from Napa.

A 1.1 resistor and a 1.1 coil would split the applied voltage 50/50. (1.1/2.2= 50% each).

A 1.35 resistor and a 1.1 coil, 1.35/2.45 = .55 ... so the resistor is dropping 55% of the applied voltage. 14 volts x .55 = 7.7 volt drop across the resistor

A 1.00 resistor and a 1.1 coil, 1.00/2.10 = .476 ... so the resistor is dropping 47.6% of the applied voltage. 14 volts x .476 = 6.66 volt drop across the resistor

The above calculation are really just for reference or just to show how a slight change in the ballast resistor ohms changes the voltage drop across the ballast resistor.
Bottom line is any of the above should work fine.

Jim

 

Another important factor is a resistor's ohm value changes dramatically when it heats up with use. With a cold resistor, any ohm-based calculations go out the window after the current starts flowing. (Light bulbs act the same way, too)

If a manufacturer gives a spec for an ohm value, that's intended for checking the component, but not really for making calculations.

Coils have their own weird effects due to all the overlapping flux fields and counter EMF. While not heat-related, a coil's apparent resistance changes under load. Criminy, I used to know all the details, but due to my limited brain capacity, I deliberately only remembered the important part: The resistance you measure at rest with an ohmmeter is different than the effective resistance under load.

 

The horse shoe connector and coil contacts can be nice and shiney but those thin wires are still only crimped to the horse shoe terminals.

After 30 years of corrosion and vibration on those thin wires it's really a wonder any current can still get through them at all!

 

Brnfree: I'll look into getting a new connector. Thanks.

Results of today's test runs:
#1 No changes except clean coil horse shoe contacts. Same problem under load @ ~ 2800 RPM. However the problem is definitely load related because acceleration on level ground was good past ~3500 RPM.

#2 Jumper around the resistor. Good acceleration under load, with no stumble. I ran out of hill before I got much past 3100 RPM. Definitely better. I didn't want to run it too long on full voltage, but maybe it wouldn't have been a problem.

#3 New Echlin ICR23 resistor (1.2 ohms). Slight improvement, it seems, but still stumbled on uphill acceleration.

#4 Voltage drop tests with new resistor (Key on/engine off):
Voltage before resistor = 12.02. Voltage after resistor = 6.88. Voltage drop across resistor 5.02. Voltage at the coil = 6.75.

Where to go to next is the question?

 

The worst stumble would probably be at low RPM with the throttle wide-open. That gives maximum cylinder pressure, which requires the strongest spark to jump the plug's gap. So, if you had no stumble to 3100 RPM with lots of throttle then it probably passed the test.

Have you tried replacing the coil? Perhaps it is breaking down and requires more voltage than it should.

 

In my case I think I had a bad connection on the ground (tach) side of the coil and I wonder if I cooked a couple coils because of the bad ground. I read somewhere that if the coil isn't fully discharging because of a bad ground (+ hot summertime temps) then that can cause them to break down.