Hello All,
Glad to join the forum. Purchased my first diesel and Mercedes a few weeks ago: 84 Turbo Diesel. Despite the fact that this car was operating perfectly I went ahead and ordered my Haynes manual a couple days ago. The car, not wanting my investment in the manual to go to waste, decided to throw me my first mechanical challenge today -- a few days before the arrival of said manual.

Symptoms:
Starts fine and operates at around 85 degrees while driving at any speed. Within 20 seconds of sitting at a stop light the temp gauge rises quickly. It gets to about 110 degrees and seems to stop around that area. If I place the car in neutral or park and give it about 1500 rpms the temperature goes right back down to the 85-90 degree range. I also have that condensation showing above the passenger-side vent below the windshield. No smell of anti-freeze or odd smell of any kind. I see no fluid leaks from hoses in the engine compartment and the engine areas don't feel particularly hot. When the temp rises the auxillary fan does NOT come on. In fact, I don't ever recall hearing it come on since I've had the car.

Suspects:
I've googled around and browsed MB forums and it seems I could possibly have 2 issues hitting me at once: heater core being responsible for the windshield condensation, and either bad aux fan or bad engine fan clutch.
If its a bad aux fan (ostensibly it should have come on when the temp reached 110?) then its curious that this problem only cropped up today after having driven it for 3 weeks. If its a bad engine fan clutch my understanding is the tell-tale sign involves a freely spinning fan when the vehicle is warm but turned off. My fan has some resistance when I spin it.

Questions:
-if the engine fan clutch is shot should or shouldn't it spin freely when warm and the engine off?

-if my temperature cools down when parked and putting my foot on the accelerator (not allowing a normal idle) would this further support the idea that the engine fan clutch is bad -- premise being it needs the increased rpm's to cool the radiator to the degree it used to be able to cool it when it was simply at idle?

-Can someone confirm that the auxillary fan should have come on when the temp got that high?

-When I "played" with Volvo 240s we shunned the Haynes manual in favor of the Bentley manual (which I thought was excellent). Is there a better manual for 300 Diesels?

Any other feedback and comments are surely appreciated. Apologize for being the parasite with such a long post. Oh, one other oddity this car has. There is a large hose leading from the engine block to the interior that has a valve installed. The previous owner said he had a mechanic install that to help control how much hot air to let into the interior of the car. So, I suspect it isn't quite a correct situation for the climate control system to be in, but don't know enough yet to say its a contributing factor...

Thanks in advance,
David

OK, you may have a couple of things going on. Easy stuff first:

Have you verified that the coolant level in the tank is correct (about half full)?

Are you losing any coolant?

The mechanical fan clutch may be slipping, an easy test is to try to stop it with a rolled up newspaper with the engine up to temperature. If you can stop the fan from spinning with a hot engine, it needs to be replaced (not unusual, I'm on my third clutch).

I believe the electric fan normally kicks on at something around 100C, so that may also be a problem, but I don't think it's the main issue.

It does sound like someone hacked up the climate control, but that shouldn't have anything to do with overheating.

Something isn't right you have a bunch of problems.

First the electric fans should come on at some point above 100C, 105C for my SDL. But it should never get that high at idle. I have no idea when they are supposed to start on a W123, probably 105C or 110C.

Sounds like your radiator, fan, and possible water pump are marginal. I'd start with a good flush, and check the fan clutch. If the electric fans are not coming on fix those, but thats not your problem.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I really don't think it's the radiator, or it would overheat under load. My 300D had those symptoms with just a bad clutch fan (even with the electric fan working correctly). the water pump is a possibility, but they don't usually fail suddenly.

It depends how hot it is where he lives. I have run my SDL without the fan clutch and it hasn't really affected anything, idle temp's were up slightly. But it was 50 degrees out as well. I don't think the radiator is working as well as it could.

If it was sudden it's not the water pump, but I have seen them with the vanes worn down to almost nothing.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

My 300D would go from 80 or 90C to boiling over during a stop light on a 70-80F day with the clutch gone, and it started suddenly. It could be something else, but my moneys on the clutch.

The electric fan only comes on when the a/c is operating. If the a/c is not operating, the electric fan won't come on. The electric fan is controlled by a temp switch at the receiver/dryer. In 1985 the 126 chassis incorporated a 212 switch near the thermostat housing that tied into the electric fan circuit.


1) Check fan clutch per previous posts.
2) Check radiator for cold spots with a digital laser thermometer.
3) Check coolant temp sending unit for accuracy when you test the radiator.
You may have to pay a radiator shop to "shot" your temps (or you should pay them a small fee). Good chance your temp sensing system is reading incorrectly and you are getting bad information.
4) If someone put a manual valve in the heater hose they are a dumb-a**.
You probably have a bad mono-valve. Pull it and check the diaphram for a hole or tear.
5) Pull thermostat, inspect it and test it on your kitchen stove with a cooking thermometer (don't let wifeage see you do this) You need to test for a proper opening temperature and proper opening space. If its not opening fully, it won't cool properly.

__________________
84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

My money's on either a bad fan clutch or a bad water pump. Will the temperature come down at idle if you turn the heater on full blast? If so, I'd say the clutch is the likely problem. If the heater makes no difference, I'd go with the water pump.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I had the same problem before. and it turned out that my clutch was already due for replacement, so i got my volvo clutch immediately. It solved my head ache

My problem of this exact description was solved by replacing the fan clutch.

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

I had the same problem with a new water pump and a somewhat new fan clutch. When I replaced the radiator (the top broke) the problem was solved.

__________________
Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback