Help! I'm burning up coils in my M43!!

m37brat · Jul 10, 2011

I have an original M43 and I have been going through coils like water. At first it only happened if the ignition switch was left on with the engine off. Today I burned up 2 coils while driving the truck to a show. They went while the truck was traveling at 45 MPH. Is it possible that the capacitor is bad? Is there a resistor in the system somewhere? Any help would be greatly appreciated.

Thanks.

Nonotagain · Jul 10, 2011

Before you fry another coil you need to verify the voltage being output by the generator. I don't know if the original system used a resistor or resistor wire to reduce the voltage to the coil after the engine warms up.

Provided that you are getting close to 12.5 volts, I'd purchase a Chrysler ballast resistor in mount it inline with the coil primary feed line.

High voltage to a coil usually eats the tungsten electrodes off the points.

I played with too many high voltage coils to remember, but only had one coil go south.

TGP (IL) · Jul 10, 2011

Makes sure the two Dist. vent lines to the carburetor air horn is hooked up and providing fresh air to the Dist.
These lines must also be connected correctly and not reversed.(disrupts air flow)
These lines are a "Must"

Also I had an issue once with my Voltage regulator causing a higher voltage and burning up the coils. 27.5 -28 Volts max. when running.
There's no resistor in the Ign. Circuit. The system is straight 24V and requires a 24V coil.

Last but not least the aftermarket china made coils are less than dependable.
Have had a few work fine,others not so.
Original "NOS" are hard to come by.

Someone else will add and elaborate.
Tom

derby · Jul 10, 2011

What voltage is the ignition system? A 12 volt system will have 9.6 volts at the coil through a resistor. Have your points been burning the faces also? I think some coils have to be mounted vertical. I believe the capacitor protects the points, but it could be shorted out and causing the problem. there cheap change it out and see. also did you switch the polarity of the replacement coil?

AN/GRC-46 · Jul 10, 2011

Before you do much more you might want to check the voltage output at the regulator. The generator output varies a bit but is smoothed out and regulated by the regulator to supply 24V to the coil. If the regulator is out of whack and putting out too high a voltage, then the coil is being overpowered.

The ballast resistor, in vehicles equipped with them, serves a similar regulating purpose. It takes the higher voltage [~28] of the generator and, smooths it out [to ~24]. As is heats up under higher voltage, the resistance increases and the output voltage reduces and vice versa. However, the circuitry allows the starting switch to bypass the resistor when starting so as to deliver full voltage to the starter.

You likely have a bad regulator, or possibly generator. Simply adding a resistor is treating the symptom but not curing the cause.

m37brat · Jul 11, 2011

AN/GRC-46 said:
Before you do much more you might want to check the voltage output at the regulator. The generator output varies a bit but is smoothed out and regulated by the regulator to supply 24V to the coil. If the regulator is out of whack and putting out too high a voltage, then the coil is being overpowered.

The ballast resistor, in vehicles equipped with them, serves a similar regulating purpose. It takes the higher voltage [~28] of the generator and, smooths it out [to ~24]. As is heats up under higher voltage, the resistance increases and the output voltage reduces and vice versa. However, the circuitry allows the starting switch to bypass the resistor when starting so as to deliver full voltage to the starter.

You likely have a bad regulator, or possibly generator. Simply adding a resistor is treating the symptom but not curing the cause.

How should I go about checking the regulate and generator? The truck has the original 24 volt system and all the wiring is closed. Can I check the output of the regulator by connecting to the wire coming from the on/off switch?

Thanks, Jim

Bill W · Jul 11, 2011

Miltary coils have internal resisters
The ohm resistence on your coil should be between 10-14ohms anything lower will cause shorter coil life ( coil runs hotter ), if you do have low resistence then you can ad a external resister to bring it up ( to check resistance put ohm tester leads across the pos and neg posts on the coil )

ALSO
You could have a bad surpression filter ( part AA in photo )
This filter is mounted in the base of the distributor where the power lead screws into ( see photo ) these get saturated over time and cause the same symptoms of a bad coil., I bypassed/modified mine ( top one in photo ) so I could still use the ( stock ) power lead plug in

AN/GRC-46 · Jul 11, 2011

m37brat said:
How should I go about checking the regulate and generator? The truck has the original 24 volt system and all the wiring is closed. Can I check the output of the regulator by connecting to the wire coming from the on/off switch?

