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Mark
Mark, Technician
Category: Car
Satisfied Customers: 1504
Experience:  Qualified City & Guilds
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How hot should an ignition coil run on a classic car with a

Resolved Question:

How hot should an ignition coil run on a classic car with a standard points setup ? Ive changed the coil as part of renewing the whole system and the new one is too hot to touch after 10 minutes running. The car runs fine but I dont want to burn out any wiring. Its got a ballast resistor and the coil is 1.5 ohms. Trouble is I never felt the old coil so dont know if this is normal or not. Thank you.
Submitted: 3 years ago.
Category: Car
Expert:  Mark replied 3 years ago.
Mark :

Hello, my name is ***** ***** I will try help you with this.

Mark :

An ignition coil will get warm in use but it should not burn you or be too hot to touch.

Mark :

If the system has a ballast resistor then check the voltage supply to the coil + terminal with the engine cranking and then with the engine running.

Mark :

It should read 12v on crank and about 9v with the engine running.

Customer:

Hi Mark. Thank you for your quick reply. My readings are backwards ha ha. Has about 9v cranking and 11.8 running. The car actually has a ballast wire (1965 American Ford) and Ive checked this as per the manual and it seems fine. If it is this wire playing up as its a nightmare to change would it be O.K. to just run a 3 ohm coil with internal ballast resistor? Thank you. Mark Couchman.

Customer:

Hi Mark. Thank you for your quick reply. My readings are backwards ha ha. Has about 9v cranking and 11.8 running. The car actually has a ballast wire (1965 American Ford) and Ive checked this as per the manual and it seems O.K. If it is this wire playing up as its a nightmare to change would it be O.K. to just run a 3 ohm coil with internal ballast resistor ?

Mark :

Ballast ignition systems were changed to run a lower voltage coil (usually 9v), and these coils could still give the same output as the original 12v coils. In order to run such a coil, the 12v ignition feed runs through a ballast, reducing it to 9v at the coil. To assist starting, a 12v feed (usually from the starter) bypasses the 9v ignition feed, giving the 9v coil a 12v feed. The result is a better than normal spark which is ideal for starting, particularly on cold damp mornings. As soon as the engine has started, the 12v feed is cut and the coil will run on the 9v ignition feed. If you think the ballast wire is good check to see if the 12v supply wire is fitted correctly.

Mark :

As you have only 9v on crank it looks like this is due to a voltage drop and not a ballast.

Mark :

Does your coil + connection have two wires going to it or just the one.

Mark :

It should have 2. One 9v and the other 12v.

Mark :

Depending on the setup the 12v supply might not show until the engine is cranking.

Customer:

Wow thank you. No. My coil only has one feed.

Mark :

Did you manage to find the other wire?

Mark :

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Mark :

Thanks.

Mark :

Mark.

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