Electronic Voltage Stabiliser

[Chris-72sprint]

Been fixing recommissioning snags over last few weekends

My fuel gauge wasn’t working !
First problem was it wasn’t grounded - easy fix.
The next problem took me several hours to resolve.
The car has an electronic voltage stabiliser - two 12v spades on one side and 2 10v spades on the other side. I connected it to ground by fixing it to a black ground cable.
Using my multimeter there was only 2 volts coming out the 10v tag even after leaving car with ignition on for several minutes.
If I put a 12v feed to the gauge the needle moved albeit higher than fuel in the tank.
After trying to improve the grounding without success I finally gave up suspecting the unit faulty.
Then when I removed the ground connection to remove the unit everything worked as it should
Can anyone explain why the stabiliser only works when using positive green power in and lead out to tachometer on the 12v side and 10v out on the other side to fuel gauge.
The only other thing to mention is my RVI tachometer is reading low - won’t go past 5000 rpm but they may not be connected
Thanks in advance
Chris

[elanner]

From your description it's not totally clear whether you have an original mechanical stabilizer or a modern electronic stabilizer.

Anyway, an original stabilizer uses current flow to create an average voltage. The green 12V simply goes into the stabilizer and out the other side, still 12v, to the fuel gauge. The current flows through a bimetallic strip inside the regulator, which warms up and finally goes open circuit, dropping the output voltage to 0v. The bimetallic strip is fiendishly designed to be open circuit about 16% of the time so that the *average* voltage of the output is 10v. Meanwhile the fuel gauge uses a bimetallic strip to move the needle so the switching on and off of its input voltage is not noticeable.

Measuring the output of one of these stabilizers is mostly pointless because you know what voltage it's delivering - 12v and 0v. The question is for how long?

Electronic stabilizers work in a rational way, providing a stable, measurable 10v. They need to be grounded. Original stabilizers happen to be grounded because they're screwed to the tacho, but that's by-the-by.

Nick

(Heck, I hope this is right - it's been a while....)

[Chris-72sprint]

Nick - thanks for your reply
I think I have found the problem

I am using a new electronic voltage stabiliser.

When the unit is grounded it outputs 2 to 4 volts.

When it’s not grounded it is passing over the same voltage as the input supply side - therefore I deduce unit is faulty

New one ordered

Thanks again

Chris

[billwill]

The tacho has its own electronic stabalizer inside the can, therefore it does not need to use the 10v regulated output. Indeed it it was the old style bi-metallic regulator the 12v pulse waveform would completely confuse the tacho.

With a modern electronic voltage stabalizer if the output current is zero or near zero milli-amps, it is likely to have very strange values of output voltage. It is after all just a big transistor working on the flat part of its output curve. It needs some output current to stabalize.

So there may be nothing wrong with yours..

[Andy8421]

With a modern electronic voltage stabalizer if the output current is zero or near zero milli-amps, it is likely to have very strange values of output voltage. It is after all just a big transistor working on the flat part of its output curve. It needs some output current to stabalize.

While in general Bill is correct, the electronic voltage stabilisers available in the market are all based on the LM7810 series of 3 terminal voltage regulators, a widely used and venerable design dating back to 1972. I am not sure I would call it modern - it is almost period correct for the Elan. The datasheet does specify a minimum 5mA load for the device to operate within spec, but even under no-load conditions the internal circuitry provides sufficient load for the output transistor to be happy, and the voltage should be close to the rated voltage.

If you are measuring 2 to 4 volts, something is adrift somewhere. There are two versions of the regulator, one for positive earth, one for negative. The old mechanical stabiliser didn't care, but the electronics do. The usual suppliers sell both versions. Make sure you have the right version for your car.

Good luck.

[Chris-72sprint]

Just some feedback on my problem which is now resolved
The fuel gauge is working again and reading correctly (1/2 tank reads as 1/2 tank )

The old (electronic) stabiliser was faulty - when it was earthed there was only a couple volts output, rather than 10v When it wasn't earthed it was passing across the full 12 volts across so the gauge moved and looked ok.

I replaced it a smiths BR1307 voltage stabiliser for Triumph, MG , Land Rover etc (12v in, 10v out), earthed it and all works perfectly

Thanks for help on this
Chris