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MG MGB Technical - Positive to negative tach question

In looking at the description in the Moss catalog for converting to the tach to negative ground, it lists Jon Twists instructions on reversing wires inside the tach. I seem to recall that the previous owner said he switched to negative ground (car was no running when I bought it).

I do not know if he reversed the wires, can anyone look at this photo and tell me?

Thanks! David

David Steverson

You need to show or trace where the connections from the 12v spade and the case are going. This schematic http://www.classictiger.com/techtips/motach.html shows a *negative* earth tach with the 12v terminal going to several components, but the earth going through a 150 ohm resistor before it goes anywhere else, which should be easy to determine. This resistor should be colour coded brown-green-brown as arrowed on the attached picture, going between the two PCBs. Note this example may be a *positive* earth tach, if the spade the resistor is connected to is insulated from the case. Your resistor looks to be brown-red-brown to me i.e. 120 ohms, but the blobby soldering and the burning on the PCB indicates that it has been modified. However it could just be that resistor has been replaced, and it is still positive earth. It's which spade that resistor goes to that will be the final arbiter.

PaulH Solihull

Thanks Paul!
That soldering may be the clue that it has already been switched. I called John Twist today, he <thought> the 2 wires crossed each other originally. If that's true, there's another indication it's been changed. He said just try and see if it works, that it will not harm the tach.
David
David Steverson

This thread was discussed between 25/10/2010 and 26/10/2010

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