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MG MGB Technical - Pertronix and Tachometer

I installed a pertronix unit on my car last night ('73 B) and now my tachometer doesn't work. I'm not sure if this is related, but I think that it is. When I was disconnecting everything from the old points setup, there was a white/blk wire running from the distributor to the coil, this wire was spliced together at the fitting for the coil with another white/blk wire. I haven't been able to figure out what the other wire was for, i.e. was it connected to the tachometer? Or did the tachometer get its reading from the white wire that goes into the coil? I have never touched a distributor before this, let alone done any work on one, so forgive my ignorance in this matter.

Thanks
Zach
Zach Dorsch

There should but one wire from the distributor to the coil, which is white with a black tracer. It dives into the wiring harness near the dist and emerges several inches away near the coil. It has no connections within the harness. It connects to negative terminal of the coil. Not sure what the other wire is -- sounds like the work of a PO. It's common to find that the wire is in poor shape and gets bypassed. Regardless, with the Ignitor it is not used and can be taped out of the way (but don't cut it in case you need to reinstall points some time down the road.)

The tach gets its signal from the plain white wire coming from the ignition switch. The circuit is switch->tach->coil. The tach should work with the ignitor installed. Not sure why it wouldn't -- perhaps you got the coil terminals reversed? Don't know if that would cause it, but I can't think of anything else....

What I noticed with my Ignitor was that the tach bounces a lot more than it did with points...
Rob Edwards

Didn't you post this elsewhere?

The 73 has a white/black from coil to points and one from coil to tach which supplies the voltage pulse. The current loop of the white via the tach to the coil ended in 72.
Paul Hunt

O.K. help me out here guys - I still have positive ground , want to convert to negative ground but what to do with the tach ? . it's the original positive ground tach , I looked inside a used one and I don't think I can touch it and still have it work .

Any suggestions aprreciated ! .

-Nate
Nate

Normally you just have to swap the 12v and ground wires over inside the tach case. The alternative is a replacement tach, but one which still uses the current loop, not the later voltage-triggered type, unless you also want to run in a new wire from the coil CB terminal to the tach.
Paul Hunt

This thread was discussed between 05/01/2002 and 07/01/2002

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