How to calibrate oil pressure gauge?

[Old Henry]

The pot is a Radio Shack 1-Megohm 500 VDC - 0.5W Linear-Taper Potentiometer part number 271-0211.



Here are the specs: http://support.radioshack.com/suppor...oc33/33575.htm

Here's the schematic drawing:
http://support.radioshack.com/suppor...oc18/18001.pdf

These stock gauges will work with a variable resistance sender such as my rheostat gas gauge sender. And can be controlled by variable resistance through a pot. But, combining variable resistance with the cycling switch of the stock sender just does not work as may have been supposed. Either one alone - yes, combined - no. I believe that is because the sender switch is turned on and off by the same wire that goes around the deflection rod in the gauge also wrapping around the switching rod in the sender and has to heat up and cool down to turn the switch on and off as shown in this diagram of the sender and gauge wiring.



Too much resistance in the circuit cuts off the current to heat the sender and gauge rods enough to do anything and just shuts the whole system down.