Thanks, Jim

The dash voltage gauge should show the relative voltage coming from the regulator. If it's more than a third of the way into the green it may be too high. You can measure it directly by pulling the lead from the gauge and using a multimeter.

I'm not sure if you can start and run the M37 on the battery alone. If so, you could remove the cable from the generator, start the engine and use a voltmeter directly on the pins. It should read about 28.5 volts.

Oldfart · Jul 12, 2011

A quick diagnostic check can be performed by measuring the voltage across the series battery pack with a volt or multi meter of some kind. At rest (engine not running and all accessories off) the voltage should be something over 24 (25 or so) While observing the volt meter, have someone start the truck. The voltage should not drop below 20 volts while cranking to start. Once started, the voltage should not exceed 28 or 29 volts and that voltage should drop down to less than 26 fairly quickly (as the battery charge is restored from the starting process.)

If the volts at the battery are in excess of 29 for any length of time, then something is not right with the charging system. Regulator output to high, bad cells in one or both of the batteries etc. can cause high output. Do check the static noise filter as well. There is no ballast resistor as the system is all 24 volt. Ballast resistors are used mostly in transition ignition systems where the coils are actually 6 volt running in 12 volt systems. The purpose was to have the ignition switch put 12 volts on the coil for increased spark during the engine crank to help the start the engine. Once started, the key switch was released putting the ballast resistor in the circuit and dropping the primary voltage to the coil. The advent of high energy ignition systems made the starting over volting of coils unnecessary and allowed a wider gap on spark plugs.

I don't know if Jatonka has the TM for all of this on his site, but it is pretty well covered in TM 9-8030 and ORD 9 SNL G-741 and to a lesser extent in TM 9-2320-212-20P

Oldfart · Jul 12, 2011

TGP (IL) mentioned the air tubes and that does make a large difference on how hot the coil get while running. These coils are incased in a sealed distributor system and the distributor has a vent system that pull air across the coil to cool it. I have seen an M37 with the vent system capped off at the distributor having coil problems. The owner of this truck opined he was not driving his truck under water so he did not need the vent tubes. He would have been better served to leave the system open to the outside air. The proper system actually draws air through the distributor to cool the coil.

Bill W · Jul 12, 2011

OF
There is a ballast system and ( as I mentioned ) its internal, early M-series ( 1951-2 ) had a external ballast resister which was stacked under the coil ( early non resister coils were shorter and sat ontop of the wafer type resister in the distributor ) these were superceded by the taller internal resister coils.

Oldfart · Jul 12, 2011

I rechecked My TM and ORD manuals again (dated May 1955 and January of 1954) and there is no mention or listing for an external resistor. Same result for the 1960 edition of TM 9-2320-212-20P. TM 9-8031-2 issued May of 1953 does not cover the distributor. While I have never encountred an external resistor in an M37 or M43, I can well accecpt that one exists or was used in early production M series. A voltage adjusting resistor built into the coil would not be something that could be checked for proper operation other than an Ohm check on the whole coil.

Toward helping with the OP's problem, here is a link that discusses ballast resistors and in the process explains how dwell can impact a coil. Ballast Resistor .

Bill W · Jul 12, 2011

Oldfart said:
I rechecked My TM and ORD manuals again (dated May 1955 and January of 1954) and there is no mention or listing for an external resistor. Same result for the 1960 edition of TM 9-2320-212-20P. TM 9-8031-2 issued May of 1953 does not cover the distributor. While I have never encountred an external resistor in an M37 or M43, I can well accecpt that one exists or was used in early production M series. A voltage adjusting resistor built into the coil would not be something that could be checked for proper operation other than an Ohm check on the whole coil.

Toward helping with the OP's problem, here is a link that discusses ballast resistors and in the process explains how dwell can impact a coil. Ballast Resistor .

