I believe the engine coolant temperature sender is malfunctioning. When my 72 warms up, the needle on the gauge starts below the first mark and settles right on the second mark. So I know the needle moves, but it never goes higher than the first mark, no matter how long of a drive I go on.

This has been covered before. Other threads have suggested checking the ohms on the sender and checking to see if the thermostat did not fail in the open position. But I'm not sure where to find the sender.

I've looked on both sides of the block, but what I'm seeing conflicts with most of what I've read.

Here is what I think is the engine coolant sensor switch. It is located on the driver's side of the block and is connected to a green wire.




Here is what I think is the temperature sender. It is located on the passenger side and is connected to two brown wires that get spliced together into one brown wire.




Based on previous threads, I think the switch on the driver's side may be connected to the fan and the sender on the passenger side is what's attached to the gauge. But I'm not sure why my sender (if I have the parts labeled correctly) has two prongs on it instead of the single prong I see listed for other 72 Vettes.

I'm fairly certain the engine was not originally built for a Vette. The engine block code is 092TAX. I can't find the date code. I'm not sure if this helps.

 

Driver's side = dash gauge

 

This is your temperature sender.. it should have a dark green single wire running to it.. Pull a ohms reading from the sender...

If you want to check the gauge, remove the sender wire and turn the key to "ON".. The gauge should go to cool... Then ground the dark green wire out and the gauge should peg hot...

Willcox

 

So what is the function of the two-prong sensor on the passenger side?

 

Actually your sender is not correct.. it should look like this one, someone has replaced it with another style. 1513321 which was a service replacement button type connector.



The other sender is your transmission control solenoid sender. It would have a dark blue and a black wire going to it..

Willcox

 

Thank you very much for the help.

 

My pleasure.. post back when you test.. I've got a very experienced gauge back ground.

Willcox

 

So here's a noob gauge question. Since the temp sender only has one prong, do I hook up the positive terminal of my multimeter to the prong and touch the negative terminal to the outside of the housing by the hex?

 

Yes... and then test it with a ground to the block. This will tell you if you have continuity between the sender and the engine.

You must have the wire removed when testing.

Here is a chart that'll tell you the expected inputs from the sender.


This chart will tell you what the gauge should read if you unplug the wire with the key on, and if you ground out the ohms wire with the key on.

 

I think I have everything I need now. I will post what I find out. Thanks for all the help.

 

I hooked up the multimeter after a brief ride home from work. The engine cooled down a bit before I could take my first reading. I let the engine idle for ten minutes before taking my second reading. I used a temp gun aimed at the block in the same spot as the sender for the temperature readings not coming from the gauge.

With the help of my 5-year-old daughter, I believe the gauge is working properly. The gauge drops below 100 degrees with the green wire disconnected and it just passed 250 degrees with the green wire grounded.

First Reading:
Temp gun 178
Gauge at first mark
124 ohms

Second Reading:
Temp 235
Gauge still at first mark
100 ohms

Based on the chart above, I would expect the gauge to be near the second line and the ohms to be around 65 if my block temp was 235 with a properly working sender. Right?

 

Well you block temperature at the sender is going to be high but you're on the right trail. Replace the sender.. But the next issue you'll have is the new senders usually won't read correctly with the gauge. This is why we made the adjustable resistor for the back of the temperature gauges. So when you change your sender in the engine you can match calibrate the dash unit to read correctly.

https://willcoxcorvette.com/corvette...resistor-68-76

Willcox

 

I see a part number in the photo of the sender you posted earlier. Is that the right part number? If so, I'll order it from you guys tonight.

 

https://willcoxcorvette.com/corvette...-in-head-72-79

 

I haven't wired my sender and gauge reads temp. quite accurate, how's that?

Dark green cable is disconnected and sender broken

 

The only way that can happen is that the gauge is wired up to some other sender. Check the other cylinder head for another sender. If you find one, remove the wire from the sender (ignition switch ON, engine not running) and see what the gauge reads then. If you get a different reading, that is the wire that is connected to the gauge. You can always run a jumper wire from the 'working' sender wire to the "correct" sending unit and see how the gauge reads then.

 

I checked out your video for the adjustable resistor. The video says that 79 ohms should put the gauge needle at 210 degrees. What tool would I need to properly send 79 ohms to the gauge to see if the setup is accurate?

 

What is called a linear potentiometer.. you can buy them on line. But if you are trying to match calibrate that all goes out the window. An IR gun works decent.. but what I've found that works best is knowing that thermostat you have in the car. If you hold your hand on the upper hose you'll feel the water get hot quickly when it opens. That will give you a quick reference point as long as you have someone in the car to tell you where the gauge is at... and it also takes time because you'll probably have to do this three times to know if you nailed it. The adjustable resistor works, and you tweak the gauge by turning the pot on it either left to lower the needle or right to increase the needle.

The problem with doing it by holding the hose is that you must have someone inside the car at that exact moment to tell you where the gauge is reading when it hits say.. 180. Then you tweak, let it cool down and do over again... (good chance to open a another beer).

Willcox

 

You just need to get two (2) 100 ohm resistors (1/2 watt is fine) at a Radio Shack or other electronic parts place. [The reisistor simply substitutes for the sender.]

Just connect the sender lead to one end of one resistor and connect the other end to chassis ground. Then turn ignition to ON (engine OFF) and see what the temp gauge reads. Gauge should read around 180*F, if all is working as it should. Then, put the two 100 ohm resistors next to each other and twist the wires at each end of them together. That [parallel] pair will create a 50 ohm resistor that you can connect to the vehicle gauge the same way as before. But, you should get a reading close to 250*F this time.

If both readings are reasonably close to desired values on gauge, then your gauge should work just fine if you get the correct sender unit for your year car. (These senders are different for different model years.)

 

Ok... I purchased the 100 ohm resistors and hooked everything up. One resistor put the needle at the mark between 100 degrees and 210 degrees. The two parallel resistors put the needle just at the end of the 250 degrees mark, so I feel comfortable enough keeping my gauge setup as is. The correct sender and 195 degree thermostat just arrived from Willcox. Thermostat has been tested in a pot of 200 degree water and it opened nicely. Now I'm ready to drain some coolant and install the new sender and thermostat.

I'm only looking to drain about a half gallon or so of coolant. Should I just disconnect the hose from the bottom of the radiator or should I use the drain plug from the bottom of the block?