The IR probe is probably good. You cannot measure the temp of the water with an open flame nearby. The open frame would create hot air and affect your reading. Turn off the frame and move the kettle to a table and do it again. It IR is a very sensitive instrument.

Measurement from my SDL.

At 82C the resistance is about 330 ohm
At 25C the resistance is about 2000 ohm.

I think you new sensor may be correct.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Yes.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

If the new sensor is correct, then why is it not registering on my gauge? As soon as I put my old temp sensor in, the gauge started working again. What am I missing?

I see that the readings you got are very close to my new sensor. Something is going wrong somewhere...

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1984 300td euro turbo

You introduced too many variables into the diagnostics. If the new thermostat is good then put it back in and start all over. Bottom line is that you need to determine whether you have an overheating problem first by measuring the temp. I do not see you have made the determination.

Also check whether the rad cap is good and is the rad still pressurized after the car cools down. If it does then it may point to a head gasket problem. Take a deep breath and calm down and start all over. PM me if you are stuck. I may or may have an answer but I will do my best.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Ok tomorrow I will put the old thermostat back in then measure with the ir gun the top of the radiator. Will post results.

__________________
1984 300td euro turbo

The easy fix. You can wire the aux fan to come on by the AC system or engine temp. Replace the brass switch on the thermostat housing with a 100°C from MB or other MB supplier. The switch on the drier goes to ground. Wire the 100°C switch to ground the circuit as well.
The two switches work in parallel. When your engine hits 100°C the fan will come on and the temp will fall back to 90°C. The cycle will repeat.
By the way, the switch on the thermostat housing you will replace is there to keep the heater from coming on until the water is warm. Don't want to get a little chill now, do we.

__________________
1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

That would be the wrong answer. The laser is just a pointer; it plays no role in the temp determination. It makes no difference if the laser is reflected, absorbed, sliced or diced.

Let's hope not.

Tangofox. Why do you hope not?

__________________
1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

He is being sarcastic. The temp cannot fall back from 100C to 190C.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

My SDL - I've had mine hard wired "on" for five years now. Ever since a MB Service Manager in AZ said...Helps the A/C too when it's very hot outside.

He was correct. Also keeps the motor cooler. I don't like it when it the motor has to get hot to have it come on...and like said here before the P&P has tons of them if they burn up.

__________________

1986 300SDL 440,xxx

I believe I read somewhere that the aux fan doesn't even come on based on temp. It's the AC that triggers it. If you're low on refrigerant it won't even come on. Please correct me if I'm wrong.

__________________
1984 300td euro turbo

You are correct. The engine temp won't normally trigger the aux fan. The rewire and switch is easy to do.
Re my post #25. That's 90°c, thanks

__________________
1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

I'm trying not to provide conflicting or contradictory advice, but I wouldn't re-replace the thermostat. If you got a new thermostat and installed it correctly (arrow up, correct orientation), then I'd leave that alone and revisit the sensor and the actual temps, and the caps.

With the engine cold, check to ensure the radiator cap is on correctly: push down and twist to get a firm seat.

Make sure the overflow tank cap is on tight.

Since you've got the correct wire ID'd for the gauge, pull that wire and inspect the connector. You want a good clean tight fit. If the electrical connection isn't right, then the reading may be off.

Since you have a spare sensor, you can do a check on your gauge. Get a bowl/cup/vessel of nearly boiling water. Put that in the engine bay. Clip the uninstalled sensor onto the gauge wire, put the sensor in the water, making sure you don't immerse the wire. Check the gauge for close to 100 c. You could verify the water temp with a meat or candy thermometer or a probe thermometer. Give the metal time to absorb the heat.

If that sensor and gauge combo is accurate, install that one. You've now reduced the variables. If it's not accurate, you can remove the other sensor from the cool engine, reheat the water to near boiling and test again. Even if if it's not accurate you'll have a better idea if it's reading high or low. Hopefully not low since it's been saying around 100 c.

Do as much as you can on a cool engine before you try to see if everything works on a hot engine.

Aiming. The visible red laser is colinear with the IR detector.

__________________
RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)