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Temp Gauge

I have a temp gauge problem on my 1959 Cadillac, I replaced the coolant sender plug but the connector didn’t look the same as the original one so I still replaced it anyway. The problem that I am having is in the dash temp gauge, when I start the car in the morning the needle begins to rise slowly to High and stays there. I installed a new radiator when I rebuild the engine 3 years ago but the problem still there.

Do you still have the old sender?
Where did you get the replacement sender?
How many wire connections on the old and new senders?

Either the new sender is wired wrong or it has the wrong (too low) resistance.

As a test if the sender is disconnected the gauge should drop to low.

Measure the resistance of the old (if available) sender then the new one, at room temperature.
Should be at least a few 10’s of ohms.

Looks like you have a mismatch between the gauge and the sending unit. You need to see if you can find the correct sending unit.

If you can’t, then you need to find an old Glenn’s, Motor, or Chilton’s manual and look up the resistance specs of the sending unit. It will give a procedure to put the sending unit in a pan of water. Heat the water to a specific temp using a candy thermometer or something similar, and measure the resistance between the connector and the body of the unit. It will also give the resistance of the sending unit at ambient.

If the resistance of the sending unit is too low, you can add a resistor in the wire to the sending unit to compensate.

If you don’t have access to all this, then you need to go to Radio Shack or similar and get a variable resistor. You need to perform the above test to get a baseline to work with. Then get a variable resistor that covers not only the range of resistances from the cold and hot values you tested, but go a little beyond.

Now disconnect the wire that goes to the sending unit and hook it up to one side of the variable resistor, connect the other side of the variable resistor to the block. Now adjust the resistor until the gauge on the dash is where you expect it to be when the engine is at operating temperature, i.e. the middle of the gauge. Next, adjust it to where the gauge just starts to move off cold and measure the resistance, then one more time, adjust to where the needle just enters the hot zone and measure the resistance again.

I should mention that you need to disconnect the variable resistor from the car and measure the resistance with a VOM each time. With these three values, you have a resistance curve for the gauge.

Now you can take this information to the parts store and try to find a sending unit that meets these specs, good luck with that. Or you can take the current sending unit, measure the value cold, measure the resistance at your operating temperature and again at what is considered hot for this engine. You may have to put the sending unit into an oven for this as the Hot temperature might be 220F where water will boil at 212.

Now with the curve established with the current sending unit and the desired curve established with the variable resistor, you might be able to find a fixed resistor that you can solder into the wire going to the sending unit that will approximate the desired curve when added to the current sending unit.

Good luck.

I assume this actually is a false gauge reading and not a stuck thermostat which is causing engine to run that hot?

If the engine is not running hot and the fault really is gauge error then this would boil down to a faulty gauge or incorrect sending unit.
Keith has given good suggestions on how to tweak the needle.

On the other hand, maybe summertime heat is causing the engine to gasp trying to propel that 3 or 4 tons of iron around. (Just kidding. I like the 59 Cadillacs; the ultimate tail fin car.)

Before assuming that the problem is the sender, I’d want to measure the coolant temperature with a seperate test gage “Teed into” the sender hole. You don;t want to risk engine damage by making any assumptions.

I’ve never worked on a '59, but if the engine temps is actually okay but you’re unable to find the proper sender, you may want to replace the gage with one that matches the sender output, at least while you do research for the right parts. The parts manufacturer should have a guide to doing so. Adding a resistor improperly (in series instead of parallel) would shift the curve but also affect the sensitivity. Adding in parallel would do the opposite. Unless you understand the difference you may be better not to jury-rig anything.

Thanks for comments, I really appreciate it.