The Stovebolt Site Forums The Shop Area Engine Shop My coil has a crack on top - should I replace?

Hi All -

This may sound like a question with a simple answer but from what I've learned in other posts, deciding on which new coil to get (resistor/no resistor) can be tricky.

I have a '40 Chevy 1/2 ton with a '53 235. It's a 12 volt system with a newish alternator, a ballast resistor on the firewall that has not been connected since I've had the truck (10 years). Recently, because it had been sitting in my garage for a year or more, I was hesitant to start it up with old gas and drained the tank, put new gas in, and replaced the fuel filter (with a large-ish Fram inline filter. Because It still wouldn't start (with new gas too), I noticed that the fuel filter was not filling up and fuel not getting to the carb. In other posts (thanks) it has been documented that inline filters seldom fill up visually but work none the less.

The fuel pump seems to work with good flow before the filter. (still need to check after the filter). But the truck still won't start. Then I noticed a crack in the top of the coil - could this be the issue? I was hopeful but plugs still have a healthy spark when observed outside the engine while cranking it over. Could it be a weak spark due to the crack?

I bought a carb rebuild kit thinking it may have been clogging in there that is keeping the engine from running but don't want to start that project until I've ruled out all other issues.

Any thoughts on the coil, or anything else, is welcomed!

thanks - Doug

Picture of the coil and the fuel filter below.

That coil must have gotten extremely hot at some point. You need a new one. Buy a 12 volt coil without internal resistance and use a ballast resistor. If you have a stomp starter, you should put a 2-pole starter switch on it, and wire it so that you will have 12 volts coming from the side post on the 2-pole switch directly to the coil while the starter switch in depressed. Once the engine starts and you take your foot off of the stomp pedal, the 12 volts to the coil will then be diverted through the ballast resistor, giving you around 9 volts to the coil. Without the ballast resistor, your points won't last until next Thursday.
You need to inspect the condition of the points in case the engine was run without a ballast resistor prior.

Yes. Buy a new one.

I agree you should replace it. I think you also questions the external resistor ballast. As I understood it the external ballast is required for when you install spark plugs that do not have an internal resistance, otherwise no. That’s what I’ve read researching the same concerns.

Thanks Carl - I've been driving the truck for about 10 years (to shows only because I had that kind of insurance) and have not had a points problem so I assume there is a resistor in the coil (?) or alternator(?) or somewhere(?) but good advice anyway. I just opened up the distributor cap and things look clean in there an not burned so not sure if that's a useful data point. I had a bad overheating problem for the first few months and lifted the water pump/fan, adjusted the timing, and got a new radiator to fix it - perhaps that was the heat that split the coil top.


In your experience is there a specific type or brand of coil that is a good fit for this type of vehicle?

Thanks again - D

All I have are antique cars. I just buy whatever coil is listed as correct. I have a preference for Standard Motor Products ignition brand parts if available. May or may not be better but had a good reputation when I started working on cars.

Search online for a NOS made in USA coil. Otherwise it's a crapshoot. Most offshore brands are proven untrustworthy. I have a Standard Motor Products 6v coil made in Mexico that is doing fine.

You are reading incorrect information. The purpose of a ballast resistor is to reduce the amount of current (and voltage) flowing through the coil primary winding and the points in the distributor once the engine is started. Makes the points last longer.

Please listen to Jethro about the made in USA part. You can find some on eBay and buying one of those made in USA coils will keep you away from the worst of the products out there. And there are some horrible imported ignition coils being made today. Good luck!

A "12V" coil primary winding is actually made to operate correctly at around 9.0-9.5 volts, which is what is available from a "12 volt" battery during a cold start due to the load the starter motor puts on the battery. Once the engine starts and the charging system tops off the battery, system voltage is around 14.5 volts. The ignition ballast resistor drops the voltage the coil sees during normal running to prevent the points from burning. "Resistor" spark plug wires were invented in the 1950s to reduce radio and television interference caused by the magnetic fields developed around copper or steel core ignition wires. The ballast resistor works in the low voltage primary ignition circuit. Resistance plug wires are in the high voltage part of the ignition system.
Jerry