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MG MGB Technical - Electrical Mystery

A nutty little problem for everyone....


When I turn the ignition to on, i.e. fuel pump runs etc... the Rev counter needle jumps up to 1000RPM. When the car is running the reading is 500-1000rpm above what it should be.

I've checked what I can at the dash end, but where are the connections at the other end? How does the Rev counter work?

I'm not sure but I think this may be related to other electrical problems. The main one being that the starter motor sometimes (and this is a really erratic problem that is impossible to recreate) doesn't turn on the firt few turns of the switch. The battery is fine, I think the alternator is okay.

Any help is much appreciated.

Actually, despite this minor issue, I took the car on a run through the Yorkshire Dales and up to the Lakes. 200mile round trip in one day, and the car ran beautifully. Classic countryside for a classic car.

Cheers All, see you at Silverstone on Saturday

Darren

80GT
D Paine

Hi Darren.

Your tacho is essentially a pulse counting meter, and counts the number of times the points open per minute.

It sounds to me as if the tacho head has an internal fault, probably an electronic prob rather than a mechanical one.

Regarding the starter motor prob, can you hear a clunk as the motor's pre-engage solenoid pulls in when you turn the key ?.

Don
Don

The 80 tacho counts voltage pulses from the coil -ve as Don says, if disconnecting this wire still allows it to jump up when turning the ignition then the problem is inside the tach. If not then problems at the points or in the coil wiring could possibly be causing it.

And if you can't hear the clunk from the starter solenoid when turning the key to startyou should be able to hear a click from the starter relay which is up near the fusebox. Depending on which of those do and don't make a noise will narrow down the problem area.
Paul Hunt

Regarding the starter: dis-connect the cables, clean the ends and reconnect with vaseline or other corrosion inhibiting goop/spray. Also disconnect the solenoid leads, clean and use the same goop when reconnecting. I predict more certain starter function. While you are down there check to see that the mounting bolts are tight.
Agree with the others on the tach but think that a possible current back-feed from improper/corroded grounds can cause the problem too. Also, both issues could be result of internal fault in ignition switch.

Mike!
mike!

Dear All

Thanks for the feedback. A slight update. I should have mentioned that the starter is a two month old re-conditioned item from MGBeeHive, so should be okay, but I'll get down and clean the leads, as they needed a good polish when I did the initial replacement.

To the Tacho, it has now started jumping around all over the place, even when stood at tickover. I'll let you know more when I get under the bonnet.

Regards

Darren

80GT + mid 70's engine (18GG I think, bought for £30 just to see how things worked, pistons still turn even though not run for 10 years apparently)
D Paine

What time are you going down on saturday Darren ?

There will be two BGT's going down on saturday, and will hit the M1 some time before lunch, could meet up on way down?.


Mal.
malcolm watson

Malcolm

I'd loved to have taken the B down in convoy, but family committments mean I'll be coming up from Herts in the family saloon.

Where in W Yorks are you? I'm in Hebden Bridge and there is usually a classic car weekend in August.

Cheers

Darren
D Paine

This thread was discussed between 17/07/2004 and 20/07/2004

MG MGB Technical index

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