Engine temp sending unit backwards? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**K10-Kansas** · 12-17-2025, 01:05 AM

***EDIT*** I had my multimeter on the wrong setting so I uploaded new pictures and retyped the post to avoid confusion.

My sending unit is showing 128 ohms when the engine is 228 degrees. Davepl posted a good chart showing that at that temp I should be getting 65 or 70 ohms. What ohm rating are you getting when fully warmed up and if your sender is working well, where did you get it from?

**pjmoreland** · 12-17-2025, 04:07 AM

The resistance of the sending unit should go down as the engine temperature increases. The wire needs to be disconnected from the sender when measuring the sender resistance. If you leave the wire attached, then the resistance measurement will not be valid.

HO455 · 12-17-2025, 09:51 AM

And your guage is upside too!

**kwmech** · 12-17-2025, 12:33 PM

Is the sender for a gauge or a light

**K10-Kansas** · 12-18-2025, 04:04 PM

Quote:

Originally Posted by kwmech

Is the sender for a gauge or a light

Oh heck, I hope there aren't 2 different types of sending units. I'll check on mine and see which it is for, thanks.

HO455 · 12-18-2025, 06:43 PM

Quote:

Originally Posted by K10-Kansas

Oh heck, I hope there aren't 2 different types of sending units. I'll check on mine and see which it is for, thanks.

There are two kinds of sensors. When testing the warning light senders they will show as open below the designated temperature of the sender and zero resistance above that temperature. They are simply a temperature controlled switch.

And as stated above remove the wire before checking with a meter.

One thing that can affect how the guage reads is how the needle is positioned on the shaft. Over the 50 plus years since new many times the needle has slipped on the shaft and that affects the needle position at normal running temperature.

**MikeB** · 12-18-2025, 10:13 PM

Quote:

Originally Posted by K10-Kansas

Oh heck, I hope there aren't 2 different types of sending units. I'll check on mine and see which it is for, thanks.

All I know is you can buy a three of them, and all would be a little different. And probably none exactly like your original. Even the originals from the factory may have varied and were paired with the appropriate shunt resistor for the gauge.

**MikeB** · 12-17-2025, 06:19 PM

Good luck getting a gauge/sender combo to work. I've spent many hours on mine, and finally got it close after playing with several different shunt resistors across the top terminal and ground. The pointer doesn't start moving until ~150-160 degrees. Then it points close to straight up at around 200, and really starts moving fast after that. So it could be worse.

One of these days I'm gonna try an Autometer gauge/sender combo if I can make one fit the instrument cluster. This may be of some help, but all temp senders are not equal. https://67-72chevytrucks.com/vboard/...d.php?t=689065

Until then, here's a gauge I can depend on.

**Steeveedee** · 12-17-2025, 06:38 PM

I have a mechanical Autometer Phantom® mechanical water temp gauge. I hung that one under the dash because I didn't want to stand on my head to take the factory gauge out. The needle was going everywhere. Probably that resistor?

**MikeB** · 12-18-2025, 02:07 PM

Quote:

Originally Posted by Steeveedee

I have a mechanical Autometer Phantom® mechanical water temp gauge. I hung that one under the dash because I didn't want to stand on my head to take the factory gauge out.

Yeah, it's always fun to pull out the panel without removing the seat. If only I was 4" shorter and 30 lbs lighter!

**'68OrangeSunshine** · 12-26-2025, 06:34 PM

Quote:

Originally Posted by Steeveedee

I have a mechanical Autometer Phantom® mechanical water temp gauge. I hung that one under the dash because I didn't want to stand on my head to take the factory gauge out. The needle was going everywhere. Probably that resistor?

Same here.
I re-engined my '71 GMC Jimmy K/1500 in 1999 with a Mr. Goodwrench 350 crate. The new electric Water Temp sender [from LMC -- I think] didn't work. I was told I needed to add some Resistors inline with the wire. That didn't work, so went with a mechanical AutoMeter gauge in an underdash kneescraper mount. Works great.

