74 Temp Guage reading hot
Thread Starter
Intermediate
Joined: Sep 2023
Posts: 45
Likes: 20
74 Temp Guage reading hot
I have replaced the sender and engine is running at 180 after driving 20 miles but the temp guage is running between 210 and the red line. I have validated the engine temp with a manual guage. everything is working but my temp guage reads warmer. Any ideas will be appreciated. Thanks, Chris
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Overheating at idle is an Air Circulation issue.
Overheating at speed is a Coolant Circulation issue.
But in your case, I suspect a poor GRD at the sender.
When the sender can not fully contact a good GRD source, the gauge needle reads at or near max.
It's getting full 12v power but a very poor GRD.
(When a gauge does not move, its usually lack of voltage)
The gauge needs ohm resistance to work properly and register in a normal sweep of the needle. The resistance is through a ground source.
So how does a sending unit not receive an excellent GRD?
By poor contact between the senders' threads and the engine block or cyl head or Intake.
You just installed a new sender. Did you put a sealant or pipe tape on ALL the threads?
Overheating at speed is a Coolant Circulation issue.
But in your case, I suspect a poor GRD at the sender.
When the sender can not fully contact a good GRD source, the gauge needle reads at or near max.
It's getting full 12v power but a very poor GRD.
(When a gauge does not move, its usually lack of voltage)
The gauge needs ohm resistance to work properly and register in a normal sweep of the needle. The resistance is through a ground source.
So how does a sending unit not receive an excellent GRD?
By poor contact between the senders' threads and the engine block or cyl head or Intake.
You just installed a new sender. Did you put a sealant or pipe tape on ALL the threads?
Thread Starter
Intermediate
Joined: Sep 2023
Posts: 45
Likes: 20
Not all the threads, but lower half. It was a cheapy from Auto Zone. I wonder if I should try the Delco one from Advanced auto. It does work as intended and goes up as the temp comes up it just reads warmer than the engine.
Last edited by chriscovell; May 20, 2024 at 03:51 PM.
Instructor
Joined: Aug 2023
Posts: 206
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From: San Antonio, TX
I had purchased an AC Delco sender unit and it always pegged hot even after 10 or so minutes when I knew there was no way it would be hot. I ended up getting one from Oreilly, I believe it was the MaterPro, popped it in and my gauge started reading accurately.
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
No sealer on any portion that goes into the casting.
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I had an issue with a sender I got from Zip Corvette. Put my original one back in and all is fine.
I would suggest you try another sender.
I would suggest you try another sender.
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Race Director
Joined: Mar 2017
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From: Virginia
Nice find! I'm saving this one...
Isn't there something the OP can do with the resistor on the back of the gauge? I have not found a spec sheet for any one-wire senders being sold now, so there's really no telling if the replacement sender is correct, or has the same issue.
Isn't there something the OP can do with the resistor on the back of the gauge? I have not found a spec sheet for any one-wire senders being sold now, so there's really no telling if the replacement sender is correct, or has the same issue.
Instructor
Joined: Aug 2023
Posts: 206
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From: San Antonio, TX
I would recommend following the steps posted a long time ago but still hold true.
Le Mans Master
Joined: May 2007
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From: North tonawanda NY
I was told the sender on my 74 used a different resistance value than most auto store senders. I questioned my reading on my car since the car had multiple owners. I had swapped engine but moved the original from the l48. anyway to rule it all out and to verify if I had an issue I installed a $25 mechanical gauge that also has a digital readout and found the gauge on my dash is in the ballpark but not in sinc with my new gauge. I have a second temp gauge and sender I bought for my transmission temp and I verified those two new gauges and senders are calibrated the same.
Ive since installed more accurate duplicate gauges for all my old stock gauges in a holder mounted at the cowl of my high rise hood. Ive left the old gauges installed and intact between the 2 I feel I have my bases covered lol.
I will add my original temp gauge is in my intake manifold and the aftermarket one is mounting in one of the heads. so...
Ive since installed more accurate duplicate gauges for all my old stock gauges in a holder mounted at the cowl of my high rise hood. Ive left the old gauges installed and intact between the 2 I feel I have my bases covered lol.
I will add my original temp gauge is in my intake manifold and the aftermarket one is mounting in one of the heads. so...
Race Director
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From: Virginia
When I was researching replacing the sender and all that, I found various takes on the the resistor. Essentially what I boiled it down to (pun maybe intended) was that the resistor could ultimately cause a false reading. While the gauge would show the running temperature, the resistor could then be inaccurately telling the gauge if it's hot or cold, then running the risk of actually over heating and causing further issues down stream.
I would recommend following the steps posted a long time ago but still hold true.
