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 ].

speace,

yes, I have had to deal with the wiring harness issues - replaced the engine harness 4 months back due to the cheap insulation on it. It also looks like the insulation on the single lead on the dropping resistor has fallen apart.

Arthur Dalton,

It does click when I jump it but I didn't get any power on the resistor. I did locate the fuse for that (on the shock tower - I really need to get a decent wiring diagram for this car) and it was blown. I replaced the fuse and now get 12V on the dropping resistor. Since the insulation was falling apart I thought I would replace some of the wiring and broke the dropping resistor terminal trying to get it apart. So now it looks like a new dropping resistor and some wiring replacement is in order. One thought I had was to removing the dropping resistor completely to aid in cooling (read that in another thread somewhere) but now sure I am ready to do that. I think I will just replace it and see hot it runs in the 90+ degree weather here. Do you see any potential problems in bypassing the dropping resistor?

Thanks again,
Bob

The aux fuse [ F22/2] that you replaced was correct. That was the next step in diagnosis.
The problem one runs into with jumping the R15 is that the wire from the low fan relay load terminal to the R15 terminal is too small for the additional current draw. The low fan with the resistor draws only 1/2 the power of high fan w/o resistance... which is why the Factory went with a lighter gauge wire for low fan load circuit. Some guys do this R15 jumper as a modification in temperate climates to gain the additional air flow for condenser. but they rewire that connecting wire to a larger gauge wire.
If that is a plan, the wire that has to be beefed is the one blk/vi connected on R15 [ single wire side] back to K9 relay , terminal #1.
I will also answer a couple of your questions for future reference and to help in your understanding.
The fuse #7 is used to feed the coil side of both relays [ high/low]. This is only the primary side of the relays. If the relays do not CLICK when checking the sensors, this is where one would look . That is why I asked about the clicking. The fuse schematic actualy refers to this fuse as K9 relay fuse, not aux fan fuse. It just power the relays, not the fans. Now , the other part of the relays is the LOAD side ..That is the contacts themselves and they have fuses seperate from all else. That is the aux fan motor fuse you have at the tower.
On your chassis , they combine the two relay in a single case , but they work the same as the dual relays of the 124 chassis 6 cyclinder cars.

Here is your relay . It should have the schematic on the top of the case.

http://catalog.worldpac.com/mercedesshop/sophio/image.jsp?title=Auxiliary%20Fan%20Relay&url=http%3A//img.eautopartscatalog.com/live/P205970192OES.JPG

You seem to have a handle on it now. I do have yor 124.034 schematic up on a screen here for diagnosing this post, so if you PM me an addy, I will send you a jpg if desired.
I would stay with the R15 unit, but I would also do a resistor bridge modification on the coolant temp sensor on the block to lower it's cut-in from 105 to 100C. This gives you the edge when high ambients present themselves. The main guy you want to protect is the engine coolant..It is the lack of a low fan with the a/c on that causes the coolant to jump b/c the condenser is passively heating the radiator and taxing it's heat transfer capacity. Low fan keeps both that and a/c high-side pressure in check.
If you have any other ???s, I will answer them.
You can do that coolant temp in a couple of min and for $2.
RS item/ Archieves.
You can also get a new R15 here from the Forum

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU23G0SK109&year=1992&make=MB&model=400-E-001&category=G&part=Aux.+Fan+Resistor

Regarding the fan speeds, I have done the resistance modification on the coolant temperate sensor to come on at 92 degrees C. I noticed, though, that the fans only come on at the low speed when it reaches 92 C. I haven't seen coolant temperatures above 100 C yet, even in stop-and-go traffic at 100 F outside, so I don't know if the high speed is supposed to kick in at a higher temp or whether I should be looking for something wrong with my fans. When the fans are commanded to come on by the coolant sensor--not the A/C sensor--should they come on at high speed only? Thanks.

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

Did the fans come on high before the sensor mod??
The mod does nothing different to any system it is installed in..it only changes the R value of the sensor at the same temp. So, it actually does nothing different except it fools the N/22 module into thinking that the temp is as spec , when in reality, what yuo now have is an "Out of Calibration "sensor..The ECU has no way to know this as it only reads OHMS, not TEMP. and has no other sensor to compare to.
In 93 , there was some aux fan changes , but I can not see them b/c they do not show K9 internals ..however, the V8s with different speeds usually has 2 dropping Resistor instead of 1 ..is that what you have.???

Okay. I was just under the hood with a voltage reader and some jumpers. Here's what I discovered: I think it is the low fan speed that is not working, because it is fairly hot here in Arizona right now and they do not turn on at all with the a/c running and the temperature below 92 C. Yesterday, I did hear the fans running when the coolant temp went above 92 C, but they sounded to me like they were running in low.

There is voltage coming out of one of the pigtail wires on top of the receiver dryer. I jumpered the pigtails (1st picture) and, with them jumpered, measured the voltage at the top terminal for the resistor behind the front left headlight. There is no voltage there.

When I tapped into the 30 amp strip fuse toward the firewall (2nd picture) and connected that directly to the top terminal of the resistor (3rd picture), the fans came on. So I think the resistor is okay. This makes me wonder whether there is a problem with the wiring that leads to the top terminal of the resistor. I am presuming that the wiring goes from the 30 amp strip fuse directly to the resistor terminal. Please correct me if I am wrong.


Is there a relay between the 30 amp strip fuse and the resistor? I tried to trace the wire from the strip fuse to the resistor, but it is bundled and tie wrapped with a bunch of others.

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

With key ON, do you hear the relay click when you jumper the high pressure sw??? [ I hope your are jumpering the sw on the Circuit side, not the sw side]

No. I don't hear the relay click when I jumper the switch with the key on. I'm jumpering the wires (on the circuit side) that came off the sensor--not the sensor iteslf. See picture #1 in my prior post. I'm using a paper clip to jumper it. I think I'm jumping it correctly. Where is the relay?

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

I'm looking at the schematic and I see two relays: an aux fan preresistor relay (K10) and an aux fan relay (K9). I can see the aux fan pressure switch that I jumped (S32), and it looks like that relay runs a separate voltage to the aux fan preresistor relay (K10), then to the preresistor (R15), then to the aux fan (M4). Am I supposed to hear the preresistor relay clicking when I jumper S32 with the key on?

It looks like the aux fan relay itself (K10) sends separate power to the fan, probably in a stepped-up voltage. My high stage fans (I'm now presuming that they are running at full speed) are working okay and come on at 92 C, where the Cool Harness is supposed to activate them.

I have no idea where either of the relays are located under the hood though. Can anyone help point me to them?

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