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Thread: 1951 Plymouth: very hot coil, no start, why??

  1. #1

    1951 Plymouth: very hot coil, no start, why??


    I have been suffering to start my 1951 Plymouth when the engine is hot. I have noticed the 6V ignition coil is overheated when I have this starting problem. If I wait some time, it cools down, then car starts. I have already replaced the 6V battery and the coil many times, and no solution was achieved. I changed the original location of the coil, attaching it to the fender, what reduced the overheating, but did not solve the problem completely.

    Currently I use a 6V Bosch ignition coil. The temperature gauge does not need to be at maximum level to starting failure, in fact it happens at temperatures above the half of the gauge indicator. I believe the engine cooling is fine, recently the water pump was rebuilt and the radiator core changed.

    I always have a spare coil inside the glove box, so I can replace the coil when the problem happens. Sometimes I just wrap the hot coil with a wet cloth, so the temperature gets down and the engine starts.

    What could be the source of this problem? What is getting the coil so hot?

    Thanks,
    JRA
    1928 Chevrolet Touring
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    1951 Plymouth Cranbrook
    1954 Willys CJ-3B Jeep
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  2. #2
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Two things come to mind. either the ballast resistor has quit or you have the wrong coil fitted. In order proceed further,how good are you with auto electrics?

  3. #3

    Re: 1951 Plymouth: very hot coil, no start, why??

    This Bosch 6V coil I am using has 1.4 ohms resistance and it is the current replacement of Bosch Super Blue coil. I have used new 6V coils sold in US (black ones) with even worse results, so I gave up of them and started to use the Bosch ones, taht has better results, but it still overheats.
    The car wiring seems to be ok, the harness is still the original one. Recently I installed an additional grounding cable, what improved the electrics performace (brighter lights, ...), but maybe there is something wrong and I do not know.
    Thanks
    JRA

  4. #4
    Rusty Rascal Dandy Dave's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    I think your wiring, or ignition switch is bad. When the defective areas get hot, they will break down and cause enough resistance to reduce the spark. Check how much voltage is actually reaching the coil when everything is hot. If there is a reduction with the points open, that is your problem. Also check it with the points closed to see how much voltage drop you have. Also, not as likely, but it could be in the distributor. Are you positive that you have a good condensor, and the points plate grounding wire is in good order. Dandy Dave!
    Last edited by Dandy Dave; March 4th, 2012 at 09:10.
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Hello,

    I just joined a forum that deal specifically with 40s and 50s plymouths and dodges. I am new the the brand and have found that very knowledgable folks belong to that group. I remember this problem being hit on several times and I think it had to do with the carb getting too hot. fuel boiling or something to that effect.

    Join this group! P15-D24 Homepage

    The forum is great and I'm 100% sure you will get someone that can answer your question without a doubt.

    Darren
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  6. #6

    Re: 1951 Plymouth: very hot coil, no start, why??

    The question is what is making the coil overheat. With a very hot engine, I can easily start the car just by changing the coil to a cold one, or wrapping the hot coil to a cold wet cloth. This cold wet cloth quicklly turns down the coil temperature allowing a easy engine start. Once I thought the problem could be on the starter, but I sent it to a complete overhaul recently. I think ithere is something in the system making the coil hot, that I do not understand.
    I am buyin an Optima RedTop 6V battery to try another option, but I am not sure this will solve the problem.
    Thanks,
    Julio

  7. #7

    Re: 1951 Plymouth: very hot coil, no start, why??

    Coils always get hot especially Bosch coils. They use internal resistance to regulate temp. When they are cold resistance is low and you get a hotter spark. As they warm up resistance increases until they reach a certain point then get no hotter. This is why Bosch coils don't need an external resistor.

    Your original coil was probably fine.

    Don't worry about the coil heat. Are you getting a good spark out of it? That is the main thing.

    If your engine has spark, gas, and compression all happening at the right time it will start and run. It can't help itself. Now all you need to do is figure out if you have spark, gas, and compression and if they are happening at the right time.

