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


JRA

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:mad:

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

1929 Chrysler 75 Roadster

1951 Plymouth Cranbrook

1954 Willys CJ-3B Jeep

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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 :)

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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!

Edited by Dandy Dave (see edit history)
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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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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

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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.

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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.

Edited by Rusty_OToole (see edit history)
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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...

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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.

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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.

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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 :)

post-76672-143138870866_thumb.jpg

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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.

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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

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Guest bofusmosby

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.

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Guest bofusmosby
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?

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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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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 ??

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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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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).

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. 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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Your Plymouth did not come with a ballast resistor or any type of resistor in the ignition. The coil got full power all the time. The Bosch coil should be fine.

The condenser, or capacitor, is supposed to be .25 mfd. A modern mylar capacitor is far better and more durable than the old tinfoil and wax paper jobs. If you get one from the electronics supply, connect the - side to the points and ground the + side. If it will not fit inside the distributor, connect it at the coil, to the same wire that goes to the points. Electrically it will function the same no matter which end of the wire it is attached to.

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OK agreed no resistor in the 6v system; not a lot left in the circuit to create a resistance high enough to cause a coil to break down other than the coil itself.

Granted that you have replaced a number of coils and that you can start with a cold coil you dont have a lot of options remaining. Making sure you have the correct dwell angle is a possibility, this establishes the coil build up and collapse timings, condensor may have high resistance, are the points burnt/corroded, or there is a gremlin somewhere in the wiring.

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I believe the car is slightly more advanced than nit was in the past. In a recent work in the distributor, the mechanic set it up a little more advanced, and I feel the car much more powerful on running up the hills. This behavior is very different than in the past, when the car struggled in 2nd gear shift to move up hill, always requesting 1st gear. Now it goes very well in 2nd gear. The speed and power in 3rd gear seems to be similar as in the past. Could this overheat the coil, creating difficulties to start the engine when hot?

I ordered a Optima redtop 6V battery to my car, to see if this helps. Do you have experiences using this battery?

JRA

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The coil does not care what the timing is set at. Another battery is not going to help

Correct, neither the timing or battery are going to impact on the coil performance, the problem will exist somewhere between the ignition switch, distributor or coil.

What condition are the distributor points in, pitted or burnt ?

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I post on another board where a guy was having the same problem. He had replaced the coil, the ignition module, the magnetic pickup and still experienced, what he felt was, an overheating and subsequent breakdown of the ignition. His carburetor was shot. He had it rebuilt and, voila, problem solved. Just sayin'............

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OK sounds like you have covered most of it, did you ever establish whether you still had a spark when the engine got hot and quit ? i.e. spin the engine over with the high tension lead out of the distributor and see if you got a spark.

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Harold is correct and so is 58Mustang. I would mount the coil where it belongs and use the correct AutoLite coil that it came with. NO ballast resistor. Hot wire the car, that will bypass the ignition switch. Clean all of the connections. And yes, use a condensor that is known to be good.

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I just had a weird thought. What if the problem is vapor lock? What if when he raises the hood and changes the coil, or wraps the coil in wet rags, the fuel pump or fuel line cools down enough to start?

This makes a lot more sense than a coil overheating. We have had a lot of threads in the past about old flatheads getting vapor lock on today's lousy gas.

Next time put the wet rags on the fuel pump and see what happens. Also check the heat deflector shield is on there, between the fuel pump and exhaust manifold. They sometimes get lost over the years.

The only thing wrong with this theory is, what is he doing getting vapor lock in winter? However, as he does not give his location, he could be in Death Valley or the Southern Hemisphere for all I know. The name Julio and the Bosch coil suggests he may not be in the US but that is not much to go on.

Edited by Rusty_OToole (see edit history)
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I just had a weird thought. What if the problem is vapor lock? What if when he raises the hood and changes the coil, or wraps the coil in wet rags, the fuel pump or fuel line cools down enough to start?

This makes a lot more sense than a coil overheating. We have had a lot of threads in the past about old flatheads getting vapor lock on today's lousy gas.

Next time put the wet rags on the fuel pump and see what happens. Also check the heat deflector shield is on there, between the fuel pump and exhaust manifold. They sometimes get lost over the years.

The only thing wrong with this theory is, what is he doing getting vapor lock in winter? However, as he does not give his location, he could be in Death Valley or the Southern Hemisphere for all I know. The name Julio and the Bosch coil suggests he may not be in the US but that is not much to go on.

That's what I was trying to say in post 5! I remember this coming up on another forum. These old Plymouths tend to have vapor lock problems and the carbs don't get the gas. I think one of the problems was due to the fuel pump heat shield missing. Another problem that was discovered was the use of fuel filters to close to the manifold. I'm no expert at these by any means, but if the spark is there when hot, then it has to be another problem.

Darren

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The car is in Brazil, so we do not have very cold winters. The coldest period im my town is from may to august, ranging from 46F to 64F. I cannot remember if this problem reduces during winter, but it is true it is more frequent in summer.

And I think my fuel pump does not have a heat shield. Does anyone have a picture of this heat shield? This pump is relatevely new, it was replaced about 6 years ago.

Best, JRA

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Julio I do not have a picture of the fuel pump heat shield. It is a flat piece of sheet steel about 10 cm wide and 30 cm long. It is bent into a J shape. It fits between the fuel pump and exhaust manifold. You will know at once if you have it or not, just look at the engine.

................Later................

Here is the only illustration I can find of the heat shield. This is in a catalog of Dodge truck parts but the heat shield is the same.

http://www.vintagepowerwagons.com/pdf/parts/03fuel.pdf

Scroll down to page 21 nearly at the bottom of the page. The heat shields are on the left. The tab at the corner goes behind a bolt at the front of the exhaust manifold and there is a long bolt that sticks up from the fuel pump to hold the other end. You can see small holes in the heat shields where this bolt goes through. If you can find a picture of the pump and shield on an engine, in a repair manual, you will see how it goes. It wraps around the front of the exhaust manifold, above the fuel pump.

If you don't want to send to the US for one you could make one from sheet metal. Thick aluminum sheet might be better than steel as aluminum reflects and disperses heat better than steel.

Edited by Rusty_OToole (see edit history)
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