M37 Gauges and Sender Resistance

by David Sherman

The question came up here a while ago, and I've been trying to find a good set of gauges for my ambulance, so I thought I'd post the results in case they can help anybody else. What I did was find a good gauge of each type and measure what resistance it took to make it read two different readings. Where I had more than one good gauge, I checked them all. This is just a good/bad test, not a precision calibration. Expect your readings to vary +/-20% or so from mine. M-series gauges aren't known for accuracy. They basically either work or they don't work.

To simulate the sender, you can use either fixed resistors or an adjustable one (potentiometer or rheostat) adjusted until an ohmmeter says it's the right value. In every case, I applied 28 volts between the hot terminal, which is the left (driver's) side of every gauge as it's mounted in the panel, and ground, which is one of the mounting screws on the back of the gauge. I used an adjustable bench power supply, but a pair of clip leads from the vehicle batteries would work fine if you're careful not to short them. Then I put the test resistance between the other (right-hand) terminal of the gauge and ground. When you do this, also check for the pointer getting stuck, which commonly happens if dirt or rust gets in the mechanism. The ground on the gauge (mounting screws) is important. It must be connected to the negative side of the power supply when testing, and when the panel is installed in the truck, it must have a good ground all the way back to the battery. Often the gauge-to-panel or panel-to-body ground is not very good.

Fuel: 30 ohms = full, 0 ohms = empty.

Temperature: 2400 ohms = 120 degrees, 1000 ohms = 180 degrees, 500 ohms = 260 degrees

Oil pressure: 30 ohms = 60 psi, 0 ohms = 0 psi. Gauge must be held with face vertical to read correctly.

The point of these tests is to be able to test a gauge by itself instead of guessing at whether the gauge, the sender, or the wiring is bad, and probably wasting time and money swapping unnecessary parts.

Note that you can also use these numbers to test the sender. Putting an ohmmeter directly on the oil pressure sender, for example, should show close to zero ohms with the engine off, and somewhat less than 30 ohms, depending on your actual oil pressure, with the engine running. You can take the fuel sender out of the tank and work it up and down by hand to check its resistance. If it's intermittent, solder a small, coiled, stranded wire from the float arm to the body of the sender. For the temperature sender, measure it cold and hot. About all you can say for sure about that is that cold its resistance should be at least 2500 ohms and the resistance should go down as it gets hotter.

If you've checked the gauge and sender as described, and you now know they're good, there must be a problem in the wiring or grounds somewhere, but at least you know your gauge and sender are good.

It's better to test them with a variable resistor than a pair of fixed ones, because with a variable one you can "dial" the pointer slowly across its full range in both directions and see if it sticks anywhere. Gauges that stick are really frustrating. A 100 ohm potentiometer is good for the oil and gas gauges, and a 5000 ohm one for the temperature gauge.

I've fixed gauges and meters that have sticky spots, but it's really not worth it. The covers are swaged on, so you have to pry them off, and then the stickiness is usually due to tiny specks of rust that have gotten stuck in the gap of the magnet inside it. Blowing it out with air strong enough to get the dirt out usually wrecks the springs, which are like the hair spring in a watch, and taking it completely apart and cleaning it is like cleaning a mechanical watch -- you have to have tiny tools, a perfectly clean bench, and a magnifier.

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