jfr's 63swc

Hello all,

Fixed my brake issues on my new car - Ray. Turns out the pulling was the front hoses were bad even though they bled properly. Replaced the hoses and the pulling is gone. With no leaks in the system and no air in the system, the brakes pedal would slowly sink to the floor after stopping. I rebuilt the master cylinder and that fixed the pedal sinking problems. Now I on to sorting other things.

The temperature gauge is pegged to the right. It does wiggle a little when the key is turned on. So, per the 1963 Corvette Shop Manual, I tested the temperature sending unit. It says to use a test light - I used a voltmeter. I assume that this would be similar - please correct me if I am wrong.

The Manual says basically to connect one lead to the battery and the other to body of the sending unit. If the bulb lights, it is properly grounded. The voltmeter showed 12 volts so I guess it is properly grounded.

Next it states to remove the test lead from the body of the sending unit and connect it to the terminal of the unit. If it lights, the sending unit is internally short circuited. Well, it lit so the temperature unit is bad. I checked a spare sending unit that I had with the same tests and got the same results - bad!

Next I tested the temperature unit on Sting (my other '63) the same way and got the same results! Only that unit/system works fine!

So, what did I do wrong?

Thanks in advance for the help.

Regards,

Jon

Nowhere Man

if you unplug the sending unit and turn your key on does the gauge needle stay at zero?

jfr's 63swc

Nowhere Man,

Thanks for the response.

The gauge is to the right - reading hot all of the time. Unplugging the sending unit does not change that. Turning the key off does cause the needle to wiggle a little, but it is still pegged at hot.

Jon

Nowhere Man

Then I would venture to say your gauge is bad and needs recalibrated

JohnZ

It will ALWAYS show internally short-circuited to ground - that's how it works; the variable resistance of the thermistor inside the sending unit (which moves the needle) changes with the coolant temperature.

With the key "on" and the wire disconnected from the sending unit, the gauge should be pegged to "COLD"; if you then ground the sending unit wire terminal to ground, the gauge should peg to "HOT".

Chart below shows the typical relationship between resistance of the thermistor and coolant temperature.

jfr's 63swc

JohnZ,

So is the Shop Manual test incorrect or was it my choice to use a volt meter instead of a test light?

Jon

jfr's 63swc

Nowhere Man,

Guess I will be pulling the dash and sending the temperature gauge in to be redone.

Thanks,

Jon

65GGvert

Just before I pulled the gauge out, I'd turn the key on and disconnect the green wire from the sensor and alternately touch it to the engine ground and pull it off. I'd do that several times in case there is a slight spot in the gauge that is binding and it might free up. Won't hurt anything to try. Don't leave the key in the on position for very long if you have standard ignition, just in case, so you don't overheat the coil if the points are closed. If it doesn't drop to low scale with the wire off, it's not the sensor. A short to ground anywhere along that green wire to the gauge will also cause full scale reading, but only with the key on. If it stays full scale with the key off, it's in the gauge movement.

Frankie the Fink

I'll have to look at the shop manual, but the JohnZ checks will give you a gross indication of the gauge's functioning. Its good if you can save an original sending unit as the repros can be quite inaccurate.

buns

Before replacing the gauge I would pull the connector off the back of it and see if the needle unpegs. Maybe there are some incorrect wiring connections.



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jfr's 63swc

Buns,

Would disconnecting the battery do the same thing?

Thanks,

Jon

vipervetteguy

My 66 does something similar and I've already replaced the sending unit. My temp gauge reads perfectly 90% of the time and then it will just swing all the way past hot. I can't determine any pattern for this action. I've learned to live with it, but it is very alarming to see it pegged every once in a while!!! (momentary panic)

cardo0

Light test is interesting and first ive read of it. Well for testing the sendor and gauge circuit there are a few recent threads here that may help. All '63 cars. Why '63 has me wonder u guys dont share info well.

https://www.corvetteforum.com/forums...-variable.html

https://www.corvetteforum.com/forums...-resister.html

https://www.corvetteforum.com/forums...emp-gauge.html


I think Wilcox Corvette has this problem covered (for the early years) and maybe your best source of help here. But a gauge pegged high sounds defective. All the sendor does is act as a variable resistor but its resistance decreases w/increasing temp (negative temp coefficeint). So if your gauge is pegged high all the time then either its broken or u have something wired badly enough to provide it power w/ign off - unlikely. What im saying is that gauge normally would need a source of pwr to read pegged high.

So i think u are correct and in need of gauge repair.

Good luck.

Frankie the Fink

Aren't temp gauges dampened ? And if that failed I would think it would make for erratic operation..