**Steeveedee** · 12-26-2025, 07:29 PM

^ Yep, Bourdon tube is pretty reliable tech.

**K10-Kansas** · 12-18-2025, 04:15 PM

Quote:

Originally Posted by MikeB

Good luck getting a gauge/sender combo to work. I've spent many hours on mine, and finally got it close after playing with several different shunt resistors across the top terminal and ground. The pointer doesn't start moving until ~150-160 degrees. Then it points close to straight up at around 200, and really starts moving fast after that. So it could be worse.

One of these days I'm gonna try an Autometer gauge/sender combo if I can make one fit the instrument cluster. This may be of some help, but all temp senders are not equal. https://67-72chevytrucks.com/vboard/...d.php?t=689065

Until then, here's a gauge I can depend on.

Lol, I guess I could wire up the multimeter and and watch to make sure I don't go lower than 120 ohms.

**dmjlambert** · 12-21-2025, 02:01 PM

Just an observation, in your first post the picture shows 128K ohms, not 128 ohms.
davepl gave us a chart of the exact readings from his temp sending in this post: https://67-72chevytrucks.com/vboard/...53#post7680753
This is about where the temperature gauge sits when fully warm

**K10-Kansas** · 12-21-2025, 10:53 PM

Quote:

Originally Posted by dmjlambert

Just an observation, in your first post the picture shows 128K ohms, not 128 ohms.
davepl gave us a chart of the exact readings from his temp sending in this post: https://67-72chevytrucks.com/vboard/...53#post7680753
This is about where the temperature gauge sits when fully warm

Thanks, I see he was getting about 70 ohms at 200 degrees. Graphs I've seen show 80 to 90 ohms at 200. My gauge doesn't start moving until I hit 120 ohms so it makes sense that my gauge just reads cold all the time. I wonder if I can change out the resistor plate on the back of my gauge.

Are you sure about the 128,000 ohms? My multimeter is showing 0.128 kiloohms which is 128 ohms. But, I'm new to all of this so I may be wrong.

**dmjlambert** · 12-21-2025, 11:31 PM

My mistake, I didn’t see the decimal point.

**K10-Kansas** · 12-23-2025, 01:53 PM

I bought a potentiometer so I could test the LMC gauge. My numbers are coming up close to what Davepl showed in his GM chart so my gauge is good but my sending unit is not showing numbers anywhere near what Dave found when bench tested a sending unit with a multimeter.

My gauge is showing that my engine is 170 degrees but a temp gun shows the engine is 220 degrees. My sending unit is not a good match for the reproduction stock gauges and isn't in line with what GM shows on their charts.

I'll have to see if I can get a better sending unit or replace the resistor on the back of my gauge with a lower ohm resistor.

**1970cstblazer** · 12-25-2025, 09:17 AM

The common currently available (last 20 years) sending units are trash. Only somewhat accurate units are old NOS AC or Lectric Limited branded sending units.

**MikeB** · 12-25-2025, 02:02 PM

That's close to how mine works now with an aftermarket sender. Mine's either AC Delco or whatever NAPA sold me. I can't remember the shunt resistor value.

My truck's operating temp is typically in the 190-200 range according to my engine compartment thermometer. So if the pointer is between 11:30 and 12:30, all is well. Still want to check out an AutoMeter gauge someday to see if I can make it fit. I've used the Sport Comp series electric gauges on two vehicles, and they work well.

**dmjlambert** · 12-26-2025, 02:16 AM

This is the one I use, https://www.amazon.com/ACDelco-G1852...dp/B000C9SM22/, and I had it turned and re-threaded to 3/8 NPT by forum member A1971Blazer, details here in this thread, https://67-72chevytrucks.com/vboard/...d.php?t=703388

**Richard** · 12-26-2025, 04:16 AM

If you have the new sending unit and reproduction gauge. I do not think the resistor on gauge is required. Maybe try that.

