Loss of spark when warm

1912Minerva

1912Minerva

Junior Member
Hi All,

I took my A Model for an outing today and a problem that I have had numerous times happened again:

I had been driving for about half an hour when I stopped to get some fuel. After paying for the fuel and messing about for about 10 mins I went to start it and it wouldn't fire. This exact pattern has happened quite a few times. I opened the points by hand to see if they spark (with ignition on) and nothing. Sometimes I can screw the ignition cable in or out a bit and I will get spark but lately all that will fix it is to let it cool down. I did, this - leaving it for about an hour and half. Came back - checked for spark at the points gap - it had spark so I fired her up and drove home with no problems.

Any ideas as to what is going on here???? I run a modern upper plate and have recently replaced the points and condenser.

Also I noticed that as I opened the points, I could hear clicking (like the sound of sparking) from inside the coil - I haven't noticed this before.

My theory is that, with the motor turned off and no water circulating, something gets warm in the distributor and creates a short, then, when it cools down, the problem goes away.

Any ideas as how to fix this would be gratefully appreciated.

Regards, Andrew.
 
You should never hear anything inside the coil. My guess is the coil. (If you did not mention the sound, I would have said condenser.)
 
I had the very same problem and eventually traced it to the coil. What happens is when you stop for gas the temperature under the hood goes up and the coil is failing at temperature. Let the car sit until the under hood temp goes down and the coil works, until the next time. A very frustrating fault to find.

Tom Endy
 
If you are useing an oil filled coil you may need it mounted with the tower and wire connections pointing up instead of mounting in the original manner. the oil cools the internal windings. all modern coils used on later model cars were mounted with the tower and wire connections pointing up.
 
If you were using an original condenser, it suspect it going open because of the heat.. But, it shouldn't on a 'modern' one..
I agree with Purdy regarding the coil.. Oil coils don't work upside down.. Epoxy units don't care..
It certainly appears you're getting an 'open' somewhere due to the heat and you're having to wait for it to cool and 'close' .
 
In case the coil thing doesn't work out, here's another straw to grasp at if you are using a repro junction box. I have heard of the mounting screws on those shorting out to the firewall when the firewall gets warm; the heat connection doesn't make a lot of sense to me but that's what they said and it might explain the sparking sound you heard in the vicinity of the coil.

I'm kind of surprised that coil failure in the primary circuit would show up like this.

Steve
 
There is one quick way to test the hot coil theory. Buy a can of "canned air" at a hardware store, the kind used for dusting your computer keyboard. Hold the can upside down and spray the coil. The "air" coming out of the can will be at 40 degrees below zero so will cool it off very quickly. It sounds like a coil with a secondary windings issue. Thats why you hear the "clicking".
 
It's got to be the coil. Problems can occur at different temperature ranges with coils. Ok when cold and ok when really hot, but midway can sometimes cause problems. I have had the same experience myself with a coil that always always cause a miss at a particular stretch of road. I blamed all sorts of things until I was told to change the coil and the problem disappeared.
 
How does one tell if a coil is oil vs. epoxy filled? I'm using a NAPA coil terminals down. Away from home so can't say what part # but I can say that it's fairly pricey. :)
 
I don't know how that you can tell if it is oil filled. I use the Pertronix flamethrower coil on our roadster. they make several versions they have a chromed and a black oil filled coil. I used the black epoxy filled one and that is what they recommended and told me why. They also make a 45.000 volt version for electronic ignition. the 3.0 epoxy filled version that I use is 40.000 volt and gives electronic ignition like performance with points ignition. The 3.0 ohm unit is for 12 volt use. The 1.5 ohm coil is for 6 volt, both are internally resisted. The original coils that came on model A's were tar filled. I don't know about the NAPA coils but I would suspect that they are oil filled. If its getting hot, that is an indication that it may be oil filled. sometimes the oil filled coils will leak some oil if mounted with the tower mounted pointing down. If it shows either symptom, I would remount with the tower pointing up. If it ever got hot and wouldn't crank of started missing and loseing power I wouldn't trust it and would just replace it. Any of the coils that the model A vendors sell would be a safe bet. I had lots of coil failures with some coils that I bought at the tractor dealer. I told them that I wanted the type coil that was used on the 8N , they were 6 volt but they didn't last very long. I use original eighty something year old coils on a couple of my cars. I have never found an original that didn't still work unless it was rusted out or had carbon tracks on the tower.
 
Hi Everyone,

Thanks very much for your replies - looks like I'll be replacing the coil (and if that doesn't fix it I'll look into the junction box theory). And yes my coil is oil filled and mounted "upside down". But hey, I'm in Australia, so maybe it's really the right way up!!

Thanks again for your help and I'll try and remember to post a message here let people know what the outcome is although it might be a month or so before I'm really satisfied that the problem is gone away.

Regards,

Andrew.
 
Not to sound like a "smarty"- but if you shake a coil you can often hear the oil sloshing inside the case. This condition gives Purdy Swoft's 1st post concerning how to properly mount a coil "extreme relevence".
 
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