During the refurb process I elected to upgrade my 1961 Triumph Herald with a 60 Amp Alternator to replace the 45 year old 22 Amp dynamo. In doing this I needed to convert the entire car's electrical system from a POSITIVE Ground system to a NEGATIVE Ground system. Most circuits, like the starter, the lights, the ignition, series wound DC motors for the wipers and blower fan don't care what the polarity is. Some of the instrumentation can merely have the the connections reversed at the terminals but not so with the fuel gauge. I wanted to preserve the standard white face Jaeger gauge so some rewiring of the gauge circuitry was in order.
The modification is relatively straight forward but involves disassembly of the gauge and rerouting it's internal wiring.
The crystal needs to be removed from the gauge. Bend out the three small tabs securing the trim ring to the gauge body and then it is best to heat the sealing o-ring by submerging JUST THE TRIM RING in boiling water for a few minutes. This softens the o-ring and the trim ring can be prised loose.
While the seal is softening you will note there are two small nuts on the back side of the gauge. These are the calibration adjustments. DO NOT loosen them now.
Once the trim ring and crystal are removed, CAREFULLY prise up the face plate retaining tab (near the E for Empty) crimped over the edge of the gauge case and gently remove the face plate being careful not to break off the gauge needle. You can bend it slightly but it is very delicate so use care.
Inside you will now see the two coils and buried behind them the shunt resistor. Fortunately, the shunt resistor doesn't care which way the electrons flow so we don't have to dig it out. Below is the standard Positive Ground Schematic and the standard gauge wiring.
What we need to do is to reverse the polarity of the coils. To do this we must CAREFULLY disconnect the exiting connections of these VERY VERY FINE coil wires and reconnect them in reverse. Shown below is a schematic with the polarity reversed on the coil and a photo of which wires go where in the actual gauge.
Once I get this modification done and pull the tank sending unit out, I'll begin the calibration process. The basic process is pretty straight forward and you will learn why you didn't loosen those two nuts on the back of the gauge.