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MG MGB Technical - Tach and Fuel gauge electrical issues

I’ve noticed that my Tach and Fuel gauge intermittently quit working. Seems to be occurring more often now. Any suggestion as to where to start looking for the problem?
James Barloon

James,
The voltage stabilizer is probably bad. It controls the tach, fuel gauge and temp. gauge (depending on year of car).

The part is a bit tough to replace, as it's way up on the inner firewall behind the dash.

Hope that helps! Dave
David Steverson

The voltage stabilizer supplies the water temp gauge and the fuel gauge, not the tach. You may have a loose ground connection at the gauges. There is a black ground wire that is connected to each gauge at the gauge mounting stud, under the thumb nuts.
John H

The grounds as the fuel and temp gauges don't power the gauges, only the night-time illumination.

It could be either the 12v supply to the stabiliser or the connection from the stabiliser to the first gauge, so don't rush out and buy a new stabiliser before proving it is that and not a simple bad connection.
Paul Hunt

Paul, I believe you are correct on the temp and fuel gauge but I do believe the tach requires a ground for proper operation.
John H

On my cars the 12v lead which powers the voltage stabilizer continues on to power the tach. If the voltage stabilizer is dead this will effect the gas gauge but not the tach which takes the full 12v. However if this 12v lead is loose or faulty before to the stablilizer this would effect both the gas gauge and the tach.
Hope this helps.
Cliff
Cliff Maddox

John H,
On my '77 B the fuel and temp gauges as well as the tach were not working. I replaced the voltage stabilizer and all 3 came to life. Was that a coincidence? or might the stabilizer have an affect on the tach with my year car?
Dave
David Steverson

Does it matter what year car James has? Aren't there at least three different types of electric tachs used over the years?

Charley
C R Huff

John H - yes the tach does need a ground for correct operation, but as pointed out by someone else the tach isn't powered from the stabiliser but direct from the fused ignition supply.

When leads are *daisy-chained* from component to component (the stabiliser and tach is a case in point) the two wires will be inside one spade connector, spot-welded, so it is highly unlikely one of these wires will be loose. If the connector is loose on a spade terminal it will only affect that component, not others further down the chain. However where the green circuit splits off in a tree and branch arrangement using bullet connectors (as it does before it gets to the stabilser), if the 'in' bullet is loose in the connector it will affect all branches from that point, but if it is one of the 'out' bullets that is loose it will only affect the components that branch feeds.

It's quite possible David Steverson joggled one of these connections when replacing the stabiliser, the stabiliser *definitely* doesn't power the tach on any year.

There *were* three different tachs over the years (two current pulse and one voltage pulse) but they were all powered with full 12v (the white unfused ignition circuit from 64 to 67, the green fused ignition circuit after that), none from the stabiliser.

And finally the *stabiliser* needs a good ground connection, which it gets from its mounting tab and screw, but if this fails the gauges read *high*.
Paul Hunt

Sorry I was not logged in when I posted this question.

My B is a 76. When the Tach and Fuel quit working everything else still does. Oil pressure, Speedo, & Water temp.

To recap where should I begin to start to get a line on the problem? It only seems to happen when driving. Once I get to a stop, I'll cycle the ignition a couple of times and they come back to life.
J Barloon

The coolant temp is fed the same as the fuel gauge, so they should both be affected equally. If the coolant gauge *isn't* dropping but the fuel is, *and* the tach is, then you have two faults (seems unlikely both wouyld come and go at the same time) or some non-standard wiring which is going to contemplate matters.

If the coolant and fuel gauges and tach are all affected together then you need to be looking at the 12v feed (green) to the stabiliser and tach, not the stabiliser itself, which would only affect the ancillary gauges and not the tach.

On a US 76 there is quite a complex arrangement where a green wire comes off the fusebox and goes to a 6-way bullet connector that feeds lots of things. One wire comes off that and feeds a 4-way bullet connector. One wire off that feeds the turn signals, and another wire daisy-chains to the column stalk for the wipers, a multi-way plug that feeds the tach, the voltage stabiliser that feeds the ancilliary gauges, and finally via another multi-way plug that feeds the heater fan switch.

If switching the ignition on and off a few times cures the problem, then it is almost certainly a coincidence as any problems with the ignition switch or its immediate wiring would affect the engine as well.

So you need to determine which of the following things are affected when the gauges quit - heater fan, wipers, turn signals and exactly which gauges are affected - and that will whittle it down to hopefully one or two connections.

Paul Hunt

This thread was discussed between 02/06/2008 and 06/06/2008

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