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I have done multiple searches (here and google) and the general answer seems to be a bad ground, but how do I find it.

I have a fuel sender unit that gives 0-90ohms, and it works.
The signal cable goes to the gauge - checked continuity
The ground cable goes to ground - checked continuity
On the Miata instrument cluster the ground goes to ground - checked continuity and the other gauges work
the signal cable goes to the sender - as above
the power line is a common line that works on all of the other gauges.

my fuel gauge reads full - ign on or off.

if I disconnect the ground at the sender the gauge drops to empty.

so my searches suggest a bad ground, but how can it be? Any of you electrical gurus got any suggestions?

thanks very much
Adam

Hi Adam,
I have no idea what differences a Miata fuel gauge may have, however, a fuel gauge doesn't necessarily need a hard ground as such. The sender unit provides the ground and the ignition B+ supplies the power. The resistance changing with the fuel level varies the ground to the gauge.

Generally, if you provide 12 volts to the power side and ground the sender side of the gauge it will read full. Remove the ground and it will read empty.

If there is a ground at the gauge, it's likely more for the light than the gauge. If this doesn't help, post a pic of the back of the gauge so the connections are visible.

Geoff

This sounds like a mis-wire to me rather than a "bad" ground. With your ignition off, you should have no current through the meter circuit. Since the gauge changes when you pull the ground at the sender, you DO have current in the circuit. Time to check the wiring diagram and buzz out the wires.

By the way, is the sender a Miata sender or an after-market unit? How about the wiring harness - Miata or after-market (or custom?) If you have a sender/pump (in tank) combo unit, does the pump run OK?

You do know that there are at least four and maybe many more completely different gauge/sender circuit designs used in the most popular cars and after-market solutions, right?

John