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MG MGB Technical - instruments not reading correctly

ive recently had a problem with my temp & fuel gauges not reading correctly, too high, too low--help!!
i swapped out instruments and sending sensors, no effect.
What can cause the readings to be so flakey?
NBH Harden

Depending on the year of the car, there is a voltage regulator on the RHS of the inside of the firewall, about 1" X 1.5". If this is faulty it would give your symptoms, especially if the temp gauge is electrical.
To test measure the output with and analog volt meter, it should go between 12v and 0v a few times a second, averaging about 10V.

Herb
Herb Adler

no voltage regulator, 1974 MGB ,
showing 12v at guages,temp shows almost hot while car at Normal Operating Temp
gas shows almost full after 190 miles (should be almost empty)

if 10 volts should be at SMITHS guages from a voltage regulator in older models ,What should be the voltage at guages on a '74 B?
NBH Harden

You have a voltage reguloator on a 74B. On the LHD 74 it is on the Left side under the dash with 2 wires color coded per the Workshop Manual. Mounts with a single sheet metal screw.

Jan
Jan Hurd

confused on the voltage regulator..
You don't have a regulator in the charging circuit (big black thing under the bonnet) but you do have one for the gauges (small tin thing under the dash) It looks a bit like a flasher unit ;0)
12 volt at the gauges is way to much. As Herb says 10 volts at the gauges is what you are look for and as smooth as possible. Buy a modern semiconductor 10v regulator capable of delivering 10v at about 5amps.
MGmike
M McAndrew

*&^$^&%&^(*& pressed the wrong button and lost my post.

With semiconductor regulators, you need one with a low drop out voltage, ie the input only needs to be about 1V greater than the output. Most ubiquitous ones have a 3 V differential, which makes them dicey.

% amps is a way too much over kill. The gauges are about 200 ohms each, giving a 100 ohm load to the regulator, which translates to a current of 0.1amp. A 1 amp unit is therefore more than adequate.

Herb
Herb Adler

You *should* have an instrument voltage regulator on a 74, in fact everything from late 64 onwards. 12v at the gauges means it has either been bypassed, or the contacts have stuck closed. If sometimes it reads low or not at all and sometimes high then it will be the latter.

As Herb says an average of 10v is what the stabiliser should put out, but it doesn't need to be as smooth as possible as the gauges are thermal and slow-acting. That's why the stabilise output switching between 12v and 0v about once per second barely shows on the gauges.

You can go electronic, but it gives you a slower gauge rise time (more of an issue with fuel than temp) than the original.
PaulH Solihull

This thread was discussed between 01/02/2012 and 02/02/2012

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