This is a 67BGT with the mechanical oil pressure/water temperature gauge
in one unit. So there should be no voltage stabilizer in the circuit.
However, I am all too familiar with the problem you describe. The
voltage stabilizer was not properly grounded on my 72B and the coolant
gauge quickly went into the overheat zone but the coolant was barely
steaming when I tested with the cap off. One of those easy fixes.
From: Gosling, Richard [mailto:firstname.lastname@example.org]
Sent: Wednesday, January 31, 2007 1:55 AM
To: Councill, David; email@example.com
Subject: RE: Todays MG heat
My guess - your voltage stabiliser is in it's dying throes. Did you
notice whether or not the fuel gauge reading also changes when you get
your apparent temperature changes? If it does, it's definitely the
This is a (cheap and easy to replace) little box that takes the rather
variable (11.5-14V) voltage at the battery and gives a consistent 10V
output, so that your gauge readings don't fluctuate with the battery
voltage. Looks a bit like a flasher relay, and hangs around somwhere
behind the dash (sorry, I forget exactly where). The casing also needs
to be grounded to work properly, so the problem could simply be that
it's not grounded. When these fail (or they lose their ground) they
generally output the full battery voltage, thus giving a high
temperature (and fuel level) reading.
N.B. If you want to test your stabiliser, it's no good using a digital
multimeter. The device works by alternating between full battery
voltage and zero, to give a 10V average. Digital meters will give the
instantaneous value; an analogue meter (with a needle on a dial) reacts
more slowly, so will show the average value (with a bit of twitching).
Richard & Sammy ('73 Black Tulip BGT)