I need some help understanding how the auxiliary fans on my 1992 400E work. From searching other threads on this site it sounds like they should come on at low speed @ 105 degrees and then full speed some time later. It also says they should come on when the AC is on. Unfortunately, they only come on at about 110 (if gauge is right) at full speed. They do not come on earlier at low speed and do not come on when the AC is running. Can someone please confirm the logic used to control the fans and what relays/fuses I should check. I did look at the Auxiliary fan resistor and the terminals look rusty, but otherwise ok. I did jumper past the resistor to see if the fans came on any earlier and they didn't.

Thanks!
Bob

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You have it a little wrong

The high fan has nothing to do with the a/c system...and yours seems to be working correctly at 105 coolant temp.
A/c fan is LOW fan only. That is triggered by a/c high pressure sw . The test for that circuit is to jumper the sw at the reciever/drier with the pigtail wires on it. That should verify low fan circuit. If fan comes ON w/jumper, then you hav a bad sw. or low on refrigerant [ the latter being the most likely diagnosis]
If NO fan w/jumper, resistor.relay/fuse are problem
First tests is jumper high pressure sw..................

The receiver/dryer has 2 switches on it. unplugging one disabled the AC completely (black connectors). Unplugging the other (white connectors) didn't effect AC (still ran and blew cold). When I jumpered those together the auxiliary fan still didn't run.

The relay that clicks when I unplug the temperature switch on the engine is the 3rd one back on the left (black). Is that the same relay that would manage the low side? The relay power according to the paper inside the fuse box is #7 and it is good. Is it the same fuse for both high fan and AC fan?

Thanks again!
Bob

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>>The receiver/dryer has 2 switches on it. unplugging one disabled the AC completely (black connectors). Unplugging the other (white connectors) didn't effect AC (still ran and blew cold). When I jumpered those together the auxiliary fan still didn't run..

As I stated , the sw that is the a/c LOW fan at the reciever is the one with the PIGTAIL wires .
Do not disconnect the wires ..just peel back at the connectors and jumper right there [ key ON, of course]

Is that what you have done???

With the black connectors on the top sensor and AC running I unplugged the sensor via the pigtail with the white connectors and jumpered the two white connectors from the pigtail together and no fan. I can't see it would make any difference to leave them connected to the switch as a short (jumper) would become the shortest path and the switch down stream in parallel would make no difference. Would this confirm the problem is either the relay or the resistor?

Don't overlook bad wiring!

This model is prone to wiring harness problems. My 92 400E has had problems with the fan resistor and associated wiring on two separate occasions. Exactly what was replaced/repaired I do not know, but they specifically said they replaced some bad wire...

Good Luck!

__________________
02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)

OK

The reason I ask that the wires stay on the connectors is b/c I actually get guys that unplug the pigtail and then jumper the wires together from the SWITCH side , rather than the CIRCUIT side.. if you know electric circuits , you may find that hard to believe, but I get it quite often..we just want to complete the circuit by shunting the sw OUT of the circuit, so I just want to be on the same page as you are..

Anyway..when you jumper the sw wires , do you hear the relay click..if yes,
then leave jumper and go to R15 [ dropping resistor ]and see if you have 12v at the wire on the SINGLE wire terminal [ as opposed to the terminal with 2 wires ].

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

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

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



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 ..

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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.

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

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.

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

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.

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

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.

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83600.pdf (57.8 KB, 234 views)

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