Hi all,
I recently fitted my 190E with a new R134A ready Denso compressor out of the USA. I fitted seals, new switches and a receiver dryer.
All fans are working and the auxiliary fan has both low and high speeds.
When I engage the freshly gassed A/C, the low speed actuation of the auxiliary fan will not trigger via the brand-new high pressure switch on the receiver dryer, but will trigger if the wires to the switch are bridged. In that case, both main fan and auxiliary fan trigger.
I therefore believe the fans for A/C only trigger if required...not on cold startup, or where vehicle temp is below 100 degrees C or where A/C high side system pressure is below 250psi...
This would make sense to me as when the car is moving, enough airflow prevails...but if stuck in traffic, fans would engage as required for temp or pressure.
Am I correct in my thinking? Anybody know?
The refrigeration tech that gassed the car has told me the new pressure switch for the fans must be faulty as the fans don't come on the minute the A/C is engaged. I am not sure his line of thinking is correct in the case of this car.
Can anybody clarify?

I don't have a 190E but I have a 190D. The A/C system is pretty much the same. The aux fan ( slow speed ) does NOT come on immediately when A/C is engaged. It only comes on when high pressure builds up and switch at the recv/drier is made. You can attach the A/C gauge to measure the high port pressure. The switch is rated at ??psi ( I think it is 20bar or 290psi ) or above. My observation is that the aux fan comes on when the pressure is high after the car is idling for a few minutes. It could happen when car is moving on the freeway but I never check. The other reason is that you do not have enough freon charged, i.e. the pressure is not high enough. Other A/C experts may chime in.

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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.

If the 201's A/C system is anything like the 126's, the aux fan shouldn't come on unless the high side pressure is at or above the trip-point of the "temp" switch. Sitting still, warm day, in traffic, the fan should usually cycle on. When moving, it will likely shut back off.

Remember that you have an engine driven fan pulling air all the time, the aux fan is just that - auxiliary.

__________________
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]

If MB meant for the aux fan to run whenever the AC compressor is engaged, the fan would trigger off the Klima circuit, not a refrigerant pressure switch.

Sixto
83 300SD
98 E320 wagon

The aux fans engage at low speed via the AC high pressure switch. High speed engages if the coolant temperature exceeds 105C (about 231F) regardless of AC high side pressure.

From what you say it sounds like your aux fans are working properly.

The mechanical fan clutch should fully tighten at about 100C and above, and should make noticeable noise at 2000 revs and above.

Duke

Thank you all for the responses. I tested the car immediately after I posted and to my delight, discovered the A/C fans are triggering as they should according to the responses here. New switch triggers both fans together and they cycle on and off. They don't appear to engage when car has speed up.
Cheers! Now on to my Daimler (Jaguar) V12 climate control repair...