Coil problems

[427James]

I seem to be having recurring problems on 2 cars - that of the live to the coil burning out.

On one car (73 911T) the coil is wired via a ballast resistor - with the resistor in place, it wont start (i have tried it with replacement resistors). With the resistor out (bridged) the car starts and after about 20 minutes the coil burns out.

On the other car (70 Corvette small block with HEI) the live to the HEI is constantly getting warm and then burning through. This is an internally earthed unit.

Any ideas? Odd that the two cars I have/look after have the same problem!

[ch427]

im no expert on the 911 but the ballast resistor usually gets bypassed on starting, this used to be via a terminal on the starter. This would apply 12 volts while cranking to a 9 volt coil to aid cold starting.
When running it then runs through the ballast resistor, are you using the correct coil for the application?

[427James]

Think so. Clearly running 12v through a 9v coils a killer though...

[ch427]

thats the only thing i can think is happening, burning cables seems to suggest too much current flowing through them.

[cpas]

Does it keep running if you un-bridge the ballast resistor once it's started? May be worth making a relay circuit to bypass the resistor when the starter is being operated.

[427James]

cpas said:

Does it keep running if you un-bridge the ballast resistor once it's started? May be worth making a relay circuit to bypass the resistor when the starter is being operated.

Nope, it dies instantly. Strange, I know. I think the ballast resistor is correct though.

[cpas]

The only other thing to try is to replace the coil with a 12V (non ballasted type) then permenantly bridge the ballast resistor. May be harder to start in cold weather but may get you over a problem.

[427James]

cpas said:

The only other thing to try is to replace the coil with a 12V (non ballasted type) then permenantly bridge the ballast resistor. May be harder to start in cold weather but may get you over a problem.

Good suggestion and will try and find one.