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MG MGB Technical - Rev Counter

Hopefully a quick question.

When I bought my '71 BGT, the rev counter didn't work, there was a note from early '90's saying that PO had smelt a burning smell, so had got the garage to disconnect it.

Anyway, being curious, I checked wiring, no burns or melted bits evident on the loom, so I reconnected the rev counter.

It worked fine at idle when I started the car up and after 10 mins, no smells or signs of burning.

I thought this was too good to be true! When I drove the car on the road, the rev counter was fine....until I indicated or braked, then the needle went mad, swinging round the counter at random until I took my foot off the brakes or turned of indicators.

I'm guessing there is a voltage stabiliser somewhere which may stop this surge of voltage which I'm guessing is causing the problem?

I hope it's simple to sort out..by the way, I've disconnected the rev counter again and am just going on engine noise to change up.

Cheers
John
J M Collins

Sounds like a ground fault from a pair of wires "fused" from heat. The tachometer shares no wires with any other circuit in the car other than the small red/white one attached to the light.
OK. That is, other than the ignition circuit.
The tach senses pulses from the "-" side of the coil in some applications and in others (for a fact pre-'68) the positive feed to the coil makes an inductive loop at the back of the housing.
I suspect you have the former otherwise the car wouldn't run!

So, check all ground connections and fuses. Then check the entire run of wire from the ignition switch to the starter solenoid, coil, and back to tach. A test light will help you find which run has the cross-connection.

Mike!
mike!

John - Does the tach go crazy when the indicators are turned on for one particular direction? If so go the the tail light/brake light/rear indicator assembly on that side and check the wires going into it. Chances are that you will find that a couple of the wires have been bent over and mashed into the back of the sockets, causing a short to ground. When this happens, the 12 volts is run directly to ground whenever the brakes are appliedor the indicator for that side is actuated. I had this happen in our 66 mgb roadster, from packing to many things in the trunk for a trip. I finally made up some wire protectors for the sockets and have eliminated the problem. Good luck - Dave
David DuBois

This thread was discussed on 22/06/2004

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