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MG MGB Technical - Tach Pegs--even w/ignition off

My 71 B's tach tends to jump intermitently, even with the engine not running. Also, whenever I turn on any electrical device, lights, signal, horn, brakes, the radio cuts out until the device is turned off. Could the voltage stabelizer be causing this? All other gauges work perfectly.
Vernon

Hi Vernon.

The tachometer is powered from straight 12V, not via the regulator.
It is just possible (but unlikely) that the regulator could be causing problems by producing interference energy due to arcing contacts, temporarily disconnecting it would prove or disprove this.

I would start by looking for poor connections around the fuse block.

HTH.. Don
Don

Vernon,

Has this problem just started or has the tach always pegged and the radio cut-out? If this is a recent developement I would suspect a mistake in re-hooking the wiring. At any rate it sounds like the green and white accessory circuit is crossed with the tach circuit.

Doug
D. Cook

If the tach moves with the ignition off then it sounds like it is either powered from the accessories position of the key (white/green for the unfused side or green/black for the fused), but if it does it with the key out of the lock it must be powered from a purple or brown circuit.

The horn is powered from the purple (fused always hot) circuit which is the bottom fuse in the fusebox and the headlights from the brown (unfused always hot). The parking lights have their own fuses which are the top two in the fusebox. The turn signals and brakes are powered from the green circuit which is the 2nd fuse in in the fusebox. The only common point with all these is the brown at the solenoid, but if there were problems in that you would have problems with all the aforementioned circuits, particularly the turn signals. I'd look to the radio 12v feed and grounding first, measuring the voltage at the radio before and after it cuts out when anything is turned on.
Paul Hunt

This thread was discussed between 06/05/2004 and 08/05/2004

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