How to test a W124 auxillary fan?

millo777

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After coming precariously close to reaching 120c in a motorway tailback earlier today, I had a look at the viscous fan, which I think is dead (can be freely spun with the engine off, and continues freely rotating for a few seconds after the engines is switched off)

To add insult to injury, the auxillary fans don't seem to be working either. At least, the don't come on when the ac is switch on, which is what I was expecting. The engine was however cold.

Are they supposed to come on with the AC, or via a temp sesnor in the coolant?

What is the best way to test them and any other related parts, relays, thermo switches etc?
 

khimani_mohiki

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The viscous fan on our 190 does the same thing, with the engine off and the bonnet open it spins with a gust of wind, doesnt seem to overheat though. I think the aux fan is controlled via a thrmo switch as well as the AC relay. It wouldnt be too hard to rig up a switch to turn it on when you want to.
 

GP801

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The auxillary fans only come on when both the AC is switched on and the outside temperature is hot. Used to think ours did not work as rarely came on in the uk but in the hot weather in Italy they are on most of the time.
 

balge

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Hi
first thing I'd try would be to stick 12V across the fans to make sure they work, then bridge the temp sensor to check that
cheers!
 
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millo777

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Thanks for the replies.

Re the sensors providing input to switch the fans on, if they are only activated when the ac is on and the temp is above a pre-set level, I assume both sets of sensors will need to be shorted to fool the fans into action.

The fans seem to be wired to a continuous 12v rather than switched, which I assume is so they can operate when the engine is switched off if required. This is why I thought it was was either temp or AC being on which triggered them, rather than both AC and temp, as the AC is of course never on with the engined siwtch off.

Re putting 12v across the fan, is there an easy way to tell which pin is live and which is earth? I'll have to construct some test wires for this, which I'm sure will come in handy in the future.

Re locations of the sensors. Is the temperature sensor the 2 pin connector on the thermostat housing? there are 2 connections on the thermostat hosuing, one with a two pin connector, and another with a single pin. Not sure why, but I was convinced the single pin one was the temp sensor.

The connectors for the AC side are the 2 mounted seperately on the AC pressure switch?

Sorry for the long post....I seem to making a meal of a very probably simple test!
 

franey

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W201 190 2.5d, W124 300d 24v, W221 S350 AMG
Remember, if the A/C pressure is below the required limit then the compressor will not engage. If that does not engage then it won't pump the refrigerant around. Low pressure = ac no work = ac fan at front no work.
As for the engine cooling fan, not sure which is on yours? should be the viscous one methinks?
 

franey

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PM your reg number to me will have a look for you.
 

balge

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Pretty sure it doesn't matter which way you power the fans, it would just go the wrong way? Only diagram I can find seems to be US style ACC, might be different?
Think there might be two temp sensors on the engine? My 230TE definitely has a two pin sensor for the engine fan....?
cheers!
 
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millo777

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Finally got the aux fans working. Cleaned all the contacts on the temp sensor contacts and the ac pressure switch and they back to life.

Probably nothing to do with any efforts on my part, and as per GP801's experience, simply a case of not encountering the right conditions for them to trigger until a hot august day!

Anyway, thanks for your help with this one.

On to the Viscous fan change.....it never ends.
 

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