You should read this whole thread from the beginning b/c it tells about the relays.

But , here it is again
The relays are in the relay box behind the fuse box You need to take the 6 screws off to get to them.
There is a relay for the high fan [which you claim is a low fan, but isn't] .and that relay is triggered by the coolant sensor that you modified the cut-in point on. That is ONE CIRCUIT. [ HIGH FAN}
There is another relay [ in the same case] that is triggered by the pressure sw you are jumping. That relay brings 12v down to the resistor, which then drops the voltage to the same fan motors , but this time they are a LOW speed fan b/c of the resistor. This is the SECOND CIRCUIT [ LOW FAN}
The point being, if you jumper the high pressure sw, you have to jumper the wires before the sw , not the wires coming out of the switch.. When you jumper these wires , the relay should CLICK.. IF it does not , then the problem is on the primary side of the relay circuit..if it does click, then you know that the relay is pulling in the contactors for the LOAD side of the relay, which should feed the 12v to the Resistor. That load side has a fuse .
IF the relay clicks and you still have no 12v at resistor, you have a fuse out or a bad relay.
You have brought 12v down to the resistor manually by-passing the relay and the fans came on, but the thing you want to do with that test is to bring the 12v to the single wire side of the resistor..That will test the resistor..if you brought 12v to the other side of the resistor, you have simple by-passed the resistor and verified nothing about the low fan circuit.
In essence, the true test is to bring 12v down to each side of the Resistor..[ one at a time]... on one side you should have High Fan, [ this will be the 2 wire side of R] on the other you should have LOW fan.. [ that will be the single wire side of R ] That will verify resistor and fan speeds
If you do finally get low fan w/high pressure sw jumper, do not condemn the sw..The problem on that condition is low refrigerant not allowing high pressure sw activation b/c it never reaches cut-in pressure.
Try those test and don't post back top terminal ,bottom temrinal, picture etc .
...that means nothing to me as I am working off a schematic. Give me a wire color or terminal #

Thanks, Arthur.

Sorry about my vague nomenclature. I checked again. When I jump the wires coming off the connector, essentially bypassing the pressure switch, I do not hear a relay click when the key is on. Of the two wires that connect to the fixed terminals of the pressure switch, there is 12 v coming out of one and nothing from the other; however, when I jump the wires and bypass the switch, I'm sure it is sending 12 v to the relay--or at least to the wire leading to the relay.

My resistor wires have been changed by the dealer, so I'm not sure the color would help you, and I apologize again for not being able to describe it with greater particularity. There is a smaller wire bolted to the top of the resistor and a much larger (almost coax-size) wire bolted to the bottom. The large wire on the bottom is black and I suspect there may be more than one wire under the insulation. When I run 12 v from the battery directly to the top terminal of the resistor (the small wire), the fans come on low. When I run it to the bottom terminal, the fans come on high. Thus, I think the resistor is okay.

It looks like a relay problem.

I noticed in your prior post to bkinnc, you said that the 400E chassis combines the two relays in a single case, but they work the same as the dual relays of the 124 chassis 6 cyclinder cars.

I'm looking at the black relay in my car (the third back on the left) (#001 542 8219). I guess that's the one that needs replaced.

http://catalog.worldpac.com/mercedes...5970192OES.JPG

<When I run 12 v from the battery directly to the top terminal of the resistor (the small wire), the fans come on low. When I run it to the bottom terminal, the fans come on high. Thus, I think the resistor is okay.>

Yes , That verifies Resistor is OK and fan motors are OK.

<It looks like a relay problem.>

Possible...I would take it out and see if there is a bad connection in any of the pins.. the relay internals have the schematic on the top of that relay, so you can do an ohm reading for each relay coil. That is a possible. But , having 12v at the high pressure sw would mean that power has to be going through the low fan relays coil..know what I mean..so the relay would more likely have a broken contactor.. if you ground the 12v lead [ which should be br/bu ] coming from the relay to the S32, that is the same as jumpering it, being that it is a switched ground circuit. { You mention in your opening statement of your last last post that you assume if s32 has 12v that the jumper would send that 12v back to the relay..that is not the case b/c it is a swicthed ground circuit and the 12v comes FROM the relay and completes the circuit through s32 to ground. So, you already know you have 12v at relay]
The reason we listen for relay cliking when jumping the S32 is b/c that confirms all is well in the primary circuit of the relay, minus the S32..but it does nothing to confirm load side circuit. It just lets us know the relay is pulling IN the contacts .. It is the relay contacts that actually bring the 12 v down to the Resistor. But you have to have 12v at the relay contacts from the preresistor fuse for that to work..so, it is no power OR bad relay..
Sometimes the pin connectors under the relay burn up from heat..you can look there too as you narrow this problem down..you are getting there ..

