Fuel gauge

[Lesliehedley]

I wonder if any the electronic chaps on here can help. I have fitted a gauge wizard from spiyda to control my fuel gauge. I am sick of the inaccuracy with the standard gauge. I have set everything up as per the instructions and it seems ok except that setting the gauge needle positions was a little odd. For instance, if I program the position of the needle for empty, it seems fine until I switch the ignition off and then back on. The needle moves to a slightly different position just lower than it was set to. I could probably live with that as it would still be more accurate than what I have currently. But this is the problem. I currently have less than 5 litres of fuel in the tank and the gauge shows empty with the ignition on. But when I start the engine, the gauge jumps to just under half a tank. I have checked all the resistances and gauge needle settings and it all looks ok. Can anyone shed any light as to what might be happening or what I can check to correct this?
Thanks

[Polly Grigora]

First test is to check what the battery voltage is with everything switched off and check again with engine running at approximately 3000 RPM

Once you've proved that the alternator is charging the battery but not over-charging (should be 14 to 14.5 volts), it might be worthwhile setting up the wizard with the engine running and battery being charged

Does the wizard have a built-in voltage regulator, I don't know

The only other cause I can think of is a faulty fuel gauge

[Lesliehedley]

I've tried two different fuel gauges and they both do the same. Could this be due to there being no voltage stabiliser for the fuel gauge. The gauge wizard people have suggested that without a voltage stabiliser, the engine running is causing a higher voltage on the fuel gauge. Does that make sense?

[Polly Grigora]

Yes it makes sense

The problem will be in finding out what voltage gauge is fitted, Wedge wiring diagrams don't show a voltage stabiliser (not the ones I know of anyway)

If there is a 10 volt gauge fitted and it is being supplied a 40% higher voltage when the alternator is charging there is going to be every possibility that it will read 40% higher than it should be doing

[Englishman]

Interesting. I fitted a gauge wizard but don't have your problem, i.e. it reads the same with ignition on and running as well as not running. What model and age of car do you have?

[Lesliehedley]

Englishman. It's a 1990 TVR 400SE.
Polly Grigora. Thanks for your help. I've ordered a voltage stabiliser that's adjustable from 8v to 11.4v. Stewart Warner gauges operate from 11v to 16v apparently, so a 10v stabiliser might not work. The 11.4v one should hopefully work.

[Englishman]

Lesliehedley said:

Englishman. It's a 1990 TVR 400SE.

Only difference I am aware of is the make of the fuel gauge, mine is VDO.

[Polly Grigora]

Lesliehedley said:

Polly Grigora. Thanks for your help

You're welcome

Lesliehedley said:

I've ordered a voltage stabiliser that's adjustable from 8v to 11.4v

Once the stabiliser is wired into the circuit you will have full control of the voltages across the gauge with the wizard in the negative side and stabiliser in the positive side

Surely success is imminent

[Lesliehedley]

Well, I seem to have solved this problem at last. I now have an adjustable voltage stabiliser fitted and it seems to be working. I had one false start when I adjusted the voltage stabiliser to give 12 volts and this still gave the same problem. In my ignorance I thought that as long as the voltage was stable, the variable resistance would change the current through the gauge and all would be well. But I was wrong. It has to be 10 volts or the gauge still doesn't match the reading with the engine not running. You live and learn.

[Polly Grigora]

Lesliehedley said:

Well, I seem to have solved this problem at last. I now have an adjustable voltage stabiliser fitted and it seems to be working. I had one false start when I adjusted the voltage stabiliser to give 12 volts and this still gave the same problem. In my ignorance I thought that as long as the voltage was stable, the variable resistance would change the current through the gauge and all would be well. But I was wrong. It has to be 10 volts or the gauge still doesn't match the reading with the engine not running. You live and learn.

Nice fix

[KKson]

I might try that myself. My gauge is all over the place.