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MG MGB Technical - Rev counter/Tachometer problem

Every time car started there elapses 20-30 seconds before revcounter functions/registers on idle but then works ok!
Any suggestions as to cause?
/Moss
Moss

What type of tacho do you have? The cars have had mechanical, current sense and I believe voltage ones later (may be wrong on that). Regardless I would clean the earth first if its electronic.
Stan Best

When I was new to MG ownership my car's tacho would sometimes work and at other times not work. I sent it away to be rebuilt and it was fine for a few days. But then it began to need a gentle tap on the glass (or driving over a bump in the road) before it started to register. So for the last 15 years I've tapped it as part of my starting routine and it's been fine. I've heard others say the same, and I recall one thread where someone (probably Paul) explained it. If yours is working consistently after a few seconds then don't worry about it.
Miles Banister

The car is a 77 roadster - believe the tacho is electronic - linked to the ignition. Seems to work fine after the intial delay every time I start the car. Not planning on doing anything with it at the moment - general interest question and thoughts that something more serious was a foot...
/Moss
Moss

I have found that the needle sticks to the pin when resting in the off position.
Take the cover off and clean the small pin. I do not know what it is coverd with but it seems to hold the needle in a stuck position.
Sandy
SANDY SANDERS

Give it a rap with a knuckle. If it jumps up then it is probably the needle sticking i.e. a mechanical problem inside the gauge, but could also be an electrical fault inside. If it still takes as long to start registering then it could be an electrical problem elsewhere. Until 1973 electronic tachs were current triggered which worked in series with the ignition so some faults at the tach would stop the engine. From 1973 on they were voltage triggered, and the wire from the coil to the tach could break but it wouldn't stop the engine (it does if it shorts to earth, but that is another story). Your problem could also be caused by loose connections in the 12v and earth supply to the tach, although being so consistent mechanical stickiness seems the most likely.
PaulH Solihull

This thread was discussed between 27/11/2010 and 28/11/2010

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