I do not know how the temp gauge works on a 544. If it is like the old 240's, this may be something to try.
This is how the temp gauge works on a 240. 12 volts goes in the temp gauge and out the other pin. The sender will heat up and vary the ground side of the gauge to move the needle. Technically the temp gauge is a voltage gauge with a variable ground. The sender unit is a resistor for ground. KOEO, remove the wire at the temp gauge. It should go to zero, no heat. If the gauge shows heat, you have a faulty gauge or shorted ground side wire from the gauge to the engine temp sensor. Do an ohms check on the temp sensor. Below is from an 1800 service page for temp sender ohms check value. Since the gauge is stuck at hot area, the gauge could have failed. Is the gauge easily removable to tap on it to the see of the needle will move on its own and not jammed internally.



Service – Volvo 1800 Pictures and global register

 

John;

Temp Indication on the 544 is not electrical, but by way of an ether "filled thermal system" pressure sensing capillary tube. You might want to check the calibration of yours (CAREFULLY, NOT twisting and compromising the capillary tube! See: SW-EM Temperature Gauge Notes ) before considering exchanging...they are available but not inexpensive!
Alternately, you can check the temp by other means and non-invasively (like an IR Thermometer) to verify your Cooling Sys is operating at the TStat setpoint, and simply mark this on the Temp Gauge face, and live with it!

Good Hunting!

 

@Ron Kwas Thanks for the info on how the filled thermal system works.

 

I had forgotten it was a capillary action gauge. Well , it is not stuck, it just takes a lot longer to go back down to the low end. However, it is rather far off on the scale. I removed the sensor end from the head and heated it with a heat gun and kept an eye on the temp with a laser temp gun. My Dad watched the gauge in the car. When he told me it was at the end of the green and into the red, the temp of the sensor was 110f. I think it would be pegged before it got anywhere near normal operating temp. I don't know how this might be resolved so that there would be room on the gauge to indicate the car was too hot.

I read through your web page that you referenced. That's a rusty looking example! When I removed my sensor to test it, the two halves ( the nut and the bulb ) they were not stuck at all. I can see where a dab of sealant could be useful. What anti-seize have you used for this?

 

I tested the temp gauge in a more scientific way. I boiled some water and jogged out to the car and poured it into a pan where I had suspended the temp send bulb and a thermometer I had verified previously. 185 f was just a little past half way on the gauge. Now I need to dig into why it was running hot. I tested the thermostat in a pan of water and it began opening just after 185f. I need to test the water pump. It’s always run a little squeaky. It was never used before firing up the engine. I guess I could pull the thermostat and see if water moves well in the radiator. Before I can do that, I need to get the new points, etc to try firing it up again.

 

Great testing. Temp sensor and gauge, working properly. It is a process, one thing at a time. About the only test for the water pump is to replace it. It is not a bad idea to replace the water pump if it looks old or replace it now so you do not have to deal with it later. Does your car have a fan shroud and is it installed correctly. Is the radiator clean where to air flows thru. Possible restricted radiator, internally. Use a temp gun to test inlet and outlet radiator hoses. This does happen, I have had customers replace the coolant during a repair or drain and refill. They poured in 100% coolant, not a 50/50 mix. This will overheat because 100% coolant is thicker like syrup. The radiator/cooling fan cannot remove the heat fast enough. Do you still have the belt driven fan or electric fan. If electric fan, is it set to run all the time that the engine is running.

 

The pump is brand new. Well as usage goes. I probably bought it a few years ago when I built the motor. I just didn't get to starting it until this year! I don't think the 544 has a fan shroud. I'll have to check, but it might be a new radiator. I'll have to see if I bought one in the last few years. The local good radiator shop guy retired a few years ago. I could have had them check it out before they closed the doors.

 

Is the newbpump one with a cast impeller or stamped sheet metal impeller? Cast impeller was OEM, which circulates coolant better, especially at idle.

 

I don’t know the answer to that. I installed it a few years ago, when I built the motor. I think is is this one, based on an invoice that CVR dug up for me. Volvo B18/Volvo B20 Water pump (Volvo original) - CVR

 

That’s the correct one.