12-17-2025, 01:05 AM	#1
K10-Kansas Senior Member Join Date: Nov 2022 Location: Kansas City Posts: 572	Engine temp sending unit backwards? *EDIT* I had my multimeter on the wrong setting so I uploaded new pictures and retyped the post to avoid confusion. My sending unit is showing 128 ohms when the engine is 228 degrees. Davepl posted a good chart showing that at that temp I should be getting 65 or 70 ohms. What ohm rating are you getting when fully warmed up and if your sender is working well, where did you get it from? Attached Images __________________ 1971 C10 body. 76 4x4 chassis cut to short frame. Crate 350 2nd Gen small block. 4BBL Holley 4160. 4L60 transmission. ~ James Last edited by K10-Kansas; 12-18-2025 at 04:02 PM.

12-17-2025, 04:07 AM	#2
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 7,023	Re: Engine temp sending unit backwards? The resistance of the sending unit should go down as the engine temperature increases. The wire needs to be disconnected from the sender when measuring the sender resistance. If you leave the wire attached, then the resistance measurement will not be valid.

12-17-2025, 09:51 AM	#3
HO455 Registered User Join Date: Feb 2016 Location: Portland Oregon Posts: 12,406	Re: Engine temp sending unit backwards? And your guage is upside too! Attached Images __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

12-17-2025, 12:33 PM	#4
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,941	Re: Engine temp sending unit backwards? Is the sender for a gauge or a light

12-17-2025, 06:19 PM	#8
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 4,035	Re: Engine temp sending unit backwards? Good luck getting a gauge/sender combo to work. I've spent many hours on mine, and finally got it close after playing with several different shunt resistors across the top terminal and ground. The pointer doesn't start moving until ~150-160 degrees. Then it points close to straight up at around 200, and really starts moving fast after that. So it could be worse. One of these days I'm gonna try an Autometer gauge/sender combo if I can make one fit the instrument cluster. This may be of some help, but all temp senders are not equal. https://67-72chevytrucks.com/vboard/...d.php?t=689065 Until then, here's a gauge I can depend on. Attached Images __________________ Mike 1969 Custom/10 LWB -- owned for 37 years. 350/TH350, 3.08 posi, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. Hedman stainless headers. Old Air installation in progress. 1982 Custom Deluxe 10 SWB -- converted from 250-six to roller cam 350 w/ Vortec heads -- sold 1981 C10 Silverado LWB, 305, TH350C -- sold, but wish I still had it! 1969 C10 (not the current one) which I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 26 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!

12-17-2025, 06:38 PM	#9
Steeveedee Ram-A-Lam-A-Ding-Dong Join Date: Jan 2013 Location: Simi Valley, CA Posts: 11,899	Re: Engine temp sending unit backwards? I have a mechanical Autometer Phantom® mechanical water temp gauge. I hung that one under the dash because I didn't want to stand on my head to take the factory gauge out. The needle was going everywhere. Probably that resistor? __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

12-26-2025, 07:29 PM	#12
Steeveedee Ram-A-Lam-A-Ding-Dong Join Date: Jan 2013 Location: Simi Valley, CA Posts: 11,899	Re: Engine temp sending unit backwards? ^ Yep, Bourdon tube is pretty reliable tech. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

12-21-2025, 02:01 PM	#14
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Engine temp sending unit backwards? Just an observation, in your first post the picture shows 128K ohms, not 128 ohms. davepl gave us a chart of the exact readings from his temp sending in this post: https://67-72chevytrucks.com/vboard/...53#post7680753 This is about where the temperature gauge sits when fully warm

12-21-2025, 11:31 PM	#16
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Engine temp sending unit backwards? My mistake, I didn’t see the decimal point.

