Your system is now working correctly. The HP switch switches on at 20 bar pressure and switches off at 15 bar pressure and yes it controls the low speed aux fan relay.

The LP switch is there only to protect the compressor if you should loose a/c gas it has no effect on the aux fans, so if pressure is lost the compressor disingages.

I notice on many forums confusion with calling the red/green switches temperature switches.

regards

The confusion is b/c the parts catalogs refers to high side aux fan sw as a temp sw, when it is a pressure sw..I think they do that to differentiate that sw from the actual comp pressure sw , which they do properly refer to as a pressure sw, whereas the aux fan sw does control the heat coming off the condenser, but that does not justify calling it a temp sw [ the specs are in Bar, not C , so I don't what they are smoking].. you will notice that they cover that mistake by adding notation of the actual electrical function in the part discription along with the pic of each sw..They now say "Aux fan cut-in w/pigtail for ac" and "Compressor cut-out " for the other.

Confusing, but that at least helps.
..I alway refer to aux fan sw as the one with the 'Pigtail leads" to aid anyone using my diagnostic post..seems to work most of the time. I also tell them when jumping it for low fan test, to leave the wire connectors on the sw and just peel back the white connector sleeves at the pigtail to jumper there b/c , for unknown reason to me, I get a lot of guys who unplug those connectors for jumper test and then they jumper the wires on the sw side, instead of the circuit side..don't know how they think that is going to register back to the relay, but they do it all the time,so I have to have them leave the connections intact so I don't go on a goose chase..it works for me.

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.

Thanks to both of you. I was operating under the belief that the low speed fans would turn on immediately after I started the engine and called for cold air, presuming my a/c system was properly charged.

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

< >>

The low fan comes on only when the high side presure is in need of additional airflow due to high thermal load condition. Like sitting at idle on a warm day..if the ambient temp is low ,you do not have a high thermal load and the aux fan is not called for and will not come on at all. Both the Low ac fan and the High coolant temp fan only come on when either the coolant temps or ac pressures are being TAXED by high termal conditions..that is why they call them Aux Fans..They are not needed or called for under NORMAL conditions . But one usually sees a low fan for ac b/c the ac system is usually only being used when such a condition exist [ Hot day].
I have done ac work in the NE and aux fan activation is much less b/c of the climate than I see here in Fla. It is the ambient temp that is the main factor in determining if the system needs additional airflow to keep the condenser pressures in check. Again, the existance of a high thermal load condition... no load/no fan.
Just remember that if one does have a hot day with the ac on and has no aux fan activation, to a Tech, that is a tip off/ early warning sign of a low refrigerant condition. But some further testing is required to make a true diagnosis..
[ Guages to compare sw. cut-in specs to actual high side pressures]

I noted that the low fans did not come on until the high side got to 250 psi, but they did not turn off until the high side fell back down to 150 psi. Is this a dual switch that allows the fans to bracket the high pressure within such a pressure range, or is the measurement taken from the low side?

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

<>

No , it is not a dual sw . It is a simple ON/OFF pressure sw.

A pressure sw has an ON [ cut-in] spec and an OFF [ cut-out] spec.
Do you think the sw can turn on/off at one pressure??

So , when you get to to a dangerous high pressure , the sw closes [ cuts in] and the now activated fan airflow brings that dangerous pressure back down..when the pressure gets down far enough that the fan is no longer needed , the sw opens [ cuts out] relieving the fan of it's duties . In other words , that cycle is over..sw cut in until pressure dropped , at which time it cut out..
If your sw did not have a cut-out spec , then once it was triggered, it would run forever, see?????????
Once the car is traveling, the presures in the condenser drops b/c you have a good airflow from car movement, at which time the high pressure sw opens and disconnects the no longer needed aux fan circuit...but stop at a light/traffic jam for a few minutes and guess what happens.????.......yup, high side pressure rise from no airflow and a new fan cycle begins.... you get it now.
This confusion comes from the fact that cheaper cars with small condensers are designed with a fan on all the time the a/c is on, just cuz they are maxed out..not the case on this design.

Thanks. That makes sense. I'm just glad my a/c is working well again. With all your help, I was able to figure out what was wrong with it.

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

Very good....

Hi Guys
It is interesting to hear what happens to make the aux fan run.
But I have a question that is related. That is when you push the A/C button on the PBC does the aux fan start to run ( I have a 88 190E ) and according to the wire dia only to thinge control the aux fan and that is the HP swt and the PBC.
THanks
Chuck