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Temperature gauge problem


dberkham
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Assuming you are looking at a typical Mopar L-6 engine: At least through the 1940s on Plymouth the temperature gauge is actually a sealed unit "mechanical gauge". There is a bulb with ether in the head (sensor) connected by tubing to the dash unit which is actually a pressure gauge. The sensor bulb and the gauge cannot be disconnected without breaking the system.

I would not be surprised if the 1950 Chrysler still used the same type of unit. If so you can't replace the sensor separately from the gauge.

Other than damaging the system and loosing the ether these are very reliable units and, on the plus side, there is absolutely nothing that can go wrong with the sensor. The shop manual for Plymouth says that if the gauge is reading within some number of degrees of correct it can be adjusted by bending the linkage between the coiled up Bourdon tube and the needle.

To calibrate it you will need to remove the whole thing from the car and put the sensing bulb in water at known temperatures. And it is very easy to damage the tubing at the gland nut that holds the sensor in the head when you are trying to remove the sensor. On my car there is a freeze/welch/core plug on the head at the sensor location and you can remove that to get to the back of the bulb with something to help push the sensor out.

However, I'd first double check that your cooling system is actually in good shape. There is often a mountain of sludge that has accumulated in the bottoms of the water jackets. And the water distribution tube often rots out which means the back of the engine runs hot (where the temperature sensor is) even though the coolant getting to the radiator, mostly from the front of the engine, is much cooler. You can clean out the block water passages by removing the freeze/welch/core plugs along the bottom and flushing while probing with some stiff wire or equivalent. To check the water distribution tube you will need to pull the water pump and radiator. Sounds like a lot but it is actually less of an issue than trying to get the temperature gauge out without damaging it.

Edited by ply33 (see edit history)
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Good advice from Ply33. Does the gauge read high when the engine is cold? Due to the design of the gauge, if the engine overheats it can damage the gauge and make it read high but you will see the needle is too far over even when cold.

Can you get an electronic temp sensor and take the temp of the engine, front and rear? Or just touch the back of the head and the front and see if the back is hotter.

If the coolant tube is missing or rusted out all the coolant streams up the front of the engine and none goes to the back cylinders.

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Good advice from both of you. That's what I love about this forum. I do have an infrared thermometer and will see if the block is hotter in the back. The gauge reads cold when first starting up but soon goes up to hot even though the radiator temp is between 160-170 degrees. Thermostat is new and functioning. Water pump has also been replaced. Don't know if the coolant tube was replaced but I will check with the mechanic who did the work. Thanks for the sage advice.

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  • 7 months later...

Temp near the sensor reads 190-a 20 degree difference from the front. Gauge reads cold on start up but soon the gauge needle goes all the way to hot. If i rev my motor at lights or coasting with clutch in, it does show a temporary, if slight, cool down. I'll let my mechanic know of your advice and go from there. Thanks.

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