Does the AC temp sensor on the side of the receiver dryer activate at high internal pressures to turn the aux fans on low or on high?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

The sw triggers the pre-resistor relay circuit,which feeds R15, which drops the voltage to the fan motors ,resulting in low speed....... BUT...if BOTH the sw for low speed AND the coolant temp high speed are called for at the same time, high fan wins...........all that the high pressure sw/relay do for the fan feed is they add the R15 resistor to the circuit..if you were to jumper the R15 while low fan was running, you would wind up with high fan.
The sole purpose of the R15 resistor is to limit [ drop] the battery voltage to the fan motors..

I noticed that the wires that are supposed to be going into the red high temperature switch in the side of the drier are both disconnected. I tried chasing these wires all the way from the drier to the radiator but then lost them. Wonder why they would be going toward the front of the radiator--not toward the relays.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Are you are telling me that the high pressure sw has been disconnected, and the wires that went to it are going towards the radiator??

Are the sw wires just hanging there???

If that is the case, then it is possible someone has added a temp sensor to the condenser/radiator fins instead of using the high side pressure sw to activate the fans.
[ you see these type switches on electric fan add-on kits]
..I would definately do some further checking as to where those wires that originally went to the original sw are now feeding...

The wires that come out of the red temperature sensor on the side of the receiver dryer are routed to two terminal clips, which have been unplugged be a dealer mechanic. The wires continue to run down toward the botom of the radiator and it looks like they may be supplying power to the motors, but I can't tell without further investigation.

Take a look at the picture: There are two white plugs to the right of the high temperature a/c sensor. One is plugged in, the other isn't (actually both of these were unplugged when I found them). The wires come out of the sensor and make a 180 degree turn toward the front of the car, diving down toward the front of the radiator and running across the radiator.

I can't figure out why my a/c even works with the temperature switch out of the loop, although it's only blowing about 50 F in stop-and-go traffic at 85 F outside. At that time, the aux fans are on high. When the car gets moving, the temp drops back down to 40 F. All the time, the coolant never got above 92 C.

I noticed this by accident when I was replacing my aux fan relay, but if one of the wires coming out of the temperature sensor touches a ground, the aux fans will run on low, but smoke and melting wire smell will come from the top mount nut on the preresistor spring.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Someone has jerried the wires and eliminated the high sw.
They have probably used a temp sw that just uses the single wire from the relay and grounds it. Ground that wire and see if you get low fan..My guess is they did not want to open the system up to change the sw , so they did away with it [ they may not have even known that the refrigerant level was probaly the cause to begin with ..]
I also think the high fan you are seeing is b/c you have modified the coolant sensor to lower cut-in and that has nothing to do w/ac fan..
Do you have low fan w/jumper ????

I turned the key on (engine off) and jumpered the pressure switch (the black spaded connections). I left the wires connected and just pulled them back enough to get a paperclip to touch both leads. I did not hear the aux fan relay click.>>

Hold up..I think I found your mistake re-reading this last post.....The wires at the blk spade terminals is the wrong pressure sw ..that is the sw for compressor, not fan..the sw ypu call temp sw is actually the high pressure sw for the aux fan..it is the red one with the pigtails attached to it..that is fans , not the blk one.
Go redo it..the one I want you to jumper for fan is the one I just had you put the wires back on..it is referred to as a temp sw in the parts book, but it is actually a high pressure sw and it is the fan one..don't go fooling with the other spade connector sw , please.

Thanks. I have been up all night working on the car and another driver (my wife) will be using it today while I sleep a few hours. I will have a chance tonight to check it out.

I was operating under the apparently-mistaken assumption that the black sensor was the pressure switch. http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23N0NM8ZV23N0NMD92&year=1993&make=MB&model=400-E-001&category=R&part=A%2FC+Pressure+Switch and the red sensor was a temperature switch. http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23N0NM8ZV23N0NMD92&year=1993&make=MB&model=400-E-001&category=R&part=A%2FC+Temp.+Switch

No wonder I'm having problems.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Yeah, that part discription throws you if you do not know the system..that is why I always tell guys the sw with the pigtail attached..that way they do not go to the wrong sw.

