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MG MGB Technical - Wrong Coil Installed?

Hello All,

I'm working on getting my 75 running better. Is there a possibility that the previous owner had the wrong coil installed? If the wrong one were installed, what would the signs be?

The car runs at a nice idle, but there seems to be a lack of power over, maybe, 3500 RPM.

There is also a possibility that the vacuum advance on the 45D dizzy is bad (it's rusted on the outside, so perhaps it is bad).

Before I go and stare at the wires and a wiring diagram for hours, or break out the voltmeter, any thoughts.

Thanks,
Chris
CRH Hoebel

The wrong coil would result in a weak spark. the correct coil is a 6 Volt coil. There is a resistor wire in the harness to drop the cars 12 volts to 6 volts for the coil. If a 12 volt coil was installed without removing the resistor wire the coil will only be operating at half voltage. You may also want to replace the condensor and check the mechanical advance in the distributor, if it is frozen it will affect performance more then the vacuum advance will.
John H

Also check that the dwell angle (points gap) is correct. Cheers - Dave
David DuBois

Thanks for the input so far. IF I find that a 12 volt coil is installed, would my best course of action be to replace the coil, or remove the ballast wire from in the loom (I have no problem doing that, as I've already been into the loom at various points to find other problems in the past)?
Thanks
CRH Hoebel

A distinct possibility, coils and ballast causes a lot of confusion.

Chrome bumper cars had 12v coils and no ballast, rubber bumper had six volt with a length of resistance wire concealed in the harness as a ballast.

By putting a 12v coil with a ballast you get weak sparking. By contrast putting a 6v coil with no ballast you get an overheated coil and points burning (if you have points).

The first thing to do is measure the primary resistance of the coil, between the spades, wires removed. Standard 6v coils measure about 1.5 ohms, standard 12v coils measure about 3 ohms. Aftermarket 'sport' coils usually measure a bit lower, like 1.2 ohms for a 6v and 2.4 ohms for a 12v. Ignore what any label on the outside of the coil says about voltage, they are often misleading if not downright incorrect. Also ignore any concept of internal ballast i.e. something inside the can, it is irrelevant, it is the resistance between the spades that is important.

A 6v coil with ballast is preferable on a rubber bumper as there should also be a special solenoid with bypass contact to deliver full battery voltage direct to the coil while cranking, to improve starting.

If you want to stick with a 12v *don't* remove the ballast wire, there is no point, simply bypass it by taking a wire from the white at the fusebox to the coil +ve in place of the two white/light-greens. Tape these two up and make sure they can't short to anything.

But first measure the voltage on the coil +ve (white/light-greens) with a ground on the -ve. With a 6v coil and ballast you should see about 6v. With no ballast in circuit you will see battery voltage, irrespective of whether the coil is 6v or 12v, which is why you need to do both tests. A 12v coil in series with a ballast will show about 9v. Remember a 12v coil in series with *two* ballasts (and I have seen this) will also show about 6v, which is another reason why you need to measure both.
Paul Hunt

Nevermind thd the last message - just found http://www.youtube.com/watch?v=pWxJmbm5V3s&feature=player_embedded
CRH Hoebel

Paul,
Thanks for the info. It'll be a great help. I also found the above link from moss.
Chris
CRH Hoebel

Well,
There is definitely a ballast resistor in the loom, because it checked out at 6 volts. The current coil measured about 2.8 ohms (not the 1.5), and clearly says one "use without external resistor."

I rewired for 12 volts and seem to have more power throughout the RPM range. At some point I may purchase the correct coil and use the original wiring, especially because it is kinder to the starting system as Paul indicated.

Thanks for all your help.

CRH Hoebel

This thread was discussed between 25/04/2009 and 27/04/2009

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