# Temp gauge calibration



## Getyourgoat (Jul 1, 2014)

My 1967 GTO has a 1972 Grand Prix 455 big block engine.
My temp gauge normally runs around 235 degrees according to the original gauge.
I have a feeling this is not accurate. The mid-range mark on the temp gauge is 210 degrees.
I’m concerned that the temp sending unit is not calibrated with the gauge.
How can I verify the true temp of the engine?
How can I calibrate the 1972 sending unit to the 1967 gauge?
What is the normal operating temp of a 455?


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## geeteeohguy (Feb 2, 2008)

Get a hand held pyrometer and shoot it at the engine, radiator, water outlet, etc. Very Accurate. The correct sending unit for your engine can be gotten from Lectric Limited. Regular parts-house sending units are the wrong resistance, and will have your gauge reading 25-40 degrees high. Normal operating temp for a 455 is 180-215 degrees. If you actually are running 235, it is too hot.


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## Getyourgoat (Jul 1, 2014)

Thanks for your reply. Can you recommend a good pyrometer?


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## geeteeohguy (Feb 2, 2008)

Nope. Pretty much anything will do, as long as it's accurate. I'd research it on line. See what HVAC techs use. We have a generic one at the shop that works just fine.


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## Getyourgoat (Jul 1, 2014)

I got a new sending unit in the mail today from Lectric Limited. I'm in the middle of doing a disk brake conversion but soon as I finish that up (hopefully this weekend) I will install the new sending unit. I will post my resuts then. Thanks for the input.


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## geeteeohguy (Feb 2, 2008)

The 389 in my '65 went from an indicated 205 degrees on the gauge to 185 (actual temp) after I yanked the NAPA sensor and installed the Lectric Limited one. Made _me _happy. Hopefully, your results will be the same!


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## PontiacJim (Dec 29, 2012)

Getyourgoat said:


> Thanks for your reply. Can you recommend a good pyrometer?


Try AutoZone if you have any of these in your area. I picked up a unit for testing and it was fairly inexpensive. Unfortunately, I have never used it. It was a "guy" moment when I saw a tool I thought would be a good addition to the tool box! hahaha. But at least I have it, right?

I suppose you might check around at any of your local auto parts stores and they will offer something that will work.


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## Getyourgoat (Jul 1, 2014)

Finally, I got my front disk brake conversion completed. Brakes work like a dream.
I installed the new coolant temp sending unit and now the engine temp appears to be running normal. I never did get a pyrometer and check the engine temp that way vs what shows on the gauge. I still plan to, however; the engine seems to be running normal and no coolant boil overs yet so I think I'm probably OK. I will post pyrometer results if I ever get around to it.


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## 68_Goat (Nov 7, 2008)

Just a word about non contact pyrometers: Most of them become less accurate the further you get from your intended target, as they are "conical", which is to say the laser is only a guide to tell you where you're pointing it. if you are shooting a thermostat housing from 12", it'll be much more accurate than 48" as it'll average everything within the target, which is getting larger the further you get from the target. Also you're measuring the surface temperature of whatever you're pointing it at, so there may be a little error, but if you're looking for comparative and not absolute numbers, it's a good tool to have. Just try to always read as close as you can to the intended target to minimize error. Then, lastly there is emissivity which is the reflectance of different materials, which can and will effect readings. Perfect is 1.0 which is a "black body" and is as good as it gets, downward with different materials such as chrome, polished aluminum, etc. If you're ever in doubt, use a small pc. of Scotch brand 33 electrical tape, which is known to be .95 emissivity in the IR world, putting it on the intended target and shooting it (as close a possible to minimize averaging from outside the tape).


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## gtoearl (Dec 21, 2009)

I am so happy I ran across this post. My 66 GTO with rally gauges has been giving me fits for sometime. I've purchased at least three different sending units and all seem to not read accurately. When I restored the Dash I had all the gauges repaired. The temperature gage seemed to work accurately for a few start ups . Then all of a sudden it started reading at about 235°. I know this was not correct when shooting different parts of the engine and hoses with my temperature gun. So I installed an aftermarket temperature gauge. Lo and behold this gauge doesn't seem to be accurate either. At any rate, I purchased The Lectric Limited sender and will install it upon arrival. I was all set to yank the dash and retrieve the temperature gage and start over again......


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## histoy (May 22, 2019)

gtoearl, did the Lectric Limited sending unit fix the accuracy of your temp gauge? I'm having issues like you experienced, so after reading this post I checked out the Lectric Limited catalog. It shows a picture of the unit and it states that the connection is correct, however, it has a spade connector similar to the light type. My car has a slip on connector, so I'm somewhat hesitant to order theirs. What type of connector was on the unit you received? Thanks!


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## gtoearl (Dec 21, 2009)

Yes my new sending unit fixed my issue. Attached you will find a picture of the sending unit and the connector that is part of the stock harness for rally gauges. Electric Limited states that their sending units are calibrated for the rally gauges cluster. I highly recommend their sending units. My gauges were restored and I went through two sending units from other auto suppliers before receiving the electric Limited sending unit. I was so discouraged that I considered taking the gauge out of the dash and sending it back to the outfit that restored them. As you know it’s a lot of work to take those gauges out of the dash. Highly recommend them. Hope that answers your question


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## histoy (May 22, 2019)

Thank you! That's how the sending unit is currently pictured in their catalog. I'll order one today.


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## histoy (May 22, 2019)

I received my LL sending unit today and my experience was nothing like yours. The second sending unit that I removed came from NAPA. It read around 210 degrees when my actual temp at the thermostat housing read 175 degrees with my thermal gun. After I installed the LL sender my gauge never went over a 100 while my engine was operating at normal temp. When I removed the wire from the sender, the gauge stayed the same. When I grounded the wire, the gauge pegged out to the max. I called their tech guys and they wanted to know whether my gauges were original or replacements? I don't know, but they look pretty nice. I explained to the tech what I observed, and he said that info didn't help him at all. He said I needed to run the engine to normal temp and check the Ohm reading at the sender, then call him back and he could tell me if the sender was working properly. I did as he asked and ran the engine to 171 degrees ( I have a 160 thermostat) and the Ohm reading was 131. I called the tech line again and got a different tech. I explained what I found, and he said it sounded unusual. He discussed it with the first guy I talked with. I asked what Ohm reading I should have a 171 degrees, but he said they only have values for 140 & 190. He told me that it's very unusual for their senders not to work correctly. He gave me a return authorization number and told me to send it back at my expense before they would send me one that would work. Right now I'm upset and disappointed with their tech dept. I cleaned up my original sender, which went to 180 degrees, but no higher, and put it back in the engine. When I started the engine it climbed to 140 degrees, then I shut the engine off until I can take the car for a normal drive.


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## gtoearl (Dec 21, 2009)

I’m so sorry for your difficulties. Like I said mine worked fine. But I did get the gauges re-calibrated and restored. I’m wondering if you have a gauge issue. Too bad they wouldn’t pay for the shipping to send it back. If it’s a defective item Maybe they’ll reimburse you once they get it and test it to see if it’s defective or not.... if you have an aftermarket gauge that may be part of the problem. If you have a factory gauge And it’s working correctly, the electric Limited senders are calibrated to the factory gauges. So sorry for your frustration... I’ve been there and it’s not fun... just a thought, have you checked the temp at the thermostat housing with another thermal gun? Once you get two readings that are the same from two different instruments then you have something to reference... If that makes sense... good luck my friend and let me know how it’s working out...


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