12-23-2025, 01:53 PM	#17
K10-Kansas Senior Member Join Date: Nov 2022 Location: Kansas City Posts: 572	Re: Engine temp sending unit backwards? I bought a potentiometer so I could test the LMC gauge. My numbers are coming up close to what Davepl showed in his GM chart so my gauge is good but my sending unit is not showing numbers anywhere near what Dave found when bench tested a sending unit with a multimeter. My gauge is showing that my engine is 170 degrees but a temp gun shows the engine is 220 degrees. My sending unit is not a good match for the reproduction stock gauges and isn't in line with what GM shows on their charts. I'll have to see if I can get a better sending unit or replace the resistor on the back of my gauge with a lower ohm resistor. Attached Images __________________ 1971 C10 body. 76 4x4 chassis cut to short frame. Crate 350 2nd Gen small block. 4BBL Holley 4160. 4L60 transmission. ~ James Last edited by K10-Kansas; 12-23-2025 at 02:04 PM.

12-25-2025, 09:17 AM	#18
1970cstblazer All stock and staying that way Join Date: Jul 2003 Location: Elkland, PA Posts: 2,500	Re: Engine temp sending unit backwards? The common currently available (last 20 years) sending units are trash. Only somewhat accurate units are old NOS AC or Lectric Limited branded sending units. __________________ 1970 K/5 Blazer CST 4WD, TOTM winner December 2024! Medium Bronze poly, 93k ACT. miles, 350, 4 speed, rear positraction, 16.5" x 8.25" HD Kelsey Hayes wheel option, tilt, tach, vacuum, AM/FM, manual throttle...Dad ordered and purchased new 4/70. Frame off restored 1970 K20, To be a clone of my Grandfather's K20, Dark Blue poly, 350, 4 speed, 16.5" X 8.25" Kelsey Hayes wheel option, tilt, tach, vacuum, AM radio. Latest project truck 1972 Cheyenne Super 20 2WD, Dark Blue/White, 90K ACT. miles, 402, TH400, 4.10 open, tilt, tach, vacuum, A/C, AM/FM, manual throttle.. A mostly original paint never rusted Texas survivor... 2017 Sierra 1500 SLT 4WD, Black, 51k miles, 5.3, 6L80E, 3.42 LS, 20" polished wheels, everything but moonroof and 6.2... 2019 Canyon SLT 4WD, White, 71k miles, 3.6, 8L45, 3.23 LS, 18" wheels

12-25-2025, 02:02 PM	#19
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 4,035	Re: Engine temp sending unit backwards? That's close to how mine works now with an aftermarket sender. Mine's either AC Delco or whatever NAPA sold me. I can't remember the shunt resistor value. My truck's operating temp is typically in the 190-200 range according to my engine compartment thermometer. So if the pointer is between 11:30 and 12:30, all is well. Still want to check out an AutoMeter gauge someday to see if I can make it fit. I've used the Sport Comp series electric gauges on two vehicles, and they work well. __________________ Mike 1969 Custom/10 LWB -- owned for 37 years. 350/TH350, 3.08 posi, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. Hedman stainless headers. Old Air installation in progress. 1982 Custom Deluxe 10 SWB -- converted from 250-six to roller cam 350 w/ Vortec heads -- sold 1981 C10 Silverado LWB, 305, TH350C -- sold, but wish I still had it! 1969 C10 (not the current one) which I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 26 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!

12-26-2025, 02:16 AM	#20
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 4,051	Re: Engine temp sending unit backwards? This is the one I use, https://www.amazon.com/ACDelco-G1852...dp/B000C9SM22/, and I had it turned and re-threaded to 3/8 NPT by forum member A1971Blazer, details here in this thread, https://67-72chevytrucks.com/vboard/...d.php?t=703388

12-26-2025, 04:16 AM	#21
Richard Senior Member Join Date: Dec 1999 Location: Southern California Posts: 3,346	Re: Engine temp sending unit backwards? If you have the new sending unit and reproduction gauge. I do not think the resistor on gauge is required. Maybe try that. __________________ Richard 1972 K10 Custom Deluxe SWB Fleetside My build https://67-72chevytrucks.com/vboard/...d.php?t=800746



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