Jerry, The dwg you show is not how the gauges work. It is not a galvanometer located within a resistive bridge. Rather, two of legs of the bridge are actually the coils that pull the magnetic pendulum / needle left and right (toward full scale and zero scale). The third leg is a fixed wire wound resistance and the fourth leg is, of course, the sender. I've got a pretty good handle on this. And I just received two more Temp gauges and one more Fuel gauge to get a better idea of what the design and specs are or were. What I am seeing with the senders is that the 52700 is probably an exact or near exact replacement for the original 4800/00 sender. But it seems neither the original 48u00/00 nor the good substitute 52700 is presently available from suppliers. The 3800 is apparently the closest match of any available senders and that is why suppliers have been suggesting it as a replacement. It seems it has a higher cold resistance but a steeper curve, such that at around the critical temps between 170 and 230 it comes close to matching the gauge. Once I finish testing the two additional gauges, I will refine the data on the senders and correlate it to approximate Temp error (rather than ohms error) when used with a good gauge. Mike Hartman and I will then publish a document showing these results. These gauges have the ability to be adjusted by loosening a pair of nuts on the back to reposition the magnetic coils to calibrate the gauge. But I am pretty sure my recommendation will be to NOT attempt that. Unless the gauge was misadjusted at the factory, or was somehow damaged, or some owner or shop misadjusted it, I don't think the calibration would have changed over time. Coils don't lose turns, and the laws of magnetism do not age. Furthermore we do not know exactly how the gauges were calibrated, although you can see various cal point indicators on the dial.
We hope to complete this in about 2 weeks or so.
Tom
We hope to complete this in about 2 weeks or so.
Tom