Go BottomMF135 Overheating coil
 | Doc37683
Join Date: Posts: 1 
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2002-07-27 40720
1974 MF135 with an AG3.152 Perkins gas engine. The tractor runs fine when running. Ignition coil gets hot and quits working after tractor is run a couple of hours. Pulled high-tension lead from distributor and verified coil was not working, i.e. no spark. When tractor sits for a couple of hours, tractor will start and everything works fine. Coil has been replaced twice suspecting a defective coil. Coil is internal resistor type, correct for this application. Although coil is not OEM I am told it is correct for this tractor. The coil is mounted in its original position next to the distributor on a phenolic mount, to prevent heat transfer. I repositioned the coil in its mount so it is as far away from the engine as possible, about three inches. There are no exhaust manifold leaks to contribute to the overheating condition. What can be causing the coil to overheat? Alternator, voltage regulator, and battery output all check good. Ignition switch, plug wires, distributor cap, and rotor are new. Plugs and points gapped correctly and timing is correct. Wiring is routed correctly per wiring schematic but wiring is original (old).
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 | Peters
Join Date: Feb 2002 Posts: 3034 Northern AL 
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2002-07-27 40723
Is the system early electronic or points? Did you replace the points? ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-07-27 40724
Every now and then similar stories are heard about 6V to 12V conversions of old Fords. The coil ends up being for a 6V system or else and external resister is missing or is wrong. I think my ford manual gives specs for coil primary and secondary winding circuits. I think I'd try to get some specs and measure the resistances. I'd take readings when the when the tractor is cold and again after if it quits. Of course, the old wiring might be a problem. My ’52 Nash-Healey had intermittent weak spark and only after it had ran awhile. I eventually discovered a coil wire was too short and had poor terminal connection. Maybe a combination of heat expansion and pulling from engine torque weakened the connection.
Sounds like you've covered all the bases I'd think of. If the charging voltage is 14V and change at the battery, there's not a heat problem and it's happened to several coils, I'd start wondering if the coil really is the right one.
Another possibility is to mount an RF suppressor. Purely theoretical, but a lot of spark could produce high voltage RF across the fairly high impedance coil secondary winding. In most cases, the condenser on the distributor should be adequate but who knows?
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 | Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2002-07-27 40727
You didn't say as to the intervals you have changed the coil. I do believe that the factory stuff is buit more for the tractors than car parts or genric fit parts but that is your choice. ....
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| ROBDEERE
Join Date: Posts: 1
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2002-07-27 40728
DOC: I think I would start with when did the problem first
start. You said that most of the ignition system has been replaced. Were you driven to replace parts because of the problem, Or because it was time for a tune up? For example if the plug wires are RF "radio Frequency" supression type with to high of a resistance value It could make your coil work to hard. Most coil & plug wires have a resistance value per foot. Example 50 ohms per foot may be typical for a car. could you retry the old wires? Next I would carefully check the primary and secondary start/ run circuits. Some ignition systems are designed to have a 2 step voltage or current limiting device to feed the coil.
start would let full battery into the coil to allow full saturation of coil this in turn would let the highest secondary step up voltage out to the plug wires to assure the engine will start. When you let go of the key some ignition systems will have a fall back device as mentioned
before. this could be resitor in the loop to drop the voltage to the coil once the engine starts and you let go of the key. A classic example would be the dual ballast resistor used on the Dodge,plymouth,Chrysler for over 40
years. Without knowing the exact schematic layout of your machine I can only guess that it is a series loop. Battery, ignition switch,possible resistor as mentioned,coil,return to chassis ground, back to battery. If anything in this loop
has high resistance or short to ground it could cause the coil to over heat. Things that I would try is to run a hot wire from the battery to the plus side of the coil and jump the starter solenoid out momentarialy to start the tractor.
please be advised DO NOT DO THIS IF A RESISTOR IS IN THE FACTORY LOOP, OR IF IT IS A ELECTRONIC IGNITION. this would just burn up the new coil. You mentioned points so this rules out the electronic ignition.if you do try the jumper wire to the coil with the other wire off and tied back, and the tactor stays running you almost have your answer. This would mean a faulty ignition switch or faulty wireing. Did you check the dwell when you set the points? If you have to much dwell I think this could cause the coil to work to hard as well. Did the new coil get damaged internally from the existing problem? Some times coils will drip out the oil used for internal cooling and still work untill they heat up. After this long list I'll give a shorter one.
1 coil wires correct resistance
2 Ignition switch ok?
3 wireing ok?
4 dwell set ok?
5 resister if used ok?
6 engine ground ok?
7 correct plugs with correct gap?
8 condenser if used ok?( typically only for)
cars with radios
9 modifications to oem system ie:
add on accesories possibly tied into ignition
primary for power?
10 diodes if used in system ok?
11 teminals, binding posts,bulkhead connectors,
all tight?
12 last and least likely, electricity can do
weird things alternator voltage regulator
I hope this helps you out keep us posted on out come if you need more help let us know. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-07-27 40731
I realized after I posted that transformers burn up if lower rather than higher frequency AC is used on primary windings. I'm not sure what happens if RF is induced onto the secondary except that the impedance would be high and potentially a high voltage standing RF wave could be present across the secondary. I suppose the RF would cross to the primary into a low voltage signal and would short to ground when the points are closed. The capacitor should provide a relatively low impedance path to ground when the points are open.
