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.

[41ford1]

"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."
Yes. I should have looked at that drawing closer. I'll have to measure the unpowered resistance of the sensor and the gauge unit. I think they are close to equal. I'd want to verify that. Right now I think the only way to "cal" these is on a test set up where the system is powered and pressure can be varied. The cal may need to include changing the arc of the gauge bimetallic in the unpowered state in conjunction with the zero and span adjustments.

Another "solution" would be finding out what resistance is required to put the pointer at 30 to 40 PSI when wire directly to 6 volts. Put the resistor in series with the sensor wire. Replace the sensor with a modern OP switch. That way as long as the OP is above 8 PSI the gauge reads steadily in the good range.

[JSeery]

Think you are on the wrong path here guys, these are not resistance devices (resistance produces the heat, but it does not work like the later model resistance measuring devices). The points vibrate or pulse, the pulse rate determines the current in the circuit. The current produced in the sensor also flows through the gauge coil such that the gauge bi-metal stir moves proportional to the sensor bi-metal strip. But this is not a constant current, it is a pulse that pulses at the rate of the contact points. Adjustments are made by changing the tension on the bi-metal strip in the gauge. Take a look at the photo in the first post, the adjustment for the bi-metal strip is located under it. There is a second adjustment for the "zero" point.

[Old Henry]

And so we come full circle back to where we started.

[41ford1]

Originally Posted by Old Henry

And so we come full circle back to where we started.

Not really. Your tests with the POT shows that the OP gauge and sensor are a balanced electrical system. Adding total current with a shunt to ground or lessening current with a resistor in series upsets the balance. You have proven what doesn't work. Where does that leave us? I think it points to the mechanics of the system. The cogs on the gauge are some of the mechanical adjustments. I don't know if there are similar adjusting points within the sensor. I've not opened one up yet. A prior post talked about cleaning the oil inlet of the sensor. Did you try that? You mentioned replacing the 80# sender. Let us know if that works better.

[estout81]

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Please reread posts 8, 38&43. This information was taken from old repair manuals. Hope this helps.

[Old Henry]

And posts 5 & 15 regarding the inefficacy of the right cog to adjust sensitivity and posts 30 & 33 on the ineffectiveness of changing the 80 lb. sender to the 60 lb. one. I believe that we really are going back around the circle.