1989 300E W/ Single fan in front of radiator.

Does this fan always turn on when the A/C is on? or is it water temp. controlled at a preset temperature?

I come from BMW land and they are switched on whenever the A/C is engaged.

Just noticed that it was not engaged with the A/C running.

Thanks.

The aux. fan engages/disengages at low speed based on AC system pressure, and it should engage at high speed when the coolant temperature reaches about 105C and disengage at about 100C.

So it does not run continuously when the AC is on, but will probably cycle on an off in low speed traffic.

If the coolant temp. gets up to 105C to engage high speed, you will probably hear it.

Duke

So it is not A/C switched, but rather water temp. switched. With the A/C on, the water temp. will likely run hotter resulting in more on off cycles. I did not know there was low and high on the fan, I thought it was a single speed, all or nothing dumb fan. Good engineering with variable speeds.

Thanks.

Just had this learning experience on a 92 400E. The switch actually has a refrigerant line running to it, so that it will turn on low speed when the AC is switched on, and high speed based on the coolant temp. I guess mine is behaving, but wish I could turn it on without the AC, so it would come on low speed without the AC on. I found out from experience that the car will cycle between 107 and 99C in 100F weather at 60 mph, but at 90 mph it stays at 95C! Wish that was a workable solution. Meanwhile, I have probably the evaporator leak on the AC, so can't run the low speed fan. How hard can it be to replace the refrigerant switch with a manual switch?

If your fan never comes on low speed, some possible reasons - 1. no refrigerant to create the pressure, 2. a bad switch, 3. a bad fan resistor, 4. a bad fan motor. With the AC on, try helping the fan blade with a pencil, if that gets it started, you'll know it's the fan motor, otherwise the resistor does burn out, and the wiring does get hot in that area. I guess a fuse could also be blown.

Manual low fan is a common mod on Benz aux fan. All you have to do is jumper the high side pressure sw at the reciever/drier and run the wires to an
on/off toggle switch on/under the dash..

Will not interfer with normal a/c circuit, but gives you that edge in traffic on a hot day..instead of putting the heater blower on.

.. or , you can just bridge the coolant high fan sw with a metal film resistor
of the proper value to make the high fan come On sooner than 105C.
Radio Shack.. package of 5for $1..

Both mods can be done for less than $5....

The fan can be acuated by either high engine coolant temperature or excessive high side pressure in the a/c system.

Good luck,

mine blu the fuse the other day, tried to turn fan by had and it was tight got it to spin a gain put another fuse in worked ok but still bought another fan and replaceed, the fuse must have gotten real hot before it blu as the fuse box had white and blue stuff all around the blown fuse,


86 300e just turned 140000
83 300d sold @ 250000 still running good