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MG MGB Technical - Voltage Regulator II

I am I right in assuming that a '66 MGB-GT does not have a voltage regulator? Do I need one? I have converted to negative earth and fitted an alternator. The fuel gauge reads “F” when connected to the sending unit and “E” when disconnected from the sending unit. I suspect that the sending unit is bad, but it and the fuel tank are new. Any ideas?
David Werblow

for all three, four(?) things have a look on Paul's site - www.mgb-stuff.org.uk
Nigel Atkins

Do you mean rugulator or stabiliser?

Is your new fuel gauge sender the correct type for the gauge?

As Nigel said, have a look at Paul's site.
Dave O'Neill2

Your alternator should have a built in voltage regulator. Obviously, the car didn't come from the factory with one because generators were used until the end of the '67 model year. You are most likely referring to a voltage stabilizer. It maintains 10 volts for the proper operation of the fuel gauge and tachometer. The unit is mounted behind the dashboard behind the radio. RAY
rjm RAY

Your model, according to my diagrams, had a 10v regulated/stabilized supply to the fuel gauge. But put a meter on the LGG wire to the gauge, you should see the voltage "flicker". Also this stabiliser is position sensitive, i.e., must be the right way up. The gauges themselves are adjustable and every time I have fitted a new sender unit i have had to re-calibrate to match the new unit.
Allan Reeling

As said originally your car would have had a voltage regulator/control box for the dynamo, but it depends how the conversion was done as to whether you still have one or not. The control box could have been removed altogether, or it could have been left in place and is now used solely as a convenient connection point for the wiring. Dynamo or conversion to alternator shouldn't have any effect on the fuel gauge.

Are you sure you are connecting the green/black wire to the insulated terminal on the tank sender and not the earth terminal that is spot-welded to the base of the sender? Connection to the earth terminal will cause the gauge to read F - in fact higher than F normally. Other than that did this happen immediately you fitted the new sender? Or a while afterwards? It does sound like the sender is faulty. How full is the tank? If it's a third full or less you can usually remove the sender safely by raising the right rear corner.

To be strictly accurate the factory instrument voltage stabiliser doesn't maintain a steady 10v. It switches system voltage i.e. nominally 12v but potentially (ho ho) up to 15v on and off about once per second or so such that the average voltage over time is about 10v. The gauges are thermal so they only respond slowly to the voltage being switched on and off, and effectively display a constant reading, although you can usually see them moving up and down very slightly if you look closely.
Paul Hunt

" It maintains 10 volts for the proper operation of the fuel gauge and tachometer."

Voltage stabilizer does not affect/feed the tachometer. The green supply to the Tachometer is from a junction BEFORE the stabiliser

Bernie
B Anderson

Thanks RAY and others.

I'll look for the stabilizer behind the radio blanking plate (no radio)-- really had no idea where to look for it. Once I find it, I'll put a volt-meter on it and see what I get!

P.S.

The control box is still in place as I used the instructions for converting an MGA when doing the B-GT.
As for what work or didn't work before going to the alternator--I have no idea, as the car was not running when I got it.
David Werblow

This is almost certainly nothing to do with the instrument voltage stabiliser and everything to do with the the fuel tank sender. If the stabiliser were faulty it would be putting out either zero volts or 12v. It obviously isn't putting out zero volts as the fuel gauge is registering full. And even if it were putting out 12v instead of the average 10v, that would only cause the gauge to read about a quarter tank higher than it should. Unless the tank is three-quarters full or greater, then if the gauge is reading full, but drops to zero when you remove the green/black wire from the sender as you say it does, then the problem must be in the sender.

You can check the operation of the stabiliser by checking the voltage on either of the terminals on the back of the gauge, or if they are awkward to get at then test the voltage on the green/black when it is removed from the sender. You should either see a steady 10v if an after-market stabiliser has been fitted by a PO, or if it still has the factory stabiliser then you should see 12v switching on and off about once per second. Only if you see a steady 12v, or 14v with the engine running, on the green/black is a problem with the stabiliser indicated. But as I say that will only cause the gauge to read about a quarter tank higher than it should.

If the gauge reads full with the green/black connected to the sender, but zero when the green/black is removed from the sender, then either the sender is faulty, or you have a full tank!
Paul Hunt

This thread was discussed between 31/10/2013 and 02/11/2013

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