How to test Ford 901 coil

Thintz

Member
My 901 starts hard and stalls after a few minutes when it starts to warm up. Ran fine end of last season with new battery and new electronic ignition upgrade. No spark when it won't start. Once running spark starts to miss and then dies away.

Thought I'd check the coil but I'm no mechanic.

Tractor is 12V negative ground.

Coil says 067 Use with primary resistance wire or external resistor. Ohm meter reads open between terminals, 9,000 to the lead terminal. I don't see an external resistor anywhere. Bottom is stamped 0307-1.

I keep reading the coil should have 3 resistance/10-20k.

Is the coil good?
 
i should mention this is gas.

New water pump and alternator belt last season too. Note to self to check the belt.
 
i should mention this is gas.

I think we all assumed that since a diesel doesn't have a coil, although I guess there's a small chance it could be a LPG engine.

Coil says 067 Use with primary resistance wire or external resistor.

I assume that you mean infinite ohms between the two small terminals? If so then the coil is definitely bad, as long as you are measuring it properly.

Since it starts when cold and has problems shortly thereafter, have you tried waiting until it is completely cold again before measuring the resistance of the coil?
 
Yes, thank you. The measurement was cold on the outer terminals. Can I bother you for suggested replacement? I've been pleased with yesterday's tractor parts.
 
Can I bother you for suggested replacement?

As the tractor has EI and we don't have the specific brand or model of conversion module, it would be best to contact the manufacturer for their recommendation.

Have you tried bypassing the ignition switch to see if problem goes away?

Have you verified continuity from battery to distributor?

What type of resistor(s) are used?
 
Ignition is Pertronix from this site.

I have continuity end to end.

I see no resistor.

Thank you,
 
(quoted from post at 08:40:55 06/24/18) i need to clarify that I meant there is no resistance between the small terminals, not infinite.

OK, originally you said "open" which most folks would take to mean infinite resistance like and open switch, and zero ohms would be a short.

What scale did you have your meter on when it was reading zero? It should only be 1.5 ohms or so if it is a coil that requires an external resistor and around 3 ohms if it doesn'r require one, so if your meter was set on too high of a scale it might look like it is reading zero.
 
i'm sure I measured correctly. Since the can says use with external resistor right on it I didn't expect to see resistance. So the resistor is missing. Would this ruin the EI? Is there a way to test the Pertronix?
 
(quoted from post at 10:20:49 06/24/18) i'm sure I measured correctly. Since the can says use with external resistor right on it I didn't expect to see resistance. So the resistor is missing. Would this ruin the EI? Is there a way to test the Pertronix?
rong expectation
 
So the resistor is missing. Would this ruin the EI? Is there a way to test the Pertronix?

Who better to ask these questions than the folks who designed and built it?

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Got a better multimeter. The coil tested at 1.4/10,000+.

Decided to replace switch wiring before the coil even though I didn't have continuity failure. Put in a good 10gage on the key and reseated the battery cable.

Starts fine. Haven't run it long, but the easy start is a good sign.
 
Sorry... meant to type "1.4 ohm coil requires an external resistor in a 12 volt system.".

Original coils designed for 6 volt systems had approximately 1.5 ohms of resistance across the primary windings. This was the optimal resistance to allow for the strongest spark without burning up the points in short order. When they came out with vehicles with 12 volt systems to improve starting and other electrical capabilities, they originally used those same 1.5 ohm coils, and they added an additional 1.5 ohms of external resistance in series, either in the form of a resistance wire or a ballast resistor, which reduced the voltage drop across the coil to 6 volts so the internal 1.5 ohms of resistance would still be correct for the proper spark without burning up the points. Later they came out with "true 12 volt" coils that had approximately 3 ohms of internal resistance that required no external resistance in a 12 volt system. These days they sell both kinds as "12 volt coils" these, but most of the 1.5 ohm coils will say "12 volts, external resistor required" and most of the 3 ohm coils will say "12 volts, no external resistor required".
 
Thank you, Sean.

It appears that Yesterday's tractors doesn't sell a 1.5 ohm resistor. It do see the coils.

I gather from pictures that the resistor, if I could find one, is placed between the coil's negative and the Pertronix.

I find it strange the the former owner ran like this. I went through 2 sets of points before converting to electronic in hopes of fixing the problem.
 
On a 901 tractor with electronic ignition, I believe that the ballast resistor should go between the switched side of the wiring block and the coil, not between the coil and the electronic ignition. I assume that there is some other 12 volt hot wire that is providing power to the EI unit itself, as the part of the EI unit that provides the spark to the primary side of the coil should be on the ground side of the coil just like the points and condenser were before it was converted to EI.

To find a ballast resistor, go to any auto parts store (NAPA, Carquest, Auto Zone, Pep Boys, etc.) and ask them for an external ballast resistor for your coil. If you're lucky, the person behind the counter will be knowledgeable enough to not have to look anything up and will simply walk to the proper location and grab one off of the shelf. If they are not very knowledgeable they will ask you for you car's year, make and model. If they ask you that, tell them that you have a 1972 Plymouth Duster with a 225 cubic inch slant six engine (That was my first car and working on that one is where I first learned about ballast resistors). All ballast resistors are the same (1.5 ohms) although some of them have 2 connectors and some have 4. You only need 2 connections and if they sell you one with 4 connections you can simply leave one connector at each end not connected to anything.
 
Now I want a Duster :)

The Pertronix instructions have the EI leads on the coil terminals. Thank you again!
 
(quoted from post at 08:28:34 07/09/18)
do not see this diagram as working.

I agree that the wiring diagram in that second link will not work in the Ford tractor being discussed. It looks like it is for use with some third party product called a "hot spark module".

However, the ballast resistors in both links will work fine when installed as I described.
 
Got a 1.6 ohm at a shop. That's all they had. Placed it in front of the coil inline with the positive. Tried placing the EI positive both before and after the resistor. Tractor wouldn't start. Removed the resistor and it fires right up.

Without the resistor it runs for 5-10 minutes and starts to misfire, then dies. If I let it stand for 10 minutes I can repeat the scenario.

My son says replace the roter and distributor cap.
 
(quoted from post at 20:09:07 07/22/18) ok, stupid question. How do I know whether this tractor is 6 or 12 V?

A 901 gasoline engine tractor was originally produced with a 6 volt positive ground system. Since we do not know the history of your tractor we cannot tell you whether it is still a 6 volt positive ground system or whether it has been changed.

The battery should be marked as to whether it is a 6 or 12 volt battery. The fact that the battery is a 6 or 12 volt battery doesn't necessarily mean that the rest of the system is actually setup to match the battery voltage.

A more terse question (sorry if this offends you) would be "What the hell are you doing working on an electrical system if you don't know how to tell if it's 6 volt or 12 volt system?
 
The battery is 12V so I assumed it was converted, but the coil had no resistor which, I gather, is a 6 V configuration. I don't know the history either and I think it reasonable to check.
 
It is 12v negative ground. Alternator tests good. Replaced the coil with one that has an external resistor and a new rotor. Bought a new distributor cap from yesterday's tractors but it doesn't fit well enough to use it. Symptom persists.
 
Far too much mass in that coil for it to heat up to not working in 5-10 minutes. Maybe an hour or two. By the way that is experience speaking. Me? I would dump the EI until I had all correct & then re-install EI if wanted. Save yourself some time, just get a NAPA IC14SB and if that doesn't fix it, look someplace besides the coil.
 
A friend commented it is more likely a sticking valve and they suggested using Seafoam in the oil.
 

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