Help needed with aircon fan speeds

[mike18hc]

My car is a 1995 C220 W202 and I've been told by the guy who regassed it that the way the electric fans work is wrong. The viscous fan operates as it should.
30 seconds after the aircon's turned on the fans operate at high speed for 3 seconds and this cycle is then repeated.
There's a resistor in front of them that should allow slow speed but it doesn't.
The resistor cables run off to near the ABS unit in the right of the engine compartment but I can't trace them further and I can't even see the fan cables!
Since the left and right fans are different...like the relays...I assume they should operate independently.
Can anyone please tell me how the aircon fans' speeds are controlled?

 

[mike18hc]

Just a quick update. I timed everything accurately.
With AC on at idle it takes 40 secs for both fans to turn on for 4 secs. Then the compressor turns off for 15 secs. Then it turns on again and everything repeats. This car only has a refrigerant pressure sensor and not a temp sensor on the receiver/drier. I guess the pressure's too high through lack of fan cooling and the receiver/drier's getting really hot.

 

[mike18hc]

I now think both fans are electrically the same but made differently (hence the different part numbers) to fit where they do, and operate together.
The 94/95 C220 differs from other C-class cars of those years because they don't have an aux fan control unit...based on partsbase.org info. I removed everything that may hide the fan cables' run and found nothing.
There's Russian info that shows the relays directly controlled by the dash a/c controls. It shows the pushbutton option but the functions may be similar to my wheeled unit. Trouble is the a/c pressure sensor is not shown so I need to check that out too.
The saga continues.
Maybe too much R134A is producing too high a pressure. It gets hot so quickly it may need the fans to cool it as soon as the clutch operates.
Can anyone with a 94/95 C220 please tell me when their fans come on and how hot the receiver/drier feels?

 

[andy27168]

Hi,
As far as I was aware the front fans do not operate individually, they both run at the same speed at the same time, the speed of the fans is variable via. a (speed) control unit in the front of the passenger side wheel arch (RHD).
This (speed) control unit varies the speed by PWM (Pulse Width Modulation), the speed is determined by the console control unit.
My understanding is that the fans turn on/off because of refrigerant pressure not temperature, I stood and watched my car's A/C system running with pressure gauges on and as the pressure rose the fans would cut in then as the pressure dropped back they would turn off.
This was on a 98 C43 AMG with Digital Climate, though the same system was fitted from about 1995 on the 202, 129, and very similar on the 210 and 208.

 

[mike18hc]

Endgame

Well, the compressor now no longer cuts out so the AC seems to have stabilised.
The resistor in front of the fans IS used when the fans start to work on this car. I thought by the noise they made that they were running at full speed but they weren't, as they were only getting 9V. When I disconnected the pressure sensor plug on the receiver/drier the fans started to run at full speed and I'll remember that noise now!
I read in an Indian website that the A/C control unit does the conversion of the pressure sensor and coolant temperature outputs to control signals for high or low speed fans. That would make sense as there's no parts listing that shows the 1995 C220 as having a separate fan control unit. 1996 yes but not 1995.
I think I have to drop this now as I have found what controls the fans and what their wiring circuit is so my query's pretty much run its course. I may not know how the refrigerant pressure and coolant temperature sensor outputs control the fans and what makes the fans work at full speed but I can live with that...until there's a problem!