YES OF
I stated how to check the ohms in a internal resister coil in my previous post. In 14yrs I've only seen one of these with that original setup, I have a e-mail in to a buddy to see if he has a picture of this setup

Also you guys, they are VENT lines NOT cooling lines that run from the dist to the airhorn., These VENT out the gasses that are formed from the points arching/sparking when they close

TGP (IL) · Jul 12, 2011

Bill W said:
YES OF
I stated how to check the ohms in a internal resister coil in my previous post. In 14yrs I've only seen one of these with that original setup, I have a e-mail in to a buddy to see if he has a picture of this setup

Also you guys, they are VENT lines NOT cooling lines that run from the dist to the airhorn., These VENT out the gasses that are formed from the points arching/sparking when they close

Your method for checking the coil with the Ohm meter is spot on.

However I disagree about the vent lines.

True they circulate air to remove the gases`as stated,but also cool the coil.
I know for a fact if there not used or incorrectly installed the coil will be short lived.

A picture of the ballast resistor would be much appreciated as Iv'e never herd of one on the M 37.

Tom

Bill W · Jul 12, 2011

Tom
Think about it
Do you really think that big metal dist thats attached to a hot block and also has hot air blowing on it from the radiator fan is gonna be kept cool by 2, 3 ft long 1/8" od" lines

TGP (IL) · Jul 12, 2011

Bill W said:
Tom
Think about it
Do you really think that big metal dist thats attached to a hot block and also has hot air blowing on it from the radiator fan is gonna be kept cool by 2, 3 ft long 1/8" od" lines

Ha,
I'll rephrase it a little.
At least it's circulating air around it.
Tom

m37brat · Jul 17, 2011

Bill W said:
Miltary coils have internal resisters
The ohm resistence on your coil should be between 10-14ohms anything lower will cause shorter coil life ( coil runs hotter ), if you do have low resistence then you can ad a external resister to bring it up ( to check resistance put ohm tester leads across the pos and neg posts on the coil )

ALSO
You could have a bad surpression filter ( part AA in photo )
This filter is mounted in the base of the distributor where the power lead screws into ( see photo ) these get saturated over time and cause the same symptoms of a bad coil., I bypassed/modified mine ( top one in photo ) so I could still use the ( stock ) power lead plug in

I just checked a couple of coils I bought form a guy on Ebay. If I am using the device correctly I am getting 5-6 ohm's accross the polls. Did I get screwed and should I sndn these back and get my money?

Thanks

m37brat · Jul 17, 2011

Now I am really confused. I checked the new coils I bought from the guy on Ebay and they all three check in at 5-6 ohms. I checked the 2 that got fried last Sunday while driving to a show and they also read 6 ohms. Is it possible once they cooled down they return to a somewhat usable point? I am petty sure that all the coils are defective but how can they get so hot you can't touch them and then they read back to 6 ohms?

????????

73m819 · Jul 17, 2011

m37brat said:
Now I am really confused. I checked the new coils I bought from the guy on Ebay and they all three check in at 5-6 ohms. I checked the 2 that got fried last Sunday while driving to a show and they also read 6 ohms. Is it possible once they cooled down they return to a somewhat usable point? I am petty sure that all the coils are defective but how can they get so hot you can't touch them and then they read back to 6 ohms?

????????

YES, I have had coils do just that, get hot, a winding seperates, breaking contact, cools down about good as new, learned this the hard way, after about replacing everything in the ignition system, still nothing, a OLD guy (I was a LOT younger then) said COIL, I said NO it is good, he said coil and explained, it was the COIL, so yes, it could be the coil as part of your problem

iv

Bill W · Jul 17, 2011

If your getting 6ohms means the resister is letting to much voltage to the coil/windings, this has been a ongoing problem with imported coils, if this was me I would ad a external resister to the dist power lead this will give lower voltage to the coil to which it will run cooler. This symtom was quite common back in the points and condenser days of automobiles, I had a 1968 Olds F85 that the coil had a bad ( internal ) resister, when it got to hot the car would shut down and about 30 minutes later ( after coil cooled down ) it would start right up and run like a champ. Alan Bowes did a great write up on this problem with the M-37 coils and if I can find page 2 of that article I will be able to tell you what ohm resister to buy/add to the power lead

P.S. TGP no luck ( yet) on the stacked coil pic

Help! I'm burning up coils in my M43!!

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