Temp Sending Unit vs Resistor - Team Camaro Tech
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post #1 of 6 (permalink) Old Aug 10th, 04, 05:05 PM Thread Starter
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*Figured I would move this to electical....

Hi All
I added repo console gauges to my '68 convertible this year. I am having issues with the temp gauge in that it is reading on the high side. At about 180 degrees (checked in several spots with with a infrared thermometer) the needle on the gauge is just about in the red. I want to bring the needle down so that it reads about 1/4 of the way up on the face of the gauge instead of almost past 3/4. I used an AC Delco sending unit I bought from Rick's. Is there way to add a resistor in line to bring down the reading - or - should I just try a different sender. If I use a different sender, what brand would work (i. e. read correctly) with repo console gauges.

Thanks!!

Dan
'68 Convertible

Dan
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post #2 of 6 (permalink) Old Aug 11th, 04, 11:26 AM
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Follow this link. Go to the Technical Section and then to the F.A.Q. Under that there are 2 articles called "Why isn't my temperature gauge accurate?" You will find instructions and diagrams to make the gage read cooler.
http://www.lectriclimited.com/mainpage.htm
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post #3 of 6 (permalink) Old Aug 11th, 04, 10:22 PM
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An infared reads the surface of the cooling system. The surface is in contact with cool air and is much cooler than the water inside.

I forget how much off infared is when reading water temp but I'd say it's 20 degrees low anyway.
If you probe the radiator cap side with a real thermometer that side is allready cooled and should read 10 to 20 deg cooler than the hot side.

I'd calibrate by putting the sender in hot water and reading the water temp with a thermometer, I think a candy thermometer can be bought in the supermarket with proper range.
David

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post #4 of 6 (permalink) Old Aug 12th, 04, 02:16 AM Thread Starter
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Thanks David:
I also used a thermometer with a probe stuck inside the radiator to test the temp. I've never hd any temp problems with my car so I'm thinking the temp is fine, it's just the guage that reads way too high - a common problem from what I've been researching....

Dan
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post #5 of 6 (permalink) Old Aug 12th, 04, 09:19 AM Thread Starter
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Ok. I'm thinking of going the resistor route to calibrate my temp. gauge more accurately, I found an old post regarding just this topic in case anyone is interested.

Temp"]https://www.camaros.net/forum/ultimatebb.php?ubb=get_topic;f=10;t=000816#000000]Temp Gauge Resistor[/URL]


My question is, do I just wire the resistor in line with the sending unit? Would the gauge of the wire need to be changed because I am adding a resisitor here?? In other words, can I just "simply" splice the sending unit wire just short of the actual sending unit itself and wire in the resistor there?? I'm thinking of using a 90 ohm resistor to bring down the gauge reading..

Dan
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post #6 of 6 (permalink) Old Aug 12th, 04, 05:43 PM
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Go to radio shack and buy a 500 ohm variable resistor. Rig up a spice to stick it inline temporarily. That will give you a way to find out if it will work or not. The linear resistance will essentially offset the reading, but not scale it.

Keep in mind that temperature guages are not linear. The sending unit is a non-linear resistive temperature sensor. So the guage face isn't linear either.

If this is a factory guage, there may be a shunt resistor bolted accross the terminals on the back. If so, you may be able to get another value to calibrate it.

Measure your sensor's resistance at normal outdoor temps. Then drop it in boiling water. Once it's boiled a while, measure the resistance. Write down the two values. Then use your 500 ohhm pot to simulate those resistances while reading the guage. Adjust the pot until the guage reads appropriately (like 212*). Take the difference to estimate the series offset.

[ 08-13-2004, 02:51 PM: Message edited by: dnult ]

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