Fan function?
 

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?

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?

Answer to #1

http://www.peachparts.com/shopforum/diesel-discussion/300740-w124-auxiliary-fan.html

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.

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.....

Get a colder temperature switch.

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

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

Got a part number? It fits?

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

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.

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

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?

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: ???????