It says that the temp switch on the receiver/drier closes at the specified temperature to complete the ground circuit for the aux fan relay solenoid. When actuated, the relay provides system voltage to operate the aux fan.

Typically, the switch on the thermostat housing functions to prevent the cabin blower from operating in the HEAT mode when the engine is cold.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

OK, then you are saying (in my situation with the AC belt removed and the condenser locked up) that the aux fan will never come on - even if the water temp goes to 115C or 125C?

For some reason that does not sound like 1980's MB engineering.

Looking up the 1981 ETM, the ONLY thing that will turn the Aux fan on is the temp switch on the filter/dryer. With the A/C non-functional, there is nothing to ever turn it on. Later cars had an additional switch in the head to trigger the Aux fan if the engine were overheating.

The design of the Aux fan circuit is completely sound. How many American cars didn't even have an Aux fan? The engine driven fan is more than sufficient to cool the radiator by itself. If you're overheating in slow traffic, you have a bad fan clutch. Plain and simple.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Good to know; that sounds right.

How do you test the fan clutch? Seems like with a viscous clutch there is a test but I know not what it is. Does the '81 have a viscous clutch - did they even have them back then?

Is it hard to replace the fan clutch? I replaced the water pump several months ago and that was easy; did I remove the fan clutch when I did that?

Your car has a viscous clutch, you had to remove it to change the water pump. It's the finned disc that the fan is attached to. A general guideline is that when the engine is hot, the fan should stop very quickly if not immediately when the engine is stopped. If it keeps spinning, or spins several revolutions, it's time for a new one. They're a common failure item.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

The engine cooling system was engineered with to work fine without the need for a secondary fan. The aux fan is there to benefit the condenser. Believe it or not.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Very interesting. I need to take a look at the fan. Too bad I did not think about replacing it when I had the fan blade off. In all the years I have had the SD I have never seen the temp gauge rise. So I will bet that you are right about the clutch having failed or being about to fail.

Love this forum.

When you're running hot like that you should be able to easily hear the fan roaring as you accelerate too. Should sound like a trash truck if it's really running hot and the fan is locked up. High temps and lack of ran roar are pretty definitive indications that the clutch has packed up.

The other thing that the clutch requires for operation is airflow through the radiator and across the bimetal spring on the front. If the radiator is clogged with dirt, or you have a layer of crap between the condenser and the radiator, it will impede airflow and can cause the clutch to fail to engage. It's easy enough to take a water hose with a jet nozzle on it and spray through the condenser and radiator to make sure it's clean.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Radiator is really clean. No trash truck sound when it was hot. Going to test it tomorrow and probably order a new vclutch; about $70 thereabouts.

At least I can do this without removing all the belts - I assume. Just get in there with a 10mm skinny wrench, remove the fan blade, then figure out how to remove the vclutch. Assume I can turn the engine over with a socket CCW to see the bottom side bolts; hope so with the belts still on tight.

Uh oh - Pelican wants to sell a clutch hub tool - am I going to need that?

If I'm not mistaken, the OM617 has 4 bolts that hold the clutch to the water pump pulley. If so, all you need is a 10mm box wrench. No special tools required.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Yes, that is what I remember but I had to grind down/skinny up a 10mm wrench to get in there - as best I can remember.

Thanks for the help. Much appreciated.

On my previous 81 240D the compressor was shot and the belt removed. The aux fan would come on when the ambient was above about 110. Would MB really have had an 81 123 with such a design but an 81 126 without?

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

That was probably more by accident than design. If the engine compartment gets hot enough to trip the temp switch in the receiver/drier, it has the same affect as hot refrigerant.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

I don't have a 123 FSM and don't really care to look through StarTek. Was answering the OP's questions and gave the result straight from the 126 FSM. Whether or not a different model had a coolant switch is irrelevant, the one in question DOES NOT. That's what's important.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Models of that vintage without AC don’t have an aux fan.

More on the W123 side of the topic - W123 aux fan not coming on

leathermang doesn’t like it

Sixto
98 E320s sedan and wagon
02 C320 wagon