Inaccurate 67 big block temp gauge

[Robert M.]

When my 67 427/435 reaches normal operating temperature, the temp gauge is pegged full hot to the right. I have checked the actual temperature with an IR gun and it is at 190 degrees. My understanding is the sending unit is bad if the gauge pegs cold. Is my gauge bad or could the sending unit still be the problem? Thanks

[Steve G.]

Re: Inaccurate 67 big block temp gauge

I can't recall which direction this works, but, key on, wire off the sender the gauge should read to one extreme, I think it's cold. Ground the wire and the gauge should go to the opposite extreme. If it does that the gauge is usually okay and the sending unit is suspect.

There are a lot of poor quality sending units out there. In the aftermarket they sold one number that covered a bunch of oe numbers, the idea being that the calibration was "close enough". There's a vendor out there, name eludes me, that claims his sending units are bang on for the application.

One other thing is that they caution not to wrap the sending unit in teflon tape when installing it as it can affect the quality of grounding. If you look at some of the better quality and oe sending units that require a ground you will see that they have a ring of sealant on the threads, but up from the bottom about 1/4". These provides that the lower threads maintain a good ground and the sealing is done above them.

[John H.]

Re: Inaccurate 67 big block temp gauge

I can't recall which direction this works, but, key on, wire off the sender the gauge should read to one extreme, I think it's cold. Ground the wire and the gauge should go to the opposite extreme. If it does that the gauge is usually okay and the sending unit is suspect.

One other thing is that they caution not to wrap the sending unit in teflon tape when installing it as it can affect the quality of grounding.

That's not so, as we have documented many times before; the razor-sharp thread form of NPT threads ensures good electrical contact regardless of the type of sealer used, and the usual sealers (Teflon paste, etc.) have no effect. Even if they did, they would make the gauge read LOWER, not higher. Click on the file below for a summary of our work on the temp gauge circuit.


Temp Sending Unit and Gauge-CF-Members.pdf

[Joe R.]

Re: Inaccurate 67 big block temp gauge

Robert,

Your problem probably lies in the sending unit in the block. One unit from a parts store will read about 40 degrees too high
A new one should be less than $20.00 so get a couple for insurance.

JR

[Steve G.]

Re: Inaccurate 67 big block temp gauge

That's not so, as we have documented many times before; the razor-sharp thread form of NPT threads ensures good electrical contact regardless of the type of sealer used, and the usual sealers (Teflon paste, etc.) have no effect. Even if they did, they would make the gauge read LOWER, not higher. Click on the file below for a summary of our work on the temp gauge circuit.


[ATTACH]56664[/ATTACH]

I've always been skeptical of it as well, but that is taken right from the 63 Corvette factory service manual (pg 12-14, par 2 under testing) and I've seen it on notices packed in with new sending units. Given that I couldn't remember what direction an open circuit moved the needle, I thought it was worth adding. On looking up the page number I see that it is in fact cold/open circuit.