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MG MGB Technical - Tach stopped working

I store my 80 MGB for the winter. Yesterday we had really warm weather so I started it just to play around. I noticed that the tach was not registering. What could cause this to not work just from sitting. Worked fine when I put it away for the winter.
TAB Brown

Tap the face. I have had some in which the needle stuck to the little post. There is a coating on that post and it leads to sticking to the needle when the car is not used for an extended time.
Sandy Sanders

I tried that while it was running and it bounced but then went back to zero.
TAB Brown

Check the power terminal. the spade lug could be loose or (in a case I had with the same thing in our MGB) the rivet that holds the spade lug to the printed circuit board could have a fractured solder joint. The touch of a hot soldering iron cured the problem for us. Cheers - Dave
David DuBois

With the engine running check for 12v (green) and ground (black) power feed to the tach, and for 12v on the black/white sense wire to the tach. The sense wire will actually have 12v switching on and off with the engine running, and what ignition system you have will determine exactly what signal you will have, but you should have *something* switching between 12v and 0v.

If any of those are missing the tach won't work. If all three are there the tach itself is faulty.
Paul Hunt 2

A voltage stabilizer going bad can also make a tach go crazy or not work at all, usually other instruments fail also....temp hand may float around, heater switch may not work.. etc....im not sure how wiring is routed on a 80 so depending on that different instruments may not function properly.
Bryan

The only time the voltage stabiliser itself can affect the tach is if shorts to ground and blows the fuse, ditto heater and anything else on the green (fused ignition) circuit. Any other problems with it will only affect the gauges it feeds i.e. fuel gauge (all cars) and temp and oil gauge depending on year.
Paul Hunt 2

In my 70 model when the stabiliser went out my tach would go from 0 to redline and bounce back. As I remember my heater switch was fed from the stabiliser then relayed to other guages.....but it was a few years back, my mind does run things together after a while. I had no blown fuse.
Bryan

I haven't had a chance to work on my tach problem yet but I just noticed that the fuel gauge isn't working either. I will have to see if the temp and oil gauge works.

TAB Brown

The oil gauge was only fed *from* the stabiliser for the first year of padded dash production, after that it was fed direct off the green circuit same as the tach and heater as the oil sender contains its own stabiliser. The electric fuel and temp gauges continued to be fed *from* the stabiliser. The heater and tach have never been fed *from* the stabiliser, they just use a common fused ignition supply. If that supply is broken somewhere, then depending where, any number of fused ignition circuits could be faulty, and that can help with diagnosis. On an 80 the heater is at the end of a run, the tach before that. It then goes back to a multi-way plug to be shared by the turn signal feed. The other side of the multi-way plug has a wire to the stabiliser which is the end of a run, and another that daisy-chains to the wiper switch connector then through two 4-way bullet connectors before getting back to the fusebox.
Paul Hunt 2

This thread was discussed between 07/01/2008 and 15/01/2008

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