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I noticed when I started my truck that the temperature gauge never got above 60 degrees so I decided to test my sending unit and thermostat. I ran it for at least 30 minutes and the engine felt hot, so I believe the reading was incorrect.

The sending unit had a reading of 5500 Ohms at room temperature and it went down to less than 2000 Ohms in a cup of hot water out of the microwave. I'm not sure what the temperature of the water was, but it was steaming.

I also put the thermostat in a pan of water with a thermometer. It started opening around 160 and was fully open by 200 or so. I'm assuming the valve does not rotate completely to a 90 degrees angle because it did not.

Does this all sound correct, and does anyone know what the resistance vs. temperature should be? My truck has a 260 degree gauge.

Thanks

Chuck

you really need an IR thermometer to do this properly. You can also use a probe thermometer from the kitchen and test the sending unit in a pan of water. If the engine is just idling, 30 minutes isn't enough to get it fully up to temperature. You will also see huge discrepancies in temperature between the block and the head in 30 minutes. 10-15 minutes of actual driving where the engine has to work will get it up to temp though, so even a couple runs up and down the street will get you a more accurate reading. The 2000ohm resistance is about right for a steaming cup of water, maybe a touch high.

Temperature: 2400 ohms = 120 degrees, 1000 ohms = 180 degrees, 500 ohms = 260 degrees
These are the resistance ratings for the water temp. Big resistance swings, so you really have to have an accurate idea of what the measuring temperature is. The gauge probably won't start reading until it hits at least 120* either so you likely just didn't get it hot enough. Best bet is to put a pot of water on the stove, get it boiling, drop the probe in there and see what your resistance is. Should be somewhere around the 900 ish mark. May be a bit higher depending on your elevation and current barometric pressure, but it should be just under 1000ohm. If it's any higher than this, you need a new sending unit

I just tried it in a pan of water while wearing some rubber gloves and got a reading of 1800 Ohms at a little under 150 degrees. Looks like it is working.

yup. So now double check with your gauge, because the gauge could be fried, but these big hunks of iron do take quite a bit of time to warm all the way up. Even in my new Jeep it takes a good 5 or so miles of good driving to get it up to temperature and well over half an hour of idling.