Suspect HC instrument voltage stabiliser.

[hitch007]

My 1975 hc viva has just recently been acting up. At first I thought it has some serious overheating issues but after replacing the rad cap, thermostat and temp sensor and rad flush, all to no avail.
What it does is when the car is started, the gauge rises normally but instead of ending up in the middle of the range, it keeps going to the very top of the 'normal' range. It stays there for a few minutes then comes bac to just over half then surges back to the top of the normal range. Up and down it goes. With the heater on full hot it stays lower as you would expect but as soon as it is turned off, it races back up again. Very worryingly when you are thinking of the repairs needed!
After trolling through the forum, it has been mentioned a few times about the voltage stabiliser on the rear of the instrument panel being faulty. I pulled mine and fuel and temp gauges go to zero, gave it a wiggle but no change.

Now.....has anyone experienced this?? Can anyone offer a confirmation about the suspect v-stabiliser?
Even better would be place source a new item

Any and all advice appreciated in advance.

Cheers
All
Gareth

[1972nail]

The voltage regulator/stabiliser is a very crude, bimetal device and is known to fail over time, gradually getting less accurate so you don't notice it until it's really bad. You can modify them to have modern 'solid state' internals, something I'm currently planning to do to mine. The symptoms you describe are typical of the unit in advanced failure.

But, your fuel gauge should show similar symptoms but to a lesser degree. If your fuel gauge is stable then there may be problems with a short in the wiring, the sender unit or the gauge itself.

[1972nail]

http://www.britishv8.org/articles/mgb-v ... ilizer.htm

Here's how to convert to solid state.

[thomas]

Good link and a neat idea. I'd add if using the cheap as chips 7810 or any 78xx voltage regulator that a small ceramic capacitor across the supply side to earth, close to the 78xx of around 100nF (104) plus a small electrolytic of about 47uF or bigger across the output, (both rated at 25V or higher) is probably needed to stabilise the stabiliser. I think on the HB which has no voltage stabiliser, the instruments are much more sophisticated, the HC cruder, and balanced that any change in supply voltage affects electrically both sides of the balance and doesn't affect the reading.

[hitch007]

Thanks for the tips fellas, yes the fuel guage is playing up. When I fill the fuel tank the needle goes to Full but droops down again shortly after. It makes sure I won't run out gas!!
Thanks for the fix link too.

Regards
Gareth