C2 Temp Gauge Calibration

[Rich G.]

I've read the archives and hear what everybody says about the inaccuracy of the gauges/senders. My 66 L79 runs 210-220 around town and about halfway between 210 and yellow on the highway. The previous owner said he used a stem thermometer and the coolant temp was 180 when the gauge said 220. After I had the car I saw an ad from LI Corvette supply that said they had "calibrated" sending units so I put one in. Not much differance. The car shows no signs of running too hot. Today I measured the coolant Temp with a stem thermometer for quite a while while the car was hot and idling. It indicated 170 at first then after idling a while went to 180. All this time the gauge said 220. What resistance should put the gauge at center? I'd like to see how far off the gauge is. I understand the curve on the sender is not linear and that the tolerance at the cold end is wider than at the hot end, but somebody must have a test for the gauge. Anybody know where to find these specs?

Rich Giannotti

[Mike M.]

I Used Fred Oliva

Try this site http://www.gate.net/~foliva/e

http://www.gate.net/~foliva/e

[Donald L.]

Re: C2 Temp Gauge Calibration

Rich the replacement sender does not have enought impedance. The sender should read at least 700 ohms at room temp. measured from the tip to shell. I think C-1s
use the same sender as C-2s and a correct sender can be had from Mary Jo for
about $40.00.Her phone is (858) 451-1933.

[Ed Jennings]

Re: C2 Temp Gauge Calibration

If you aren't worried about judging, an accurate sender is available from Autozone (and probably others) for 6 bucks. Ask for a Wells TU5.

[Richard S.]

Re: C2 Temp Gauge Calibration

Ed, thanks for the information. Do you know the specs. of this unit so I can measure to be sure I don't get a bad one right out of the box. Thanks.

[Ed Jennings]

Re: C2 Temp Gauge Calibration

All the ones that I have tested, or that others have reported to me read real near 700 ohms at room temp. The spare I have reads 631 at 70*, which is a little lower than I thought it would be. I seem to remember it being about 690 ohms when I bought it, but the temp in the garage may have been a bit lower too. I'll check and see what the unit on the car reads at the same temp and let you know. It is pretty accurate.

[Ed Jennings]

Addendum

Rick, I checked the sender on the car and it was reading 670. At the same temp (?), the spare sender reads 640. While the 640 is probably ok, I would take a digital ohm meter shopping with me and find a sender that reads as close to 700 ohms as possible, given a temp of around 70*

[Richard S.]

Re: Addendum

Ed, Thanks very much for taking the time to give me the feedback on those senders...it is very much appreciated.

[Ed Jennings]

Re: Addendum

Yer welcome.

[Rich G.]

Re: Addendum

With some offline help from members of this forum (UJ) I got the graph of temp vs. resistance for the proper sending unit. I got 3 resistors and substituted them for the sender one at a time. The good news is that my gague conforms (at those 3 points anyway) to the curve.

The points were 160 ohms = 120 degrees (C2 gague with 210 center)
Next 82 ohms = 190-200 degrees and 43 ohms = top of the red

You can see from this that what others have said about adding series resistance to "calibrate" the sender appears to be true and can be dangerous. If you are hot and approaching 43 ohms and have a 33 ohm resitor in series your gauge will probably read around 210. Not what you want when the engine is really in the "red".

Rich Giannotti #38594