    You can check spark by disconnecting a wire off one spark plug and holding it next to the engine. If you get a good spark, great. If not you can pull the coil wire out of the distributor and do the same test. If the coil fires and the spark plug doesn't that tells you the distributor is at fault. Otherwise there are other places to look.

    To find out if you are getting gas look down the carb and work the throttle. You should see gas squirt into the carb.

    Your problem can be diagnosed easily if you break it down and check one thing at a time in a logical manner. Replacing random parts, or guessing over the internet from 1000 miles away is an inefficient way to track down the problem.

    Find out if you have spark and gas and get back to use.

  8. #8

    Re: 1951 Plymouth: very hot coil, no start, why??

    I am assuming you are using 6 volt coils on a 6 volt car. If you have 12 volts in there you could be overheating the coils but not if you are on 6 volts.

    It is possible for a coil to wear out and fail when it gets hot. But to get 2, 3 or more bad ones in a row would be very unlikely. Not impossible but very unlikely.
    Last edited by Rusty_OToole; March 4th, 2012 at 11:23.

  9. #9

    Re: 1951 Plymouth: very hot coil, no start, why??

    The car is 6V, and the coil and battery are also 6V. At cold start, there is spark, so the engine starts, but I really need to check it when I do the hot start. The fuel is ok at the carburator, once the cold wet cloth trick at the coil allows easy start when engine is hot. Somehow there is something overheating the coil...

  10. #10
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Quote Originally Posted by JRA View Post
    This Bosch 6V coil I am using has 1.4 ohms resistance and it is the current replacement of Bosch Super Blue coil. I have used new 6V coils sold in US (black ones) with even worse results, so I gave up of them and started to use the Bosch ones, taht has better results, but it still overheats.
    The car wiring seems to be ok, the harness is still the original one. Recently I installed an additional grounding cable, what improved the electrics performace (brighter lights, ...), but maybe there is something wrong and I do not know.
    Thanks
    JRA
    OK but what about the ballast resistor ?

    If your wiring is still standard then you will have a ballast resistor which reduces the coil voltage whilst the engine is running and increases the voltage durimg start, if it fails you will have a higher than normal coil voltage after start. Usually they are located on the engine rear bulkhead, a rectangular ceramic block about 2 inches long, I think I see yours just above the votage regulator; try changing that and see how you go.

  11. #11
    Senior Member r1lark's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    I'll say up front that I have little experience with the late '40s/early '50s Mopars. But would a 6v Mopar even have a ballast resistor? Perhaps mistakenly, I thought ballast resistors are primarily used with 12V systems to knock the voltage down at the points thus increasing point life.
    Paul
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  12. #12

    Re: 1951 Plymouth: very hot coil, no start, why??

    Thanks Chris, do you mean a small ceramic box, light blue color, at the right side of the voltage regulator (see picture)? I have owned this car for 13 years and never touched on this part. Is it the ballast resistor? Does 6V system have a ballast resistor? Thanks, JRA
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    Re: 1951 Plymouth: very hot coil, no start, why??

    The coil looks to be very far away from the distributor with a very long coil wire,they are supposed to be mounted on the loom for the plug wires above the distibutor and no a 6 volt does not need a ballast resistor.
    Last edited by junkyardjeff; March 5th, 2012 at 00:01. Reason: more words

  14. #14
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Quote Originally Posted by JRA View Post
    Thanks Chris, do you mean a small ceramic box, light blue color, at the right side of the voltage regulator (see picture)? I have owned this car for 13 years and never touched on this part. Is it the ballast resistor? Does 6V system have a ballast resistor? Thanks, JRA
    Well not what I have seen in the past, but if its ceramic and wired to the ignition system then that would be it.

    All the other Mopar products I have ever seen have a rectangular 2 inch long type resistor, but its possible that someone has replaced the original, and I cant think of any other reason for a ceramic piece as you have, other than to act as an ignition ballast; see if you can find another and change it anyway.

  15. #15
    Rusty Rascal Dandy Dave's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    The first use of a Ballist resister in a Mopar was when they went to 12 volt. My 57 Dodge Pickup has one, and it is a single unit. The dual unit came about in the 60's. Dandy Dave!
    Last edited by Dandy Dave; March 5th, 2012 at 05:47.
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  16. #16

    Re: 1951 Plymouth: very hot coil, no start, why??

