W124 400E Aux Fan And A/C Questions - PeachParts Mercedes-Benz Forum

zeronero · #1 07-21-2006, 10:51 PM

Hi all, I'm down in Los Angeles for the summer, and as usual 100+ degree temps are the norm resulting in my use of the a/c way more than usual. This has brought some things having to do with the a/c to my attention.

While driving on the freeway at high speeds the a/c works fine, but when I'm on the streets or stuck in traffic which is about 90% of the time the a/c blows only moderately cold air. At first I just thought that it was something directly apart of the a/c system that was doing this. Yesterday while on the freeway in traffic the compressor shut off, so 100+ degree air started pouring in. I checked under the hood to verify that it was off, and when attempting to use the HVAC controls to turn the compressor on and off the compressor stayed off. I had to restart the car for it to come back on. While reading some posts someone mentioned that it could be from the high and low pressure parts of the system having issues. Further reading seemed to suggest that the aux fans during low speed driving should always be on when the a/c is on, mine don't every come on unless the engine gets too hot, in high speed mode I believe.

From what I've read I've come to the conclusion that the reason for my a/c not blowing cold enough air at low speeds is that the aux fans aren't coming on, and the reason for the compressor shutting off is that it was the system protecting itself from maybe the high low pressure issue, something which probably has to do with the aux fans not working. Does that make sense or am I totally off?

Also, can the aux fans be modified to come on after the engine is shut off to cool the engine bay on hot days for 5 or 10 minutes? I noticed Hondas do that and it seems like it might help disperse the heat.

dkveuro · #2 07-22-2006, 12:18 AM

If the fans are not running but the a/c compressor is...this will cause high side pressure to trip the cut out/over temp' switch.
You should wire the fans to be on 100% with the a/c compressor, in those temperatures ...a must if it's running 134.

You should see two WHITE plastic connectors near the drier.....bridge these wires to activate the fans..

If with fans running full time and comp' cuts out intermitently.......replace the Klima relay and check the date on the OVPR. Older than a few years, replace it too.

The speed sensor in back of a/c compressor is known pattern failure especially on reman's from aftermk't sources.

Sometimes the pc in the dash panels fail...solder breaks.

You'll need an after run relay with a heat sensor for an up to 20 minute fans 'on' after shut off engine.

.

400E · #3 07-22-2006, 12:43 AM

A very common problem with MB aux fans is the low-speed resistor fails. You can find this resistor behind the left headlight.

Low-speed fans are designed to run only when refrigerant pressure exceeds a certain limit, so in contrast to many other makes, your aux fans will NOT necessarily come on every time the AC is on. If you do wire them to run whenever AC is on, there is some concern (from other lists) that they won't last as long, and they're not cheap to replace.

High-speed fans come on when coolant temp exceeds a certain limit (don't recall exactly how high).

FYI, if your 400E is like mine (and I think it is), you don't have a Klima relay or over-voltage protection relay -- these functions have been incorporated into the 4 circuit boards in the protective metal case on the right side of the firewall (i.e., where the battery would normally be in most other W124s).

MrCjames · #4 07-22-2006, 01:24 AM

Quote:

Originally Posted by 400E

A very common problem with MB aux fans is the low-speed resistor fails. You can find this resistor behind the left headlight.

Low-speed fans are designed to run only when refrigerant pressure exceeds a certain limit, so in contrast to many other makes, your aux fans will NOT necessarily come on every time the AC is on. If you do wire them to run whenever AC is on, there is some concern (from other lists) that they won't last as long, and they're not cheap to replace.

High-speed fans come on when coolant temp exceeds a certain limit (don't recall exactly how high).

FYI, if your 400E is like mine (and I think it is), you don't have a Klima relay or over-voltage protection relay -- these functions have been incorporated into the 4 circuit boards in the protective metal case on the right side of the firewall (i.e., where the battery would normally be in most other W124s).

I agree with everything you have stated. Inside the module box, one of the controls units is responsible for controlling the AC compressor as well as several other devices. This control unit is known as the Base Module (BM), it is the one fitted with fuses. At this time I would not worry about it being a problem because it sounds like it is doing it's job because the compressor is cycling. You might want to begin by checking the refrigerant level, if it is low it could explain the unpleasant temperature you are getting at the vents while idling.

Now the aux fans are another story, it sounds like you need to check the ballast resistor as 400E suggested. As well, you may want to remove the fuse box cover to expose the relay section, two of these relays control the aux fans. One is for hi speed and one is for low speed, some of the fan relays have built in fuses 15A (blue) 30A (green). If you discover your relays are fitted with fuses than I might suspect a new 15A fuse may solve the low speed fan problem.

FYI-the blue 2 pole switch on the intake manifold controls the aux fan high speed circuit. Fan cut in voltage is roughly 1.5 VDC and the cut out voltage is roughly 1.75 VDC. A 2k mini potentiometer (15 turn is best) adjusted to roughly 1450 ohms and then soldered to each leg of the sensor lead will help provide for better fan function. The low speed fans will cut in a little bit sooner and they will stay engaged a little longer................the advantage to using the mini pot is that you can make subtle adjustments whenever your driving conditions change. NICE modification.

mbshop · #5 07-22-2006, 10:53 PM

i had one 400e that drove us nuts. no power to the aux fans.
checked everything. had power out of the relay.
finally we just tore into the wiring.
what the heck?!! found a fuse in the middle of nowhere.
not shown in the wiring diagram either. factory installed.
so just in case, look for a lonely fuse holder on the left
side of the eng compartment.

george

Arthur Dalton · #6 07-23-2006, 03:20 AM

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That would effectively change the sensors value for High fan , not Low fan. Low fan is triggered by a/c high side pressure and changing the cut in of the coolant termistor sensor has no effect on low fan circuit.

zeronero · #7 07-23-2006, 03:09 PM

Well I decided to take a look in the engine bay to try to find the source of the problem. I thought a good place to start would be with the aux fans since I know for a fact that they do not work in the low speed setting.
I tried to find the white plastic connectors near the drier and think I found them but can't tell for sure. I also tried to find the low-speed resistor behind the headlight but I really didn't know what to look for. I then decided to look at the relays behind the fuse box and couldn't tell which relay's went to what. I took some pictures of what I the drier and the fuse box, maybe someone could point the proper relays out to me.

I also noticed a small black box just behind the brake booster that I thought might have been related to the fan, when I popped the cover off there was a blown white colored fuse. I have no clue what this goes to but it still has power going to it. I took a picture of it as well, under the cover there's a place for two fuses but only one space has wires going to it. If anyone knows what this is let me know, it seems as if this car has random fuses spread around everywhere. It's the box in the middle in the picture.

zeronero · #8 08-13-2006, 03:54 PM

Finally it's fixed, as mentioned before it was the wire to the low speed resistor that was the problem. The wire leading to the resistor on the top was completely burnt up and barely connected. I cut and cleaned the wire, crimped on a new connection and it solved the problem. When I first turned the A/C on and waited for the fans to come on something around the resistor started to smoke but then stopped in about a minute, I think it was just burn off of the left over burnt wiring. Now all seems well and the A/C works much better in traffic and slow driving.

Thanks to all who replied.