I have been trying to figure out why my benz is overheating when climbing up hills or going fast. I think it may be fan related.

A few questions....

1. When does the aux fan engage (besides when AC is on)? At +100C?

2. When does the engine fan clutch engage? Does it run all the time? At cold start up, my fan is spinning and I cant stop it with a towel. Ive heard that it should be free-spinning.

3. Do the fan and aux fan share a control switch? Where is this located?

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'87 300TDT 150k

I am not familiar with the w124s but I can answer on 2 & 3:

2. Never test the fan clutch by trying to stop it or impede it.

The easiest way is to watch it when you shut down the engine. It should continue to rotate a bit after the motor stops turning. If it stops dead with the motor, it's bad.

3. You ask about the fans sharing control... the fan clutch is a purely mechanical/hydraulic device (there is another word for it that I can't remember, not enuf coffee in me yet) and has no electric or electronic input.

It's helpful to quantify what you mean when you say 'overheating'. Where does the gauge get to when it runs hot?

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Answer to #1

W124 Auxiliary Fan

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

120C+++++ ( I will pull over before this happens, but it will redline if I let it)

Only when going up a big long hill, or really gunning it on a flat highway.

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'87 300TDT 150k

Could be the thermostat is on its way out, the radiator is partially internally plugged or choked with dead flies...

Could be you have an airlock in the system.....

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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....

Get a colder temperature switch.

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CENSORED due to not family friendly words

New radiator and thermostat. Nothing to indicate head issues.

Yeah, I've been thinking about replacing the temp switch.

The red one is the best, eh? 100C/110C

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'87 300TDT 150k

I'm using one off a BMW, 91/99c

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CENSORED due to not family friendly words

Got a part number? It fits?

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'87 300TDT 150k

Actually, Im not sure I would want a 91C switch. The car usually runs right around 90C.

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'87 300TDT 150k

If it's overheating at speed then it likely isn't a fan issue. Most of the airflow through the radiator then is being pushed in rather than pulled by the fan. If it has a good temperature in town or at lower speeds, then the fans and thermostat are likely fine. Most likely is as stated above, radiator plugged internally or with debris.
One long shot is that the front mounted pusher fan may be wired backwards. I had this on a Subaru with a similar frustrating overheat issue, I didn't find the problem until I was standing over it doing some testing and realized I was having hot air blown onto my legs when the fan kicked on. The front mounted fan should push air through the radiator into the engine compartment, not pull it out the front.

I had this problem on my old Lincoln years ago. The fan clutch was shot, but replacing only helped a little. A fresh radiator solved the problem completely.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

ÜRO PARTS Part # 61311378073
You will also need the BMW pig tail to splice into your wiring harness. It uses the same threads into the cyl head.
I'm also using a 71c thermostat so it runs right at 80c most of the time.
Vernet #280964664017

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CENSORED due to not family friendly words

OK, so I was running my AC today and when I pulled over the aux fan was not spinning at all. Bad fan? Replace?

And I when I killed the engine with the shutoff valve, the engine fan stopped turning immediately. Bad fan clutch?

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'87 300TDT 150k

How I test fan clutches in cars: Cold, engine stopped-fan should be able to spin freely by hand. Next, run the car enough to where the fan should definitely be on. Stop motor and immediately check to see how hard it is to spin the fan. If it's kind of hard to move, then you're good. If it spins freely like it did in the cold test, it's time for a new fan clutch.

Aux fan not spinning with AC on: Is is getting power?

Running with AC on in Helena, MT in November: ???????

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.