Series III fuel gauge

I assume your fuel gauge isn't working from this. Are you trying to measure the output from the fuel gauge supply regulator or the lead to the tank unit? The regulator (on the back of the speedometer) keeps the supply constant so the gauge reads the same as the battery voltage fluctuates ( e.g charging after start up) and supplies a regulated 12 volts but the voltage (and current ) to the tank unit are very small ( I don't think you really want to put 12 volts in the tank for obvious reasons) and any dirty connections etc. will consequently affect the reading of the gauge. Check if the gauge moves when earthing the tank unit lead to the chassis as the most common fault seems to be corrosion at the tank mountings and not completing the circuit.

HTH Martin

Thanks. Yes, I was reading from the lead to the tank sender, while disconnected from the sender. And the other meter lead to a good contact on the chassis. Bother, don't have an impedance figure for the meter to hand, but 4.5v certainly implies a pretty huge resistance for the gauge and wiring downstream of the regulator, dropping 7.5v -- I rather doubt that a 1960s fuel gauge would have a useful response to currents measured in microamperes.

I'll check tank-to-chassis. Since I'm getting a voltage, the fuse hasn't blown, but check the contacts there...

Yes it has. It's a wire wound coil round a bi-metallic strip which bends and moves the needle when heated by current passing through it on series 3 and the '60s one is magnetic deflection with a variable resistor in the tank unit which wears through the coil in time. If you earth the lead at the tank with the '60s one it will read empty instantly but the series 3 one will go to full slowly. You said series 3 *and* '60s so I don't know which one you have got.

Yes, they do I think, but I've never needed to measure them. But with the reading you have it appears that all is well down to the tank and it will be the tank unit itself which is faulty or just a bad earth on the tank.

Martin

I echo what "Oily" says about the earthing. Thoroughly clean up the top of the sender unit and the 6 (?) little bolts that attach it to the tank. My gauge was under-recording for 2 or 3 years then finally ceased to register at all. Cleaning things as above got the gauge working fine again.

Dave

Yes, getting that done fixed things.