I would recommend following the steps posted a long time ago but still hold true.
Here's the procedure to diagnose temp gage problems:
Tests
A. Disconnect the green wire at the sending unit and turn on the ignition switch. The gauge hand should stay against the left side stop pin.
B.Ground the green wire disconnected from the sending unit. With the ignition switch still "ON," the gauge hand should swing across the dial to the right stop pin.
Corrective Measures
If the gauge hand does not stay to the left as in test "A", either the wire is grounded between the dash unit and the engine unit or the dash unit is defective.
Test further by disconnecting the dark green sending unit wire at the gauge. Turn on the ignition switch. If the gauge hand stays on the left hand stop pin, replace the disconnected wire. But, if the gauge hand still moves, replace the dash gauge.
If the gauge hand does not swing across the dial as in test "B", there is an open circuit in the wire between the sending unit and gauge, the gauge is defective, or current is not reaching the dash gauge.
Test further by grounding the sending unit terminal (dark green wire) of the dash gauge and turn "ON" the ignition switch. If the gauge hand now moves, replace the disconnected wire. If the gauge hand still does not move, connect a test lamp to a ground and to the power (pink wire) terminal of dash unit. If the lamp does not light, test the wire between the ignition switch and the dash unit by connecting a test lamp to the "Accessory" terminal at the ignition switch and to a ground. The test lamp should burn.
If the gauge hand operates correctly as in tests "A" and "B" but the gauge does not indicate temperature correctly, either the sending unit is defective or the dash gauge is out of calibration. Replace sending unit one of known accuracy. If gauge reading is still incorrect, replace dash unit.
If the gauge hand is at maximum at all times and test "A" and "B" indicate that the wiring and the dash unit are in condition, the sending unit is bad and needs replacement.
If the gauge hand will not move, the dash unit is bad, or incorrectly installed. Correct the installation or replace the gauge unit.
Tests
A. Disconnect the green wire at the sending unit and turn on the ignition switch. The gauge hand should stay against the left side stop pin.
B.Ground the green wire disconnected from the sending unit. With the ignition switch still "ON," the gauge hand should swing across the dial to the right stop pin.
Corrective Measures
If the gauge hand does not stay to the left as in test "A", either the wire is grounded between the dash unit and the engine unit or the dash unit is defective.
Test further by disconnecting the dark green sending unit wire at the gauge. Turn on the ignition switch. If the gauge hand stays on the left hand stop pin, replace the disconnected wire. But, if the gauge hand still moves, replace the dash gauge.
If the gauge hand does not swing across the dial as in test "B", there is an open circuit in the wire between the sending unit and gauge, the gauge is defective, or current is not reaching the dash gauge.
Test further by grounding the sending unit terminal (dark green wire) of the dash gauge and turn "ON" the ignition switch. If the gauge hand now moves, replace the disconnected wire. If the gauge hand still does not move, connect a test lamp to a ground and to the power (pink wire) terminal of dash unit. If the lamp does not light, test the wire between the ignition switch and the dash unit by connecting a test lamp to the "Accessory" terminal at the ignition switch and to a ground. The test lamp should burn.
If the gauge hand operates correctly as in tests "A" and "B" but the gauge does not indicate temperature correctly, either the sending unit is defective or the dash gauge is out of calibration. Replace sending unit one of known accuracy. If gauge reading is still incorrect, replace dash unit.
If the gauge hand is at maximum at all times and test "A" and "B" indicate that the wiring and the dash unit are in condition, the sending unit is bad and needs replacement.
If the gauge hand will not move, the dash unit is bad, or incorrectly installed. Correct the installation or replace the gauge unit.
My 80's temp sender is accurate to the gauge, for now. My Bubba solution is to use 2 and 3 wire senders instead, and my own circuit, to read the temperature, pressure, and other things. I can find the spec sheets for those, and test that what I get is accurate.
Not much that helps the OP. Perhaps try a Lectric Limited sender, and let us know if that fixes it? I'm done buying undocumented parts for my own cars, though.
Le Mans Master
Joined: Jan 2001
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chriscovell
The adjustable resistor works like a dream to dial in our temp gauge.
You did not state why you replaced the original sending unit???
If you still have it, and it was just reading "off", then you can try reinstalling and dialing it in with the adjustable resistor. Here is my story. Willcox is not around anymore but other vendors also sell them.
https://www.corvetteforum.com/forums...-question.html
The adjustable resistor works like a dream to dial in our temp gauge.
You did not state why you replaced the original sending unit???
If you still have it, and it was just reading "off", then you can try reinstalling and dialing it in with the adjustable resistor. Here is my story. Willcox is not around anymore but other vendors also sell them.
https://www.corvetteforum.com/forums...-question.html