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MG MGB Technical - Tach Question?

My tach on my 78 MGB bounces to a higher rpm (about 500 rpm) intermitently when I have the head lights on.
With lights off it hold steady?
Any ideas?
Jimmy Chew

If it does it with the lightswitch in the first position as well as when the headlights are on it is probably something to do with the ground - or lack of it - on the tach body. The tach electronics and internal illumination share this ground. All the gauges should have a black wire, usually secured under one of the U-clamp knurled wheels.
Paul Hunt

I have a 1977 mgb, and I think I need an excorsist rather than a mechanic to fix my tach. When starting the car it doesn't come on, but it will work after about 5 minutes of running. Even if I only shut the car off for a minute, the tach won't work when i start the car up. Any ideas?
Kate Lawrence

I also have a 77b and the tach doesn't work until you either rev the engine or let it run for a while. Haven't figured out why yet. I'd be interested if anyone has any ideas as to why.
Craig Smith

My tacho was playing up in different ways & paul gave me a lot of advice; as he says problems with the tach is nearly always a poor ground. When I looked at my tacho, a 1972 internal loop types, there was no ground wire.

Apparently the ground on some tachos, such as mine was through the body of the tacho & down the frame supports or braces where the knurled nut screws down onto and then to the back of the dash!

I then attached a small length of wire from under one of the knurled nuts to a known good ground (I used the ground for the wiper moter quite close by on right hand drive cars) - you can use any. Hey presto - tacho working fine again.

Therefore, whatever your problem is & before buying a replacement try this. It only takes a few munites.

Iwan
Iwan Jones

A lazy tach is often a stuck needle. When revving the engine give the tach a good knock with a knuckle. If it jumps up ...
Paul Hunt

I just thought I'd add something here.

If you have a dead tach, or a fruity one, and don't wish to spend alot of money getting a new one, I -can- repair and recalibrate tachometers. I can get you within +/- 50rpm at 5500 RPM. (And also fix bizzare issues!) I will only charge a nominal amount for my time to do so! ($35cdn for repairing/calibrating and a cleaning a partially working tach. A totally rusted tach may or may not be fixable)

If you want to know more, just drop me a line at my e-mail.





Gord Bestwick

This thread was discussed between 10/01/2002 and 13/01/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.