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MG MGB Technical - Tach needle drops to zero

The tach in my 1980 B has started dropping to zero & then returning to the correct RPM. It will do it at any RPM or engine temp & doesn't seem to be related to driving over bumps in the road because it will even do it at a full stop.
Any thoughts ?
Dave Pearn

Dave. What do the other gauges, fuel and cylinder head temperature, do when the tach drops? Do they also start to drop? If so, the third fuse from the top needs to be checked out for loose connections or corrosion.

If the engine quits when the tach drops, or changes its exhaust note, you are looking at a low tension ignition circuit problem. If the exhaust note remains steady with the engine still running properly as the tach drops to zero, it is a green wire connection and the most common problem, for me, is with the third fuse from the top.

Les
Les Bengtson

My tacho has these random events too. Sometimes it's right, sometimes it's "slow", sometimes it's off. Other gauges are unaffected.

It may be off/wrong for weeks at a time, then right itself.

Sometimes tapping the glass makes it right, sometimes it doesn't.
Geoff E

Believe it or not, i "fixed" my erratic tach by unplugging and plugging in the ground spade connector 5 or 6 times. Corrosion was killing it.
T Lea

Could be a bad connection in the 12v or earth supplies, or indeed in the signal wire from the coil -ve. Could also be dry joints internally. Check the green and black spades on the back are secure, you could temporarily rig-up a voltmeter between these, and also between the white/black signal wire and earth, and look for wobbles in these at the same time the needle drops. If all of those show a steady reading, it is internal.
PaulH Solihull

Thanks for all the input. I will try all the suggestions when it's back on the road next spring.

Dave Pearn

This thread was discussed between 13/11/2010 and 16/11/2010

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