<<I noticed in your prior post to bkinnc, you said that the 400E chassis combines the two relays in a single case, but they work the same as the dual relays of the 124 chassis 6 cyclinder cars.>>

Yes, ..the big difference is the 2 relays of the 6 cyl. chassis are seperate relays and they have the LOAD fuses right on top of the relays..kinda nice and easy to check..... but the V8 have seperate fuses outside the fuse box..and I am not so sure that the fuse link you have used to jumper down to the resistor for your fan test is the pre-resistor fuse..I think that one is the Blower fuse..look over by the shock tower.
Also, you mentioned you have a schematic..can you tell me what schematic index # you have??...and what does your schem show for pre-resistor fuse..I have '92 version and it shows Aux fuse holder F22/2. Not sure about "93 as there were some changes.

I write this assuming the reader to know the basic functions of a standard relays primary side [ sensors activated] and LOAD side [ high Amp/Motor contacts].
If you know that , then you can see what is going on with both circuits and understand that the Benz aux fans design is in fact 2 different circuits , with there own sensors/relays, and fuses ,etc.... BUT, the 2 circuits SHARE the same fan motors ..THAT is what confuses everyone..

My schematic came from the Service Library Manual 83-606. It's not that helpful.

Oh..I see..

Well, we kinda hijacked this thread from Bob, but I think he doing something for the holiday as I have not gotten a reply from him in regards to if he would like me to jpg the schematic to him.
I have it ready to go, so as soon as he replies , I can also send it to you ..it is for his "92 version 400/500E, but I think it will help you out too.
Let me see if he replies..

Thanks. I'm sorry, Bob. Look at what you've attracted--some other guy with a troubled 400E. I would love a current jpg copy. Thanks again.

Very good..
If he does not return, I will get one to you either way.

By the way, on my car, the 30 amp fuse next to the relay box sends power to the aux fans. That's what I was using to get 12 volts to the top and bottom of the preresistor to test it. The 30 amp fuse next to the driver's side strut tower sends power to the blower motor under the windshield.

Here is schematic 93 and up. It looks like K9/1, K10 setup only used from 94.
Your car is 93 and should have a single K9 module with two relays inside.

OK , then that would be the N/22 aux fuse holder..

Just wanted to follow up - yes was otherwise occupied with holiday activities. My parts should be here tomorrow (resistor) and hopefully I'll get some time in the evening to replace the wire with the bad insulation and the resistor. I will report back after the repairs to let you know how things went.

Bob

Following up....Replaced the resistor and fixed the wiring issue and now I get low fan when I have the AC on and jump the pressure switch. Unfortunately it doesn't come on unless I jump the switch so I am off the the shop to get the AC charge checked and potentially replace the switch.

Thanks for all the help,
Bob

Look at the eyesight at reciever/drier..if it is foamy, you are more likely to be slightly short of refrigerant then to have a bad sw. If so, just a topping off usually takes care of that. If one has decent a/c cabin temps ,but not enough pressure on the high side to trigger the sw, it needs only a few ounces added to get to fan cut-in spec.......also be aware that one can also have the correct charge and no low fan if the AMBIENT temp is low..high side pressures do not get that high at low ambient temps and that condition is NORMAL..aux fan is only called upon at high thermal load conditions ..unlike many cars , the low fan does not ALLWAYS run when a/c runs, but rather , only when the systems is being taxed. The fan circuit is only there to keep the high side in check from runaway high pressure by adding airflow across the condenser..that cut-in spec is considered in it's design..and often mis-understood.
I see many guys change the switch and then claim it was the fault b/c the fan now works ..of couse it now works..you just got the refrig. back up to full charge when you changed the sw..a topping would have done the same ...the difference would only be in the $$$$$ amount it cost you..:)

I retested it today (outside temp ~ 75 F) and after the AC ran for 5 minutes or so the fans did come on. When looking in the sight glass I occasionally saw flurry of bubbles but for the most part it stayed pretty clear. I will test again today when the temperatures get to ~88 F. It does blow very cold inside the cabin.

On a related note, the AC always has cut off if I wind up the motor (like when passing). Usually anything above 5K RPM will cause it to stop blowing cold for 3-5 minutes. I figure the high pressure switch must be tripping, but I would that that would potentially indicate an over charge situation. Maybe I will just take it to a shop and get the level checked - we'll see how it goes.

Thanks,
Bob

I think you are OK
The comp cutout is in the design at WOT and high rpm..