Even if the idea is sort of backwards, I hope new ways approaching the problem are suggested. Doc checked most of the typical problems before posting. When I first started reading the post, I thought ah ha, the old missing heat shield problem. Not so I found.
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| Doc37683
Join Date: Posts: 1
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2002-07-29 40778
Thanks to all for responding to my post. I will respond to a few of comments.
The tractor has a battery ignition system with points.
It has always been a 12V system not a conversion.
The coils I have replaced are from a Massey Ferguson dealer and are called Heritage line distributed by Agco Corp. I cannot find any specifications for the coil to check it against. This coil is an aftermarket part and has the MF part number on it.
I see no sense in testing the coil because I know it is failing due to overheating. The coil is physically HOT and I mean HOT. I don't think it heating up is related to engine temperature. It is hotter than the engine and is isolated from the engine.
The tractor is original with no modifications.
The question is what is causing it to overheat?
I have ruled out voltage as a problem by confirming the alternator, voltage regulator, and battery as functioning normal.
The only parts replaced to correct the coil overheating have been the coils and the ignition switch. I don't think I am getting defective coils.
The ignition wires are new and are correct for the tractor.
In the next couple of weeks I will test all the items mentioned in your posts and retest what I have checked before. Also, I will replace wiring in the ignition circuit. I will post my results. Thanks to everyone for your help. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-07-29 40783
AGCO and MF are really the same company as you probably know. I have heard several stories about manufacturers getting wrong after-market parts connected with their part numbers, so I wouldn't absolutely assume that the coil is right. Sure would be handy if somebody else with a 135 would measure resistances of the coil circuits. Resting current (with points closed) is another spec that is sometimes defined.
If engine heat can be discounted as an explanation, then abnormal current flow in the coil is about the only other explanation. Most of the reasons for high current, such as high load and high primary voltage have been checked. Incorrect primary circuit DC resistance seems to be one of the few explanations remaining.
A little ohm's law could come up with the size and wattage for an external resister that would be appropriate if it were actually a 6V coil, but that would be just a fooling around kind of experiment. Another possibility is to identify the actual manufacturer of the coil and get a spec for max primary current .
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| Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2002-07-29 40784
Doc;
A coil will over heat if it has a dead short on the high voltage side. This could be a defect in the coil or a short through the distrubutor. I think we have all seen coils crack were the ignition has been left on and the points are closed. It may be that the dwell angle is out and it is staying closed too long but this is unlikely. Check the inside of the new rotor, maybe this was molded incorrectly and the metal is touching the distributor shaft?
Peters ....
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| DH83
Join Date: Posts: 1
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2002-08-20 41436
DOC,My dad and I owned a MF165 for over 20 years and the
only thing on that tractor that didnt Quit working was the ignition system,it had a external resister for the coil,I
remembered touching it one time and it was very hot.
I would put a external resister on it.One simple thing to
check is the polarity at the coil + ign - to the dis.
Did somebody install resister spark plugs and wires?
The way it sounds you already checked all this stuff?
I still have my MF manual if you need any spec's
Good Luck.........
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| Doc37683
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2002-09-20 42698
Update. Finally got time to work on my tractor. Checked and rechecked everything, I mean everything. Found nothing out of specs or out of norm. Replaced all wiring in the ignition system except for the plug wires that are new. Bushhogged my field and the tractor did not shut off. The coil was extremely hot though. I have another new coil that I installed and will see if it runs any cooler. Although the engine did not shutdown I don't think the coil should be this hot. All parts are correct for this tractor. The coil has an internal resister. I will attempt to communicate with Massey Ferguson to see if they can offer an insight. I have spoken with the head mechanics at two MF dealers and they have offered nothing beyond what I have heard on this forum and I had already done everything they would have. Thanks to every one for your input. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-09-21 42714
Doc: Here's an idea that just occurred that you might pursue if you're very curious about the coil. First is that I would ensure that there isn't a missing heat-shield (in absence of anything else, adding a shield might help). I assume the points and plug gaps are set. Then, I'd measure the current in the primary circuit when the tractor is at typical operating rpm. I'd ask MF tech types if that's a normal current. If MF says the current is normal, I'd identify the coil's manufacturer and ask the manufacturer for the coil’s specs.
I believe that factory parts divisions periodically substitute suppliers of OEM parts. There's a slight possibility that either a supplier or the parts division was a little optimistic about specs and requirements, and a coil with marginal specs for the application is in the parts system. It may be stretching a bit, but if nothing can be found that's wrong, then I guess this is the sort of thing to think about.
An alternative might be to just go out and buy a bigger coil. An automotive type might mount easily enough, and coils are pretty generic things. As a '50's highschooler, one of the first things many kids did when they turned 16 was to go out and buy a Mallory coil and put it on the family car. We thought that would make it go faster (the beginnings of disillusionment in adult life). You would want to ensure that a coil is adequate for whatever the circuit is drawn though. Adding an external resister also would make the coil run cooler but would reduce the spark.
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