I broke my fuel gauge!

[Fingolfin]

I have a knack for fiddling with things I maybe shouldn't fiddle with...

Mog's 1275 upgrade project requires a different speedometer due to the changed diff ratio. The correct speedometer is black, but I didn't really want to change the sender and use the voltage stabilizer, so I got it into my head that I could just change the faceplate on my current bronze fuel gauge for the black one, and all would be well (and it would be aesthetically correct).

Changing the faceplate went fine, but when I connected everything back up, my fuel gauge (which had worked prior to taking it out of the dash) now didn't read! This is the type with the twitchy instant-response needle, and there was no needle twitch.

I did a bit of investigating. Grounding the sender terminal on the gauge and turning the key on got the needle to twitch and jump to about 1/4 tank - but I've read that grounding that terminal should cause the gauge to read full.

Any idea what I did to the gauge? Is there a way to repair it, or should I ask in "wanted" for a replacement unit - or bite the bullet and use the full black-gauge system (voltage stabilizer, changed sender, etc)?

[Fingolfin]

This is actually the second one of these bronze gauges I've gone through - the first one failed mysteriously without my intervention. They seem to be a bit fragile.

[IslipMinor]

The gauge in the speedo is not particularly fragile, it is actually a voltmeter that measures the voltage difference between the battery/fuse box and the terminal on the fuel tank sender. The sender is very fragile - if you connect 12v battery voltage to it for even an instant, it will destroy the variable resistor windings immediately.

When the tank is full the sender resistance is low, the voltage difference between the battery and the terminal is high, so a 'high' or 'full' reading on the gauge. The opposite happens when empty, the sender resistance is higher, the voltage difference is lower, so a 'low' or 'empty' reading on the gauge. And all points in between!

Your test of the gauge is correct, connecting the 'sender' terminal (with the green/black wire connected to it) of the gauge to earth or body should give a 'full' reading on the gauge. No damage can be caused to the gauge doing this, but be very careful when working on the back of the gauge to avoid an accidental flash of battery voltage directly to the sender terminal, with the sender connected.

Do you have a voltmeter? If so, measure the voltage between the other terminal on the gauge, the one with the green wires attached to it, and earth - do you get battery voltage ~12v with the ignition turned on? If you do are you getting a good 'earth' connection when checking the gauge?

[Budgie]

A similar thing happened to me when I had the ignition on and I had the speedo out while "fiddling" with some bulbs at the back. My cars negative earth and I lost my indicators and fuel gauge went down to nothing while the speedo was resting on the bodywork. I put everything back, replaced a fuse in the engine bay. That was quite a few years ago now and to this day, my gauge will only show 50% of what is actually in the tank at any one given time !!! god knows what I've done / damaged but I'd love to rectify it and get a proper fuel gauge working again.

[Trickydicky]

As you like a bit of a fiddle Fin here is how to make a solid state sender unit, http://www.britishv8.org/articles/mgb-v ... ilizer.htm

[Fingolfin]

Thanks, Tricky, but I'm trying to avoid using a voltage stabilizer to begin with.

I'll break out the multimeter tonight and check that voltage across terminals. For what it's worth, I already sort of did that with a test light - the live terminal gave a bright light and the to-sender terminal gave a dim light.

But since I've already disconnected the sender and shorted the gauge to ground, and only got 1/4 tank, doesn't that mean the gauge is bad (even if the sender is too)?