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MG MGB GT V8 Factory Originals Technical - Tale of two Tachos

Running a 72 V8 Roadster - Tacho (converted from 4cyl) start to read erratically and now will only register sluggishly and to about 50% of true revs . I have another converted tacho but the fitted one has red/yellow and black wires going directly into rear of it . The spare has a bullet and a spade . Are the two compatible ? And can I use the spare to check by running from the coil and earthing without removing or cutting the wires to the existing one . Needless to say the Mr PO Mouse has had a few nibbles at various bits of wiring....
Andy
ap lestocq

Neither sounds exactly correct for a 72. It was for the 73 model year that the tach changed form being current-pulse triggered (RVI on the face-plate) to being voltage-pulse triggered (RVC on the face plate).

Originally the current-pulse type had a pickup externally on the back and a continuous loop of white wire came out of the harness, was wound through the pickup, then went back into the harness. Later versions had an internal pickup and connections on the back - a male bullet for the white wire coming from the ignition switch and a female bullet for the white wire which went off the the coil +ve. There was a standard-sized spade for the 12v supply (originally white then green), and a small spade for the earth/ground wire (black).

Voltage-triggered tachs have the same spades for the 12v and earth, and a single male bullet for the trigger wire from the coil -ve, which should be white/black.

If you break the circuit at the tach and the engine stops then it is a current-pulse tach. If it doesn't then it is a voltage-pulse tach, which sounds like your spare.

Anything could have been done with a conversion, which may even have a much later electronic ignition system which don't always work well with the earlier current-pulse tachs. There were never wires going through holes in the body of the tach, so that is non-standard to start with, and you need to ascertain just what those wires are for before you can fit an alternative.

If the tach is now reading 50%, that would equate with what ever mod was done to make it an 8-cylinder tach having fallen off. OTOH it could be something completely different, the first thing I'd do is confirm that you still have a good 12v and earth supply to the tach.
Paul Hunt

This thread was discussed between 16/08/2008 and 17/08/2008

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