M series coil failures

[Bill Nutting]

I don't know if this has been covered before, but My M37 distributor gets hot when running the truck slow. Like in a parade for example. On a hot day the coil gets too hot and quits. When I open the hood and let it cool down, it starts and runs again. The coils I have in stock are the chrome Chinese ones. They just don't work when they get hot. So I am wondering if anyone else has had this issue. If so, has anyone ever tried to put a ballast resister in series with the 24 volt lead to the coil? I am thinking that by dropping the voltage to the coil it may run a bit cooler... Or am I just crazy?

 

[John Mc]

Well, I thought the purpose of the coil was to generate high voltage for the spark.

I read somewhere about the imported coils being a bit on the short side, so not making good contact with the housing to conduct the heat away. Someone mentioned putting a copper washer/shim in and coating it with heat conductive paste.

No personal experience with this, just passing along what I read.

 

[foxtrk2]

do you still have coil inside distributor and if so are the fording air lines still connected to the carb if they are make sure there clear they do flow air into the coil to help with cooling

 

[Bill Nutting]

do you still have coil inside distributor and if so are the fording air lines still connected to the carb if they are make sure there clear they do flow air into the coil to help with cooling

Yes, I checked the vacuum and supply line. They are so small that they aren't effective for removing heat from the coil cavity. Someone told me they are there to remove ozone created by the arc of the points. I have built an adapter that will allow me to use a 12 volt external coil. I plan to install it this week end and see if it works. If it does, I'll post some pics.

 

[Bill Nutting]

I installed my coil replacement adaptor. I’m using an American made 12 volt coil. I power it with a DC to DC converter. Truck runs good even when it’s hot out and I’m going slow. That’s when I had most of my problems. So my heat issues were the coil. Moving it out of the distributor solved the problem. Down side: no longer waterproof, won’t survive an EMP.

In the pictures, the brass bolt is covered with multiple layers of heat shrink tubing. It extends up through the inspection port in the top of the distributor housing. I plug a spark plug wire on it and connect the other end to the coil. The body of the adaptor is a 1 1/2 “ PVC pipe cap cut down to mimic the height of the original coil. The copper cup is a copper pipe cap and is poisoned to accept the spring from the distributor cap.

I know it looks like a hack job but so far it’s working good.

 

[Bill Nutting]

Here it is installed. It’s not easy to see the top of the distributor but you can see the red rubber spark plug cable end that is plugged on to the brass 1/4 20 bolt. As you can see, I installed a ballast resistor in series with the “12” volt supply wire. The DC to DC converter I used has a 13.8 volt output.