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MG MGB Technical - coil resistance

72 mgb roadster. Checked resistance across the points of the coil (Bosch GT40) and found only 0.6 Ohm. Should be 3.0 Ohm. Car runs sweet but assumed low resistance indicated part failure of coil. However when coil checked off the car, got a 'normal' reading of 3.0 Ohm!

Put coil back - rechecked - 3.0 Ohm if just one wire connected, 0.6 Ohm when both connected.

Suggests a fault somewhere else in the wiring - perhap a stray earth?

Any advice appreciated

John Crighton
J H Crighton

I would guess that you have an electronic ignition conversion rather than the old points, because if the points were open you would should read only the coil resistance. So, I believe that you are also reading the resistance across the ignition box, which according to my cobweb-laden memory of formulas should be 1.33 ohms.

Wayne Pearson

One must always disconnect at least one wire from a component when taking resistance measurements to prevent any sneak (parallel) paths from changing the reading of the resistance of the component under test. Cheers - Dave
David DuBois

Wayne. Thanks. In fact I still have points ignition. The reason I checked the coil at all was because I want to change to an electronic system and Petronix advise their system need at least 1.5 Ohms.

Dave. Thanks also. I take your point. I had cross checked the coils in my other car (54 TF) and a friend's MGB. Both showed 3.0 Ohms whether or not the wires were in place.

Clearly then I DO have a sneak current. The question then is 'Is this something I should worry about, and is it something I can do anything about'!


john c
J H Crighton


I'm the wrong guy to be answering electric questions, but I'll hazard a guess anyway and say that if you check the resistance while the coil is hooked up, you will read different values if the points are open than if they are closed.

C R Huff

The Rubber Bumper model MGBs have a ballast inside of the wiring harness and need a 6v ignition coil which measures a primary resistance of approximately 1.5 Ohms. There should be two white / light-green wires on the positive (+) terminal of the ignition coil on these cars. The Chrome Bumper model MGBs do not have a ballasted ignition system and need a 12 Volt ignition coil which measures a primary resistance of approximately 3.0 Volts. There should be a single white wire on the positive (+) terminal of the ignition coil of these cars. The problem is that some cars have been molested by D.P.O.s (Dumb Previous Owners). Consequently, you really have no idea what you have, especially if an electronic ignition unit has been fitted. Get the wrong combination of ignition coil and ballast / no ballast, and you will have either an overheating ignition coil that burns your contact breaker points, or a weak spark. However, there is a simple test that you can perform in order to divine what type of system it is that you have.

Both ballasted and non-ballasted ignition system will show 12 Volts at the positive (+) terminal of the ignition coil until ignition coil is drawing current and the contact breaker points or the trigger are closed. Only at that time will ballasted systems see a voltage drop at the positive (+) terminal of the ignition coil, while unballasted systems will still show 12 Volts. This is very difficult to measure with some electronic systems if they are designed only to draw current through the ignition coil when the trigger is being fired by rotation of the distributor. Testing a contact breaker points system is just a matter of turning the engine until they are closed, or simply tapping a known good ground (earth) onto the negative (-) terminal of the ignition coil while you measure the voltage on the positive (+) terminal of the ignition coil. For an electronic system it is probably be best to remove all the electronics connections from the ignition coil, just leave the ignition feed from the harness to the positive (+) terminal of the ignition coil, and then connect the known good ground (earth) to the negative (-) terminal of the ignition coil as before. You need to know what your ignition coil primary resistance is first. Only if the primary resistance drops to either 6 Volts with a 1.5 Ohm ignition coil, or stays at 12 Volts with a 3 Ohm ignition coil, do you have the correct combination of ignition coil and ballast / non-ballast. The one other thing to be wary of is that some aftermarket electronic systems utilize a brief high-voltage pulse to the ignition coil from the electronics, and these ignition coils can be of a very low resistance, perhaps only a mere .1 Ohms, and these should never be connected direct to 12 Volts and ground (earth).
Steve S.


Good point. In my case, resistance with:

wires off = 3.0 Ohm
wires on (points closed) = 0.7 Ohm
wires on (points open) = 3.0 Ohm

When points are closed the wire to the distributor (-ve terminal) is earthed. So the 0.7 value is pretty much what you would get simply measuring the resistance between the +ve terminal and any ground.

I found also that I can get an audible connectivity signal between the two terminals when one or both the wires are off, but not when they are both on. This is consistent with the resistance readings.

There obviously must be electrical connectivity between the two terminals, but something about the wiring is disguising that fact (at least to my multimeter).

My poor old brain is having difficulty reconciling all this!

PS: Steve - thanks, my system is unballasted 12 volt points

john c
J H Crighton

If you measure the resistance with the wiring connected and the points closed you are also measuring whatever components are connected between the white and green circuits and the body, which includes the fuel pump and all the electric instruments, which are effectively all in parallel across the coil, as Dave says.
Paul Hunt

This thread was discussed between 10/07/2009 and 16/07/2009

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