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MG MGB Technical - swinging tacho

Swinging is probably not the word but the needle of my '72 tacho,RVI 2430/00, swings wildly about paricularly when switching lights or direction indicators on. !!

Earths OK, voltage reading about 13.8 drops to say 13.3 whith headlights on. Tried parelled wire from coil to bullet terminal on back of tacho but keeps on oscilating wildly with only a few seconds of correct steady readings.

Any ideas?
melvyn simonson

Melven - You are on the right track. Check grounds and power connections to the all the lighting circuits and the ignition circuits. It sounds like you are having a problem with a voltage drop somewhere that is most likely to be caused by bad gronds or connections. For information on getting a good ground connection, go to the following web site:
http://www.omgtr.ca/technical/General%20Technical/grounding.htm
Good luck - Dave
David DuBois

Sounds like tach grounding problems to me. This is a common problem when the dash lights are switched on, without them it can 'see' a ground through its own bulb in series with one of the others to the ground on that gauge. With the indicators affecting it as well it looks more like the common point where the dash light, cigar lighter and courtesy light grounds join together, or where the single black from that common points bolts to the body along with grounds for the heater fan and wipers. This is usually high up on the firewall behind the dash, close by the wiper motor on RHD cars at least.
Paul Hunt

Thank you for your comments.

I took the tacho to a local MG repairshop and they tried it on anither MG. It worked fine. Tried various wiring setups but only when I removed the Radio noise Suppressor condenser from the -ve on coil to body di it start working properly!!! No idea why it shopuld suddenly fail but I will leave this aftermarket item off now.
melvyn simonson

This thread was discussed between 28/03/2004 and 31/03/2004

MG MGB Technical index

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