I am new to the forum and seeking advice from the resident experts. Normally a search will get me what I need but I can't seem to track this one down.

I recently installed a set of headers and side pipes to my 1979 corvette. During installation I realized I would have to remove the temperature sender located between 1 and 3 on the head. Doing so I broke the male connection off it and had to replace it. I purchased a new one part number TU65 and finished the project.

Now however, the temperature gauge reads hot and I can't figure out why. Concerned the engine was indeed overheating I put my infrared temp sensor to it and it is not overheating. In the sake of caution I even replaced the thermostat. With the car cool, the gauge will start at 100 and as the car warms up just never stops climbing until it hits 280, but never pegs out completely. The car builds coolant pressure, the coolant is circulating, and it is showing no signs of overheating or head gasket failure. Radiator hoses warm up properly and there is no oil in the coolant.

I thought the sender may be mismatched to the gauge so I started there, since the sender is the only thing I replaced on that circuit. I get correct ohm readings at operating temperature (150 ohms +/- 10 ohms) from between both the terminal and the sender housing and the terminal and the block. This demonstrates that it is grounded and functioning properly. So I am getting a 280 temp reading at the gauge with 150 ohms which is not correct. Research says that 150 ohms should put me in the 170 degree range (or thereabouts). Why 150 ohms at the sensor gives me a 280 degree reading is beyond me, but indicates the gauge is to blame.

I then tested the gauge. When removing the feed wire from the sensor it drops to 100. Grounding it pegs it out. So rudimentary tests indicate the gauge is functioning. I don't have a potentiometer to conduct a test of the gauge and may have to buy one, as all I can come up with is the gauge is incorrect or improperly grounded.

Is it time to pull the gauge cluster and confirm ground? If the 90 ohm resistor on the back of the gauge is fried, would that give me an errant high or low reading? Why would this happen when I've never had issues before but when I replaced the sender I have issues? Logic dictates since the sender is the only item replaced, the sender is to blame. However, the sender but it tests out properly, or close enough that I am ok with it.

Advice?

 

To start with what is a TU65, thats not an OE number and have no way of confirming if you have the correct sender. Read this link

http://willcoxcorvette.com/instructi...Revised_R3.pdf

 

Everyone that posts a LINK to Willcox always brings up the same thing....merely a "home page", which has NO LINK at all......so what's up with that??

 

This problem comes up often with replacement temp sending units....the replacements often are not calibrated correctly. My guess is that the replacement unit is the issue, especially if the broken one worked fine before, not the gauge. I would try a different replacement sending unit first before looking elsewhere. I believe corvette vendors sell the proper sending unit for a 79. Where did you get the replacement?

http://www.ecklerscorvette.com/corve...3tfBoClwbw_wcB

When I rebuilt my 78 L-82 in 2014, I replaced the 882 heads with AFR aluminum heads and the thread size hole for the OEM sending unit was smaller than the GM temp sending unit. I was aware of this issue with replacement sending units so I had the AFR head hole tapped to accommodate my OEM temp sending unit...glad I did......works perfectly like it always has since day one when the car was new.

 

Hi MelWff,
TU65 is a 'WELLS' number.
I have a TU5 in my 74 and it works correctly.
Pete.

 

79/80 Temperature sending unit is different than 72-78...Gauge on earlier C3's reads 200 degrees at 12 O'clock position...220 degrees @ 12 O'clock on 79/80 which requires a different sending unit

 

My 72 is 210* at the 12 O'clock position.