    I think that light blue piece, that could be made of ceramic, is actually the horn relay. It says Delco-Remy on it. The coil was previously on the top of the engine, as in the original setup, but the coil heat problem was worse. When I changed to the fender, only additional 4 inches of wire were added, and the overheating coil problem reduced a lot, but it is still there...All the connections and terminals were properly welded, including battery and grounding cable, to avoid any loss.
    JRA

  17. #17
    The History Guy bofusmosby's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Being a novice with old cars, I can't say. BUT, being in electronics as a profession it sounds like something is drawing too much current through the coil. The output of the coil goes to the center of the distributor, which is them transferred to the individual plugs. What I am concerned with is the the other wire that comes from the coil that goes through the distributor. I believe that your coil problem is a reflected problem, being caused by possibly a bad distributor. Would the plug gap being set wrong cause this? Not sure, but I don't think so. Sorry, I wish I could help, but I'm just trying to use some logic with my own knowledge to figure this out.
    Jim

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    Senior Member roysboystoys's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Replace the condensor, preferably with a used , known good one.
    It's always the condensor!Roy

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    The History Guy bofusmosby's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Quote Originally Posted by roysboystoys View Post
    Replace the condensor, preferably with a used , known good one.
    It's always the condensor!Roy
    I agree that should be the first move, but why a used one?
    Jim

  20. #20
    Senior Member roysboystoys's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    I worked in an Ignition shop for over 20 years when 'Tune ups '
    were common practice.The most common defective new part was the condensor.
    Many cars came in running fine , but some would not run after new points and condensor.Reinstall the old condensor and it would run fine.
    I got to where it was best to leave the old one in.
    If you put new defective one in you can have the same problem.
    Use one from a good running car with no problem ,and try to duplicate your
    past problem.
    Just my suggestion.
    Roy

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    Re: 1951 Plymouth: very hot coil, no start, why??

    Quote Originally Posted by JRA View Post
    I think that light blue piece, that could be made of ceramic, is actually the horn relay. It says Delco-Remy on it. The coil was previously on the top of the engine, as in the original setup, but the coil heat problem was worse. When I changed to the fender, only additional 4 inches of wire were added, and the overheating coil problem reduced a lot, but it is still there...All the connections and terminals were properly welded, including battery and grounding cable, to avoid any loss.
    JRA
    No the light blue thing is metal, and as you correctly surmise, is the horn relay, whats got my attention is the white square thing just to the left of it, looks held in place by a zip tie ??

  22. #22
    Rusty Rascal Dandy Dave's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Quote Originally Posted by Dandy Dave View Post
    Are you positive that you have a good condensor, and the points plate grounding wire is in good order? Dandy Dave!
    I said that way up there. And I forth that motion on the condenser a second time. Dandy Dave!
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  23. #23

    Re: 1951 Plymouth: very hot coil, no start, why??

    That square metal piece is the turn signal relay.

  24. #24
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Your Plymouth is 6 volts POSITIVE ground. The coil should have the wire on the positive terminal going from the coil to the distributor, and negative terminal should be coming from the ignition switch (via the relay). Another thing I'd check is to be sure that the coil body is getting a good ground . The inside of the coil mounting clamp should be clean and the attaching screws not rusty. It wouldn't hurt to run an additional ground from the coil bracket to the battery just to be sure.

    You have a nice looking Cranbrook. I've had several and presently own a '51 business coupe and '52 Cambridge (both 'someday' restorations).

  25. #25
    Senior Member 58Mustang's Avatar
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    Re: 1951 Plymouth: very hot coil, no start, why??

    Quote Originally Posted by Harold View Post
    . Another thing I'd check is to be sure that the coil body is getting a good ground . The inside of the coil mounting clamp should be clean and the attaching screws not rusty. It wouldn't hurt to run an additional ground from the coil bracket to the battery just to be sure.
    I do not believe that a coil needs to be grounded. The case is just a cover and is not part of the circuit. I have started old cars with an old coil just hanging from a wire or flopping in the wind. No problem
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