The first rule of thumb in the marine world is never trust old guages, they give you a general idea that is about it. I have seen tachs on a twin engine boat side by side saying 4k rpm and 3.7k rpm, but in reality the engines were turning 3,840.

Just because your 20 year old temp guage says 100C doesn't mean your engine is running at 100C. You need to check the accuricy of the guage before spending tons of money. For all you know the guage could just read 10C high and you may not have a problem at all.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

i believe i read someplace the MB guages do not react to every degree C change more like 5 or 10 degrees...

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

I couldn't agree more with getting your temp checked by some other source. I used an IR Heat Gun and I found my gauge reads 13*C too high! So now when I'm driving around in city traffic in Orlando, my gauge will rise to 110 or so, but I know im my mind that is only 97*C.

Supply voltage will cause incorrect temp guage readings. I'm not sure where the regulator is but its sure to have one close if not in the guage itself.
Just a thought.

Also,
How difficult would it be to remove the sender and place it in boiling water nearby. Make sure there is perfect connection to any extension wire added to perform this test as added resistance will flaw the test. Clean the sender body with a scotchbrite and use a ss hoseclamp to connect the ground. I'd use at least 16ga stranded wire.

Maybe use an electric hotplate sitting on the airfilter or a safe stable area.

Gauge should read close to 100C at boiling.

BTW. Mine runs just above 100C on a hot day by the gauge. Never puked up any coolant. So I don't worry about it.

All right, I'm breaking down and getting an IR thermometer off ebay. I have come around to your all's way of thinking, helped by the fact that my temp needle has started to bounce about 5-10C .

Also I took it on a HIGH speed tune up today and the temp was at 90 or less the whole time, so I'm tending to think it's ok. These benzes love to be abused every once in a while .

I hope my results are good. Will report back with actual vs. displayed temp.

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"I would rather be ashes than dust!
I would rather that my spark should burn out in a brillaint blaze
than it should be stifled by dryrot.
I would rather be a superb meteor,
every atom of me in magnificent glow,
than a sleepy and permanent planet.
The proper function of man is to live, not to exist.
I shall not waste my days in trying to prolong them.
I shall use my time."

-Jack London 1876-1916

1983 300CDT (running WVO since 12/05)
1981 300SD (parts car)

Well at least all the stuff you replaced was good PM stuff on a 20 year old car. Now you can idle with your AC on in traffic without fear.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

The temp switch I wired direct is just behind the thermostat housing. There are two switches side by side. one is vacuum on the left. The one on the right is a 100C close to ground. I wired that to groung, and close the Aux. fan relay mounted in a plack plastic box on the driver side fender.
There are two relays under the pastic cover the relay to the rear is for the aux fan. Find the wire in the cable coming in that goes to the ground peg and tap into it. Bring a hot line from the forward relay into the rear one. the forward relay is hot when the key is on. Make shure the key is working OK I have to back mine off a touch to make the heater system come on. (and the relay power)

Weeell, I was tempted to come to that same conclusion... because I lasered the hot side of the radiator @ only 75 degrees C, but my gauge still said 85 C....

but then I did some more checking, and theres a sender looking thing on block behind the IP, and when I lasered that and the lower block in general it was 86 C.

so I'm thinking my gauge is just right.

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Cheers,
Robert

I didn't point it at the radiator, I pointed it at the temp sensor you are referring to, and it read 83*C when my gauge was reading 98*C.

1984 300d Turbo 147k
Take The Top Water Hose From Radiator Off And Fill Hose With Water You Will Hear The Air Come Out,

Do any of you experience this. I have a 2 Lit 83 W123 I noticed 2 funny conditions:

1. Without air-con if I drive constantly at 150Km/hr and above the temperature will rise constantly up to 110 degree and still climbing, but if I lower my driving speed to 130km/h or below, the temp gage will show it dropped back to 100 degree.

2. When I switch on the air-con, regardless of my driving speed, the temp will rise instantly up to just below 120 degree. When I troubleshoot, I discovered that the moment the air-con is switched on the radiator magnetic fan will cease operation. However when I disconnect a terminal from the air-con drier sensor switch the radiator fan resume in full speed again.

The aux fan is good tested with direct battery connection.

I have been driving with the air-con off and yet to find the solution.

Chong