I've tried changing the sendor. It's not the sendor, it's the gauge. Who does calibration work on a temp gauge? Thanks.

 

Most replacement senders including the current G/M (Delco) are not calibrated correctly for our cars. "lectric limited has one that is. There is a lot of info on the NCRS board archives about that. I forget the ohm reading but there is a way to test the sender at approx 70 deg. some people use a resistor in line to achieve correct reading.

 

The below is how I calibrated the temperature gauge.

- Remove the sensor and put a pipe plug in the hole.
- Immerse the sensor in a vessel of water along with an accurate thermometer. I used an industrial grade thermometer and a large coffee can. Both the thermometer and sensor should be suspended so they don’t touch the walls of the vessel. Use coat hanger wire or something to make a hanger.
- The process here is to heat the water slowly and monitor the temperature gauge in you vette and compare it to the thermometer. You want the engine running so that alternator voltage is the reference voltage for the gauge just like it would be if you were driving. It’s higher than battery voltage and will make a difference in your readings. I used a coleman stove on a little table next to the drivers side fender. I also made some electrical wire jumpers so that the sensor wiring would reach the sensor in the can of water.
- Heat the water up. When the vette temp gauge needle lines up with a calibration mark on the gauge, record the thermometer temperature as this is the true temperature of that calibration mark. You should take two readings. One sitting in the drivers seat normal driving position which will include some parallax error because your looking at the gauge from an angle. You may also want to record the reading looking perpendicular to the gauge for future reference and overall accuracy.

My 79 vette has the first calibration marked as 100 deg and the third calibration mark as 220 deg. There is also a second calibration mark half way between the 100 and 220 deg marks with no temperature indicated. The next mark above 220 is the start of the orange danger zone. All units of measure here are in degrees Fahrenheit.

The results were:

Gauge reads 100 Thermometer reads 130
Gauge needle 1/3 way to second mark Thermometer reads 160
Gauge needle 2/3 way to second mark Thermometer reads 170
Gauge needle at second mark Thermometer reads 180
Gauge needle 1/2 between second mark and 220 mark Thermometer reads 185
Gauge needle at 220 mark Thermometer reads 204
Gauge needle 1/4 way to danger zone mark Thermometer reads 212

I’m at sea level so 212 is as high as I could go without pressurizing the vessel. Note that my gauge reads 16 deg to high at the 220 mark. Yours may or may not be so bad or it may be worse. I also did a similar thing with my 160 deg thermostat and found it started opening at 160 and was fully open at 170 deg. I can now look at my temperature gauge and know when the thermostat is starting to open and when it is fully open. This can be useful especially when setting the thermostat on your electric cooling fan. With 95 deg ambient, AC on, sitting in traffic for an extended period, my temperatures never go over 190 deg. Most of the time it doesn’t go over 180.

 

Larry is right on target about that sensor. A forum member named Jason from Vette Instruments did a great job on mine. I can't seem to find where I put his contact information. Maybe someone else knows him too.

 

Lots of good info here already. I have heard of the differences in sending units not calibrated for the original gauge but can not verify this. Sender units are faily cheap though and easy to install for comparision.
Also location will make a difference. I'm guessing but the cyl head location between #1 and #3 sparkplugs (or even #6 & #8) will read different than up beside the T-stat housing on the intake manifold. Now i once saw someone install a temp sensor on top the T-stat housing (in a thermal switch hole) which would be kept behind the T-stat and cool until the T-stat opened to flow wtr ('bout 180*F).
A simple check would be to use a hot plate to boil some wtr and dunk the working end of the sensor in while reading at the gauge - should be very close to 212*F. U can add wire to extend the sensor wire but will need an additional wire to connect the threaded base to car block or frame (ground). And really 212*F tells u when ur in trouble - and then 150 & above should be close then too. IMHO relative accuracy is good enough as long as i know when its boiling - this ain't NASA tech.
If u really need help try www.corvetteinstruments.com as they did my gauge cluster and advertize calibration.
Good luck and post ur results please. cardo0

 

Thanks for all the replies.

My temp gauge works, but peaks out at about 125. On my gauge, 200 is at 12 o'clock, and I suspect normal operating range should be close to a verticle position for the needle.

I bought the Lectric Limited sendor for a 72 and the bore was too big to fit in the head between plugs 1 and 3, so I got the 81-82 sendor, which does fit. It cost $32 from Zip, and reads a little lower than the sendor I pulled, so I put the old sendor back in and will return the new one.

I'm thinking there is more to calibration than finding 212 and resetting the needle. Right?

Please tell me more about the resistor approach. Sounds easier than sending the gauge and temp sendor to a vendor.

 

It's true that adding resistance across the sender terminal to ground will cause a higher gauge reading, in fact with a little experimentation using different resistors, you could cause the gauge to read correctly at normal operating temperature. The problem you will have is this is not a good fix because the gauge will only be accurate at normal temperature. the resistor fix is not proportional over the entire range. For example, at the time of a cold start the gauge reading will be higher then normal because you will be reading the added resistance. Having the gauge calibrated to the sender is the only accurate way of repair, or replace the gauge.

Edit: I also want to point out that a resistor added in series will lower the gauge reading. Could be a decent fix for a temp gauge that reads high (but still not proportional throughout the entire range). I have always been under the impression that most of the time these gauges read low, so the resistor needs to go from the sensor lead to ground to cause a higher reading.

 

What year car do you have?

 

VetteInstruments@aol.com

Jason

 

Thanks J! I'm going to put it someplace where it won't get lost. Now all I have to do is remember where I put it?

 

1972

 

It appears that the 80/81 sendor is not compatible with your 72 temp loop. An adaptor and the 72 sendor might be the answer. If you can add a resistor it will re-range the loop and this is also an option.

 

If you want to test your temperature gauge at temperatures above 212 F, use corn syrup. Pour corn syrup in a pan and heat it. It'll go much higher in temperature than 212 F. For a thermometer, you can use a candy making thermometer that is available from any cooking store.

You could also heat up anti-freeze (ethylene glycol). This is a really not a good idea, especially when corn syrup will actually work better and corn syrup is harmless. Ethylene glycol (EG) is extremely toxic. For sure, don't bring EG into a kitchen, if that's were your doing your temperature guage testing.

 

For that matter, you could probably use cooking oil and go as hot as you would want.