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Old 08-24-2002, 11:59 AM
Registered User
Join Date: Jun 2002
Location: Charlotte, NC
Posts: 102
Aux Fans - My solution

Forum members:

I have received a tremendous amount of valuable information reading posts on the forum. In light of that, I thought I would post this in hopes it would help other members.

I bought my MB about 3 months ago - right at the beginning of summer. I soon discovered MANY differences in design philosophies between MB and japanese car makers. Some I like, and some I don't like. One of the things I particularly don't like is the way the cooling system functions ... in particular - the fact that the temps fluctuate widely between 85 and 105 C.

My first post was a question about aux fans (after reading several hours of prior aux fans posts) and got a very in depth description of the way the circuitry is designed. This was very helpful in acquainting me with the design, but left me somewhat un-impressed with the philoshophy behind the design ... that is that it is OK for the car to run at temps of 100C+.

The responses from that first post started me on my quest...Modify the circuitry so that the engine temps stay within a narrower range. The biggest contributor to temp swings seemed to be the AC. If you have read any of the Aux Fans posts you already know that the aux fans are enabled in the low speed by the closing of a pressure sensitive switch on the receiver dryer. This switch has a threshold level that appears to be reached only when the AC is working "above normal" and producing highside pressures at or above 280psi. However, when the AC is not working quite that hard, the switch is not closed, and the fans do not come on, and the engine heats up, etc....

My desire was to have the fans come on in low speed when the compressor was running. I found a schematic on this site showing the circuitry of the Automatic Climate Control and proceeded with my design.

The prerequisites of the design were.
1. I did not compromise the integrity of the existing circuitry.
2. The mods could be installed by anyone.
3. The mods could be "undone" very easily.

One of the mitigating factors in the ensuing design was that I did not have access to the factory maintenance CD. Therefore, I will readily admit that there is a better way to do this which I will address at the end of the post.

Conceptually what I designed was this.
Since the pressure switch on the receiver/dryer engages the fans, I would simulate that event when the compressor clutch has power to it. thereby tuning on the fans in low speed.

1. The plug that goes to the compresses has three wires. The wires provide power to the clutch (1 wire), and feedback from the compressor to the AC control that the compressor is indeed rotating (2 wires). The plug can be "opened" very easily. I unsoldered the wire from the pin that feeds power to the clutch (blue or green) and removed the pin. I bought a new pin from MB and soldered this wire and a "new wire" to it and put it back into the plug, closed the plug and re-connected the plug to the compressor. I now have my new "signal" wire in place.

2. The next step provides access to the pressure switch so it can be sumulated to close. There are two wires on the pressure switch and on my car (92 300E) they run about 6 inches before connecting to a wire harness. I unplugged the wires and attached adapters to the male ends of the connectors on these two wires. The adapters are 1 female to 2 male and provide the ability to attach back the original wiring harness, and the two new wires that will make up part of the new circuit. MAKE SURE THEY ARE INSULATED!!!!! There is 12 volts on one of the wires and on both wires when the pressure switch is engaged.

3. I then ran 3 - 14ga wires to my new circuit. The new wire that was added in step 1 (the compress clutch), and two new wires that will be attached to the adapters added in step two. The circuit merely takes the presence of 12V on the clutch to turn on a relay that simulates the closing of the pressure switch on the receiver dryer.

4. I went to Radio Shack and bought a 12V relay capable of handling 30A and mounted it near the battery. There is a nice open area there and is somewhat isolated from the heat of the engine. After mounting the relay, I ran the wire from step 1 (compressor clutch) to the relays activation terminal, and ran the two wires from the pressure switch to the relays contacts. That leaves one terminal left on the relay, and that is the ground. I found a small hole already drilled by the strut mounting and attached a lug to the hole with a machine screw and ran a wire to the ground terminal on the relay. MAKE SURE ALL THE CONNECTORS ARE FULLY INSULATED!!!!!!!!

5. The "turn on" voltage and current for the relay are 6V and 160ma respectively. In order to minimize the current drain on the circuit feeding the compressor clutch, I put a 50ohm 1 watt resistor in series with the ground wire mentioned in step 4. The value of 50 ohms was derived from the specs of the relay. The coil portion of the relay has a resistance of 66 ohms. Adding a 50 ohm resistor in series will leave just about 7 volts across the coil and will therefore engage the relay.

Now when the compressor is on, the fans run in low speed. After 3 weeks of testing, I have had no problems.

It is quite easy to disable the circuit. Simply remove the wire from the compressor clutch going into the relay.

It seems a better way to do this whole thing would be to locate the wire feeding the compressor clutch and add another feed to the low speed aux fan relay in the fuse box. Not knowing where a lot of this stuff is kept me from doing this.

This mod coupled with the mod that adds a parallel resistor to the temp sensing unit that turns on the aux fans in high speed (located somewhere elsewhere on this site) should keep the engine temps confined to a narrower fluctuation range.

I realize that doing this will conflict with philophies of a lot of members.

If you would like more details, I can provide as well as some pics.

Hope this is helpful.

If anyone would be so kind as to tell me where the O2 sensor is on my car I would appreciate that info.


David Lee
92 300E - 116K miles - Sold
77 Euro 350SL - 67k miles
94 Explorer 147k miles
2009 Hyundai Genesis - 65k miles
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