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MG Midget and Sprite Technical - Odd odd problem - help!

Ok, this is an odd one, and I am unsure where to start...

When I switch on the instrument panel lights, the tachometer dies - it goes straight to zero!

The tachometer works when the lights are on but the panel light switch is off.

The car runs with not problems at all - it is a bog standard 1971 midget with no wiring issues. The strangest thing is that the tach WAS working with the panel lights on

As it happens, the tachometer panel light does not work so first thing to do is to replace it... but does anyone have any other ideas?

J.
James30

The first thing to do is check the earth connection to the back of the Tacho. Sounds like maybe the extra load placed on it when the panel lights are switched on is the straw that beaks the camel's back. Well we can only hope so! :)
Robert (Bob) Midget Turbo

There must be a wiring fault somewhere - maybe the red/white wire that feeds the bulb in the tacho is shorting onto the case. You need to get under the dash with a strong light and look carefully at the wires. Pull the panel lamp out from the back of the tacho and check it out. Does the tacho work properly with this bulb holder just dangling? Let us know what you find.

The tacho will have a green wire feeding power from the ignition circuit, two white wires which provide the signal, the red/white wire for the panel lamp bulb and the ignition warning bulb which has a white wire and a brown/yellow wire.
Mike Howlett

Yes, Bob is right, and I should have thought of that too. The metal case of the instrument must be earthed to the metal dashboard. If there's any doubt, feed another extra wire from one of the fixing screws to a good known earth.
Mike Howlett

I think Bob has it.

My guess is that the tacho earth is disconnected, so the tacho is earthing through the panel lights and side lights. When the panel lights are switched on, there is 12v on the red/white, so the tacho can no longer earth through the lights.
Dave O'Neill 2

Bob, Mike, Dave, many thanks for the pointers. Will investigate as suggested - I have my fingers crossed!
James30

When there is an "odd odd" electrical problem in a car, there is generally a bad earth at the root of it.
David "earthy" Lieb
David Lieb

My favourite used to be fried choke cables. When the engine earth used to break, the engine tried to earth throuigh the choke cable and dash.
Shawn

This thread was discussed between 31/07/2011 and 02/08/2011

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