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Chicagoland MG Club: Driveline
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  Chicagoland MG Club:Driveline
John Schroeder MGB Tach Conversion
by John Schroeder

I recently converted my 1967 B to negative earth. Everything was going well until I got to the tach. My original tach does not have a ground lug to rewire the positive earth and feed. Usually you can unsolder the resistor from the spade connector and switch it with the green wire that attaches to the ground lug. Since my tach did not have a ground lug I had to do some additional wiring.
tach side view showing resistor close up of resistor
The first thing I did was unsolder the resistor from the spade terminal. I then soldered a length of black automotive wire to the resistor.

The next step was to run a length of green automotive wire from the solder terminal on the PC board to the spade terminal.
resistor with black wire added green wire between two terminals
circuit board isolated from metal frame with insulating washers Then I had to isolate the PC board from the original ground screws. I made two plastic washers by drilling two holes in heavy plastic from a blister package. I then cut around the holes to make the washers. I carefully inserted the washers between the metal block and the PC board and reinserted the screws.
new wire loop from resistor
I then tinned the end of the black wire that was attached to the resistor, formed a loop in the end and wrapped it around one of the screws. I then tightened the screws and reassembled the tach.

new wire gropunded My tach was in excellent condition or I may have continued looking for a tach that could be easily converted. After figuring out what to do, it took me about two hours to rewire my unusual tach. I saved the cost of a replacement tach which I would have had to convert. Plus, I enjoyed the satisfaction of solving a problem many people said couldn't be done.
tach with rear circuit board removed

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