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MG MGB Technical - Voltage Stabilizer

The gauges on my 1968 MGB have incorrect readings; the Tach, Speedo, Fuel, and Temp. The Tach reads way high, in the orange and red at 45 mph and higher, full tank of gas reads 1/4, speedo about 10-15 mph high, and temp reads very low. Before tackling each instrument, could it be a faulty Voltage Stabilizer? I believe some or all of the gauges are current dependent.
Thanks for advice.
Eddie
Eddie Haynes

The only thing running off your voltage stabiliser is the fuel gauge. The other instruments don't use it. Regarding the tachometer, does your car have an aftermarket electronic ignition system fitted in place of the contact breaker? These can sometimes cause problems with the tacho.

The fuel gauge can be calibrated - see Paul Hunt's excellent site http://www.mgb-stuff.org.uk/electricstext1.htm#adjust, although whether it can be calibrated that much I don't know. It's worth a try.

The speedo - on an old car that has probably been in the hands of many owners there are a number of possibilities.
(A) Is it the correct instrument? There is a code number on the face which is actually the Turns per Mile (TPM) of the cable and is in the region of 1000.
(B) It makes a difference if you have overdrive or not - has your car been changed from non-O/D to overdrive?
(C) And over the years there were some differences in the teeth on the drive gear. It's quite possible your car has had the incorrect replacement instrument fitted, or conversely has had a rebuilt gearbox with a different drive gear. This web site helps explain it all http://www.mgexp.com/article/speedometers.html

On a 1968 car the temp gauge should be mechanical, in which case you can do nothing except replace it. Is there a long coiled spring covered tube coming from the engine right into the back of the gauge? If so it is the mechanical type.
Mike Howlett

Thanks so much for the detailed info. I will approach each piece of advice you have given me. I am the 2nd owner and I am in touch with the local original owner.
The car does have an aftermarket electronic ignition; it is non-overdrive. The speedo was replaced or rebuilt at one time. There has been no previous work on gearbox.
Thanks again. I will take your info and tackle each gauge.
Eddie
Eddie Haynes

A 68 i.e. Mk2 with the padded dash North American spec does have electric temp and oil gauges, which like the fuel gauge are fed from the instrument stabiliser. The tach was never fed from a stabiliser. The oil gauge was in fact an error, as the sender contains its own stabilisation, and during 68 the oil gauge was fed directly from the green circuit the same as the tach (in 1972 it reverted to a mechanical gauge albeit still separate from the temp gauge).

So if the oil gauge is similarly low then the ancilliary gauges almost certainly have a problem with the stabiliser, but even if the oil gauge is OK it could still be the stabiliser if your oil gauge has the modified wiring.

The original 'stabiliser' is rather amusing in that it actually switches 12v on and off about once per second averaging about 10v - although when you first turn on the ignition it outputs 12v for a few seconds until it starts pulsing on and off. So with a good tank of petrol, put a voltmeter onto the green/blue wire going to the temp gauge sender in the head - removed from the sender if the engine is hot, turn on the ignition, and see what you get. Because the gauges are relatively low resistance this will allow you to 'see' what the voltage is on the light-green/green from the sender, instead of having to get to the backs of the gauges which I understand isn't easy.

The stabiliser, which has green wires in the B terminal and a single light-green/green on the I terminal, with a threaded stud sticking out of insulated surface by the terminals, might be high up behind the dash on the right-hand side. These are occasionally replaced by solid-state devices when the originals fail, and these should give a constant 10v, right from ignition switch-on.
Paul Hunt

Thanks for the technical tips.
Eddie Haynes

Worth checking the float in the fuel sender hasn't sprung a leak. I went through 2 aftermarket senders with plastic floats before I finally replaced the float with a piece of cork. Fuel gauge has been perfect ever since.
D O'Brien

I don't believe that the oil gauge is attached to the stabilizer, it has its own electronics/regulator in the unit that screws into the oil gallery. Cheers - Dave
D W DuBois

From information I have a North American 68 i.e. early Mk2 did have the oil gauge connected to the voltage stabiliser, but this was an error for the reasons Dave says. It was altered to be powered direct off the green circuit, same as the tach.
Paul Hunt

FWIW, I just replaced the plastic float in my B because it was *completely* full of fuel. It was probably no more than 2-3 years old.

Fortunately I had a brass float on hand because I ordered two of them when the new-ish float in the CGT filled up completely. Bit of a trend it seems....
Rob Edwards

This thread was discussed between 13/07/2014 and 02/08/2014

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