So , basically , I think you had a bad connection at R15 and you may still be very slightly short of refrig, but not by much b/c fans are coming on after a while..
You now want to jumper the correct sw [ peel back sleeves at WHITE connectors and jumper right there ]
That verifies circuit for low fan and now you just have to contend with proper pressure of high side to activate the low fan..and as I said from the get-go, most low fan problems that PASS the jumper test are short on REFRIGERANT.. it is that simple....also, as SB points out in another recent thread, this short refrig/no low fan also attributes to higher than normal coolant temps b/c of the passive heat exchange between radiator/condenser brought on by no low fan..customer looks at you a little weird when you tell them their overheating car needs a little refrigerant, but that's what it needs, just to get the airflow of low fan back into the scheme of things..
Let us know....................

I jumpered the pigtails on the red pressure switch (the one I originally thought was the temperature switch) and I could hear the relay clicking. When I started the engine and jumpered the wires, the low speed fans came on. So, it looks like the wiring is correct. Now, I just have to contend with the less-than-adequate cooling at idle. Thanks again.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Now, the a/c problem:

I bought a gauge set and hooked it up to my a/c system. I serviced the system as per the manual and it took about half a can of R-134, but now I'm seeing something I don't quite understand. When I turn the key on and short the pigtails on the red pressure switch, I'm hearing the aux fan relay click, but when I start the car with the a/c calling for cold air, the low speed fans do not come on right away--even though the low side pressure is at 30 psi.

I watched this cycle that repeated itself continuously at idle: low side 30-40/high side 150 and climbing. When the high side reached 250 psi, the low speed fans came on. The high side pressure slowly went back down to 150 psi and the low speed fans would turn off. All the while, the low side was bouncing around between 30-40 psi. It seems that the high side would climb back up to 250 psi, the low speed fans would come on at that point, the high side pressure would slowly (about 2 minutes) drop back down to 150 psi and then the fans would go off. The a/c was blowing 40 degree air at idle from the vents.

Any ideas?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

oops, I think you mean the low side aux fan (controlled by the A/C HP switch) as the high side aux fan is certainly controlled by the engine coolant temp switch.

No ooops..time to learn some more.

The HIGH SIDE fan is controlled by the pressure of the HIGH SIDE, but it is a LOW SPEED fan
And the coolant temp of the engine controls the HIGH SPEED FAN..

You have your a/c pressure SIDES and your fan SPEEDS mixed up...
A refrigeration systems has two sides.. HIGH/LOW and that refers to the pressure of the system

The aux fan has two SPEEDS ..HIGH and LOW and that refers to how FAST/SLOW the fan motors RPM is..

To make it simple:

If the high side pressure goes up due to high thermal load conditions, the high side a/c pressure sw closes and activates the LOW SPEED AUX FAN/CIRCUIT.

,,and if the engine coolant gets to 105C due to high thermal load conditions, the coolant temp thermistor sensor activates the HIGH SPEED FAN/CIRCUIT.
That's all..

Thankyou, I understood the aux fan to be of a constant speed but its rpms were determined by which side of the resistor was powered by either of the two relays - one relay activated by the a/c HP switch circuit and the other by the engine coolant switch. Am still learning.

That is correct .. You have it down. They are two different circuits, but they both share the same fan motor..
And the resistor circuit is ac pressure controlled, with the resistor dropping the voltage , resulting in LOW fan..whereas the engine coolant temp circuit by-passes the resistor, giving full 12v to the same fan motor, reulting in HIGH fan.
Two easy diy test to verify these fan circuits is :
For Low fan, jumper the pigtail pressure sw at the ac drier.
. that test verifies low fan fuse, relay, resistor , and connecting wires.
For HIGH fan circuit, you simply remove the connector at the engine coolant temp sw/thermistor..that defaults the CC unit and results in HIGH fan. That test the high fan relay, fuse.wiring and CC unit..it does not test the resistor b/c the R is not in the high fan circuit.

These are very simple, fast tricks of the trade for circuit verification and are done whenever one suspects fan problems.