MGB Coil resistance

[wkilleffer]

Hello everyone,

My car's a 1974 MGB with an 18GK engine and twin HS4 carbs. I've been running a Pertronix distributor with its matching coil for awhile, but just got the car's 25D back from Jeff at Advance, and it looks better than what it probably did when it was new. I'm going to go back to using points and condenser with the rebuilt distributor.

I still have the factory Lucas coil that came with the car when I first obtained it. I checked the resistance to see if it was still useful, and here's what I found:

With a small handheld analog meter, it registered around 6700ohm across the secondary, and the little meter's not capable of giving a reading for the primary.

On the new-to-me Sears Engine Analyzer, the primary shows 5.6ohm and the secondary shows 5100ohm.

Either way, that looks too high given some of the other numbers I've seen. I haven't checked the Pertronix coil, but will sometime this weekend.

Am I right in believing this original coil shouldn't be used?

Thank you,
-Bill

 

[Sarastro]

For points, the coil primary should be 3-4 ohms and secondary around 10,000. Assuming that the analyzer is giving you correct readings, the coil is not what it should be.

Virtually all automotive coils have the ~3 ohm primary or lower. If that's a matching Pertronix coil, Pertronix must be into some weirdness that I'm not aware of. You can get a decent coil for $25, so the simplest solution is just to replace it.

Also, any cheapo digital multimeter should measure those resistances with reasonable accuracy.

 

[Coastalman]

My '77 B has a ballast beside the coil. Does that ballast have a set resistance?
Charlie

 

[Sarastro]

Not always. If you have a ballasted coil, it PROBABLY has a resistance around 1.5 ohms and a ballast resistor of 1.5-2 ohms. But there exist coils with lower resistance, and higher ballast resistance. And, it's not all that critical. If the total resistance (coil + ballast) is between 3 and 4 ohms, you should be fine.

 

[Coastalman]

Thanks! I'll check it out.