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MG MGB Technical - Exitable tachometer

In recent weeks I have noticed the tachometer on my '72 BGT has started to behave a little excitedly at times. Although the instrument generally works well, it occasionally will 'blip' with the needle flashing around the dial before returning to normal. This does not have any impact on the running of the car and even happens when the engine is off but the ignition is on.

This problem first started a few weeks back and although it does seem to have got a little worse it doesn't cause any problems apart from being a little annoying. Is this likely to be a symptom of the gauge beginning to fail or should I look elsewhere for the likely cause?

Oh, the car is fitted with a pertronix Ignitor electronic ignition unit and a Lucas gold Sports Coil (DLB-109) - I don't know if this might be significant. The car runs sweet-as-a-nut otherwise and never misses a beat or misfires.

Grateful for any thoughts or opinions on this.
Mike BGT

Hi Mike- I've a 71 split tail does the same, but quite exactly at 3000 rpm, no matter which gear. The tach needle steadily swings up to 3, then starts flipping back and forth, till asmooth out around 4500. I've checked and rechecked everything many times over the years. Changed out different tachs, tried points and Pertronix, verified the volt reg is working ( weired device I say, with readouts from 0 to 13 volts over a few seconds), the tach is 2x grounded, all for not. I've had a few threads here, and much advice, but no solution. Now I live with it. The car is perfect otherwise! Are you saying with the igny on and motor off, the tach needle blips around? Way weird senor! Vic
vem myers

Thanks for the reply Vic.

Yup, even without the engine running, the needle will "blip". It's as if the gauge is getting a brief surge of current at random intervals.

I still have to do some more investigation on the dashboard wiring, a PO has left it in a bit of a mess. Lucas wiring is great isn't it?
Mike BGT

First thing is that the voltage stabiliser feeds the fuel and electric temp (where provided) gauges but is nothing to do with the tach.

If you get the blips with the engine stopped with the ignition on then either there is a bad connection in the 12v (green) and ground(black) connections to the tach (although in my experience interruptions here only cause the needle to flick a few hundred rpm) or more likely the problem is inside the tach electronics.

Pertronix (hawk spit) and its ilk can cause problems with the current-pulse tachs used up to the 72 model year but that is more a general overeading, the voltage pulse types used after that (73 on) were less susceptible. I suppose the Pertronix *could* be firing randomly, but that would almost certainly affect running. If the tach still does this with the wires removed from the LT terminals of the coil then it won't be the Pertronix.

Lucas wiring is fine, it is 30 years and what numerous POs have done that is the problem.
Paul Hunt 2

Have a pertronixs fitted as well. Car run like a top but does exactly as described above. I figured it was a tired tach, but maybe it is the pertronixs unit. Wiring my car is original and no sign of DPO.

Shareef Hassan

This thread was discussed between 09/10/2005 and 12/10/2005

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