A clear picture is imerging. This AM it was pretty warm and the climate control came on right away. Makes me think that the sensor (a) works since there was a change over cold temps and (b) the sensor may be only for the climate control (as suggested by tangofox).

Sounds like only the AC can turn on the aux fan.

When it gets hot, I may try to rig a switch that will allow me to turn on the fan at will.

Thanks for your help.

__________________
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

yes, that's it. I wasn't really clear on my last post - the sensor on the t-stat housing is the one that cuts out the heater blower until the coolant temp rises. The aux fan comes on with A/C, and with temp sensor on the receiver/dryer I think.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

This can get confusing, because the 603 is what you were originally thinking. It can turn the fan on 1/2 speed from the pressure switch on the dryer and it also has a 105°F. coolant switch that turns the fan at full speed when tripped.

No. The aux fan is actuated by the temp switch on the receiver/dryer. The pressure switch allows the a/c compressor to run. Or more specifically, it prevents the compressor clutch from engaging if there is inadequate refrigerant pressure in the system.

You sure about that?

Check out Dave's comments in this thread:

The #14 head on the 603, really bad? For sure?

Additionally, it would make far more sense to monitor refrigerant pressure. The concern is the high side pressure gets too high, so the fan engages to bring the condenser temperatures down, and the pressure falls with the drop in temperature.

Reference the subject vehicle, yes. Perhaps your comments were in reference to some other unspecified model. The thread that you referenced was discussing an '86 300 SDL. I am not sure how that relates to questions posed by the originator of this thread.

On the '82 300 SD, there are two switches on the receiver/dryer-the r/d temperature switch and the compressor low pressure cut-out switch. The temp switch closes at 126 degrees F, energizing the aux fan via the aux fan relay. The low pressure cut-out switch opens if the refrigerant pressure drops below 10 psi, removing power from the a/c compressor. The pressure switch is not directly tied to the operation of the aux fan. The fan is either off or on; it does not run at multiple speeds.

My comments were specifically with regard to the 603 engine. The thread starter's thoughts on the system were identical to the function of the system on a 603 engine.

I stated "603" in my original comment.

The fan being switched by temp, not pressure is the case of the setup in my 1981 300SD w126. The pressure switch only kept my AC system compressor from working (back in the days when it leaked). The Temp switch kicks the fan on when it gets terribly hot however as far as my electrical analysis goes, unless the AC system is getting hot, the fan won't turn on. The 603 of course is different but you have to consider you have a bigger, more sensitive engine to cool off. The 617 should have no problem with a good clean system keeping the engine cool under most reasonable circumstances. Also, anyone who realizes how "barely"powered the stock alternator is in these cars (atleast the w126 body) probably realizes how bad of an idea it could be to have the fan running often. The germans probably knew how to properly build their cars so I imagine if you are needing a booster fan with regular driving you probably have other problems (radiator, t-stat, nesessary cleaning)

My engine hasn't broke 100 C yet and the booster fan has only come on, to my recolection, when the AC was on for a long time and I was sitting in the driveway for some time while it was high 90's-low 100's outside.

If you are not daily stuck in hot standstill traffic WIHOUT your AC on, I wouldn't go attacking the wiring before doing some good preventative maintenence to the cooling system. It takes more power (and causes more wear technically) to run the AUX fan when you are using the same amount of engine power, mechanically, to already run the mechanical clutched fan. I'd rather get everything tip top before running that motor an overly large amount.

Just my thoughts


EDIT: Just to make it clear, the 1981 300SD has the 617 turbo in it.

This is exactly right.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

The aux fan was coming on and turning off randomly. I pulled the plug on the sensor mounted on the front side (closest to the radiator) of the themostat housing (not the acc coolant temp sensor) and the fan stopped running. I believe that in fact if you have this sensor it's function is to turn on the aux fan at high coolant temps.

I need to replace this sensor/switch or probably just pull it and clean it to see if I can again get the proper control again.

In the meantime it is disconnected for now since the car wouldn't hardly warm up with the fan running all the time. Of course this is probably a sign that the t-stat is going too and is stuck partially open becasue it sometimes will not even get up to 80C while driving down the highway.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

"The consensus seems to be "

Wouldn't it be better to consult a wiring diagram ?