My 84 300D had an issue where the indicated coolant temperature stayed below 60 F. I first suspected the reading, i.e. the sensor and/or cluster gage. I found little firm info after hours searching, so am posting info here to save others much time. I ended up having to run my own mini test program.

Various sellers (Peach, rock, ebay, Autozone, ...) list the following PN's for the relevant sensor:
Mercedes 005-542-26-17-MBZ (no longer avail)
FAE 005-542-26-17-M323 Temperature Sensor for Temperature Gage
URO Parts 005 542 2617 Water Temperature Sensor
Meyle 0140540013, Engine Coolant Temperature Sensor
Standard Motor Products TS-612

I found no specs on resistance vs temperature for any of these sensors. This sensor mounts on the L side head, aft (right-side of photo). To add to the confusion, some engines have other coolant temp sensors for the "glow plugs hot enough lamp" (~83+) and the "hot enough to run cabin blower" function. The former is on the L side head, fwd and the later in the thermostat housing. There is another temp sensor in the AC filter/drier, but a simple "bolt", not in a fluid port.

My first thought was that I inadvertently swapped the sensor wires in the head, since they can physically interchange, but am now sure now I have them right because:
1) Aft sensor has much lower resistance: 1040 ohm vs 4090 ohm at 8 C, so swapping makes the dash gage bottom out.
2) My 82 engine (now in 85 300D) has only the aft sensor, and it reads 82 C fine when warmed up.
3) The wires were connected as shown when I started. Removing the small connector affects the dash gage (bottoms out), plus it is green (per schematic).

I was initially side-tracked because several posts said the glow plug temp sensor wire routes to the glow plug relay. Since the wire from my small connector runs into the glow plug harness, I ASSumed it was the gp sensor. The gp sensor (large connector) wire runs towards the firewall, and is clipped to the VCV. The confusing routing is probably because the later sensor was an addition.

I was also misled by the following Peach Parts post:
12-16-2007, 12:18 PM
tballx
COOLANT TEMPERATURE SENSOR (B11/9 or B11/10) RESISTANCE
At -20 °C 15.7 kOhm
At -10 °C 10.0 kOhm
At 0 °C 5.9 kOhm
At 10 °C 3.7 kOhm
At 20 °C 2.5 kOhm
At 30 °C 1.7 kOhm
At 40 °C 1.17 kOhm
At 50 °C 830 Ohm
At 60 °C 600 Ohm
At 70 °C 435 Ohm
At 80 °C 325 Ohm
At 90 °C 245 Ohm
At 100 °C 185 Ohm
Tolerance ±5%
I believe the one in your thumbnail is this one. If so, I just replaced the same one in my 1984 300SD. The values above are from autozone's online manual for my model.

Another post had a very similar table w/ comments:
For 86-93 300E. Should be same for 75-93 300D.
Ohm's range from the temp chart in the /Engine Manual-Combustion Manual-07.3 Mechanical/electronic gasoline injection system (KE injection)-121 Testing electrical components of KE injection system. page 60

My glow plug temp sensor is a closer match to the table above. I could find no PN's or specs for it either.

I tested my gage while the cluster was out of the car, using various resistors and found a much different table:

Resistor, Indicated Temp
(ohm), (C)
392, 40
158, 55
100, 65
67, 81
50.3, 92
40, 100
20, 120

At operating temperature, with the dash gage indicating 59 C, my IR meter (Harbor Freight) read 53 C at the copper base of the sensor. Therefore, the sensor-gage combination appears to read correctly. The sensor response (based on IR meter, includes a data point 2-4-2014 w/ new T-stat):

Temp, Sensor Resistance
(C), (ohm)
85, 59.3
53, 157
8, 1040

I then suspected the thermostat. I expected to find it in pieces (as I once did in my 96 Voyager), but it looked fine. I put it in hot water, along with several others. It began opening at 40 C and was full-open at 85 C. A new T-stat started opening at 75 C and was full-open at 80 C. For more comparison, I also put 2 T-stats from my old Mopars in the pot. They all started opening just below their stamped values and opened fairly sharply with temperature. I wouldn't consider using them since their flow area is much smaller, plus a slightly smaller flange. BTW, my IR meter read the T-stat surface temperatures thru the water very close to an immersed thermometer.

Both old and new T-stats are marked "Wahler, Made in Germany, 80 C". The bad one "12 98", new one "03 98" (date codes?). The new one was in a Stant 35588 package and marked Mercedes 176 F, 80 C. In sum, the old T-stat appears defective. It has 2 springs, so maybe one isn't responding thermally anymore. I always thought T-stats either break or stick, not get a sluggish response.

Final result, my dash gage now reads 85 C when the engine heats up (w/ new T-stat), my heater is warmer, and I hope to get better mileage and longevity from the engine. I don't know how long that car ran cool since my son had it away for 3.5 yrs (though only 25K miles so minimal added wear).

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