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).

Attached Thumbnails

 

Those are switches, not sensors. "Resistance versus temperature" does not apply.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

AFAIK, glow plug temp sensor for 84 300D is in the glow plug relay, not in the block. Which one were you referring to?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I stand corrected, "switch" is a more exact term than "sensor" for the "Aux Fan temperature measuring control component on the AC filter/drier" and the "coolant water is hot enough that climate control can now enable the cabin blower (per user request)" temperature sensing element.

I referred to the temperature sensor on the left side of the photo (fwd end of head). It apparently has a resistance signal (probably a thermister type device, but could be an RTD). I didn't trace where its wire goes, but it starts out running aft to the VCV. Based on other posts, I assumed it is for the "we predict the glow plugs are hot enough for the current engine temperature, so try to start it" lamp in the dash cluster. However, thinking more, I now suspect it is part of the EGR emissions controls. This is a 1984 car (probably CA). I didn't check the block stamp, but should be a 1984 engine since has the (stupid) rack position sensor on the injection pump.

BTW, my 1985 CA car has that same round connector, but it is unused now since I have a 1982 engine in it that doesn't have that sensor. I notice nothing missing without it (EGR bypassed anyway). My 85 car also has 3 spade terminals in a common sheath in the middle of the firewall (just below the weather-strip) that I couldn't figure out where they should go. I find no mating cable or component in my 85 engine parts, nor see anything similar in other 85 CA engine bay photos. Perhaps it was never connected.

For clarity, the switch in the thermostat housing is also a switch.

"Those" was the proper term.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

I just went through the same thing. My temp wouldn't go above 60C unless I was on a long hill. Changed out the thermostat and now it is a consistent 85C. Much better!

tangofox007,
I caught that and revised my post before I even saw your follow-up.

Tmadia,
We both had good luck, and "changing thermostat worked" was the crux of my post, and now at least 2 successes.

I am usually 50:50 trying to fix these types of problems. When I found the thermostat looked fine, it was looking like the lower 50 until I pushed on and tested it. I remembered to not repeat the trouble-shooting of my dad. We suffered thru ~5 yrs in Florida without AC in the station wagon because "blower didn't work". My dad had pulled the fuse, it looked fine, so he put it back and gave up (too expensive to change blower). The fuse wire was actually melted right by the terminal where you couldn't see it (glass fuse, not a Euro type). It melted not from excessive current, but from resistance at the fuse holder (Florida = corrosion). Thus, don't ever assume a part if OK because it looks good. Always test or swap parts.

A thermostat is the cheapest and first move on nearly any cooling system problem.

I would sure follow that approach if I was working on other people's cars for money. Indeed, I would change the thermostat, radiator, hoses, and temp sensor. That would surely fix the problem and I could charge book parts prices and labor hours. However, I have found it better to find "root cause" first, which is what I do on my cars, since my time and money. In this case, it wasn't the temp sensor/gage combination, which I initially thought most likely.

The following is culled from the Mercedes-Benz Service Technical Data Passenger Cars books, April 1985 edition, December 1988 edition, and October 1993 edition.

OM615, 616, 617: T-stat begins opening @ 80 +/-2 deg C, minimum stroke 8mm @ 94 deg C

OM601, 602.91, 603.91: begin opening @ 85 +/-2 deg C, minimum stroke 8mm @ 94 deg C

OM602.96, 603.96, 603.97: begin opening @ 80 +/-2 deg C, minimum stroke 8mm @ 100 deg C

OM604, 605, 606: begin opening @ 85 +/-2 deg C, minimum stroke 8mm @ 100 deg C

The bypass disc diameters, which may indicate compatibility of thermostats with multiple engines (if, for instance, you are tempted to try a hotter or cooler thermostat):

All OM60x: 40mm (looks like some options are available here)

All OM61x: 37mm (sorry, no soup for you!)

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

True statement. However please be wary of the fragility of the thermostat housing bolts on the OM616/OM617. It is stupid easy to snap these off and then you just went from an easy repair to a PITA.

If you are going this route be sure to obtain some AeroKroil and treat the bolts daily for several days before attempting to turn them. And when you do turn them use a 1/4 drive socket and very gentle torque. Another pro tip, very gently and slightly tighten them first, then start to loosen.

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The OM 642/722.9 powered family
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)
2008 ML320 CDI (Older son’s DD)
2008 E320 Bluetec (Younger son's DD)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech
----------
gone but living on
1983 240D (body to greazzer, engine to t walgamuth)

Another pro tip...

Spray aerokroil on the ends of the bolts, not the heads so much...

The housing is replaceable if needed.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Thanks to everyone for adding useful info. I'll add that since this post, I found that the unused "large round plastic housing w/ cable" on the firewall of my 1985 went to the "rack position sensor" on the injection pump. My replacement engine & IP was newer and doesn't have that component.

Another tip I learned since. If your T-stat housing has never been removed, now might be a good time to remove from the head and change the large "bypass hose" on the bottom. I used blue silicone hose on both 300D's (ebay, vaguely recall 42 mm ID, but measure don't flame me). While housing is off, a good time to unbolt the timing chain tensioner since most internal springs have worn flat on one side (Peach Parts ~$7). Of course, good time to replace the upper timing guide rail, but always pick your battles carefully and allow a large block of time.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans