Hey, first post here. Looked on search for an answer but couldn't find what I was looking for.
I have a 92 300D with a temperature gauge issue. When I got the car last year the temperature gauge was reading a steady 65°C once warmed up. I at first assumed the gauge was right so I replaced coolant and replaced the thermostat. That didn't work so I replaced a couple coolant sensors including the sender on near the lower radiator hose. That didn't work so I started reading temperature with an infrared heat gun. A few months ago I was reading 80°C at where the upper radiator hose attaches and about the same at the base of the lower temp sender. So I am pretty confident the temp is correct. Checking the temp in winter has been less successful though any I'm getting lower readings at both locations.

BUT I'd like to use the heater fan "auto" setting as a double check that its actually getting as hot as I think it is. Does anyone know at what coolant temperature the heater fan is supposed to kick in?

Not familiar with that engine, but the "auto" setting should adjust the fan speed depending on input. What do you have the temperature set to?

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83 SD

84 CD

Does the auto on your CCU do anything? My auto will switch on at any engine temperature and will be pegged to high air flow until the cabin temperature has been reached. The sensor by the lower rad hose on the passanger side is used for the glow plug circuit. Do you know if your auxiliary cooling fans run at all or does the engine temp never get hot enough? I would suspect the fan clutch maybe is not releasing.

Welcome to the forum

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Like I said I don't think the temperature gauge is right.
Yes, the auto function the CCU works correctly (according to what I understand as correct operation). On "AUTO" the fan will not turn on until I'm a mile from home and the engine temperature has started to come up. Maybe if I turned the CCU thermostat up all the way it might turn on earlier. The on high and low the fan turns right on regardless of engine temperature.

Have a look at this link.

Pelican Technical Article - Mercedes-Benz W124 - Coolant Temperature Sensors Replacement

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Thanks for the link. Okay, so this is a little different than my engine, but it does mention the sensor for the temp gauge is a single prong. I replaced two single prong temp sensors, one down by the thermostat (lower radiator hose) and one by the upper radiator hose. There is another single prong on the drivers side of the block but I doubt that's it. I've read a lot of mixed stuff about which sensor it is that sends the signal to the gauge. I haven't tried testing for which one actually does it.

It is the single prong on the drivers side. The 2 prong sensor a few inches away is for controlling the auxiliary fans. For the CCU temp gauge at 80 degrees you should read 325 ohms. If you unplug it the gauge should stay at zero I seem to recall 2.5k ohms is -20 degrees.

My engine warms up in under 15 minutes under load at -20 degrees C outside.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10