Can anyone tell me how to test the auxilary cooling fan? I need to know where the the sesnor is and when it is supposed to run. I have tested the fan itself and it spins although the bearings sound a little noisey. Is it supposed to come on when the AC ( nwhich does not work) is turned on, or is it just turned on by the tempreture senser alone? I have just got this car and do not want to crack the head! Thanks

To test to see if the aux fan is functional, you need to jump the plug that attaches to the compressor. There should be two plugs to choose from. One will be the low pressure sensor (I think) and the other will be the aux fan. I have done this to my car, and the aux fan will run when jumped. It will not work otherwise. I know my compressor does not get power (I have no idea where to track this problem down from) so I figure that the fan will not run without the AC system being functional *shrug*

Wish I could help more...

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Fantom71 aka Dale
Madison, AL

91 190E 2.6 5 speed 220k miles and this car still scoots!

The aux fan on the 1987 300D / TD works a bit differently than earlier models. The fan will come on LOW speed depending on refrigerant pressure, and on HIGH speed when coolant temperature reaches 105C.

Should the aux fan come one every time the A/C system is on? Where is the pressure switch I need to check, its on the compressor? And there's another temp switch for coolant temps as well?

My A/C is cold when I'm moving, but when going slow in traffic, it barely feels cool (but cooler compared to when in EC mode). So I pulled over and noticed that the aux fan wasn't on and the condenser coils are super hot.

My coolant temps have always been below the 2/3 line (is that 105deg?).

I checked the fuse, its fine. Is there a relay I need to check as well? When I have a chance, I'll put 12v to the fan and see if the fan works.

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1970 280SE Cab
1983 300D Euro 4-speed
past MB's:
1971 280SE 3.5 s/r
1981 300D
1985 300TD
1987 300TD

The aux fan does not come on when AC is first engaged. Unless the temp switch tells the ECU that the refrigerant is HOT.
The three terminal temp switch is on the head, near the front, it looks like the attached. The pressure sw is on the receiver/drier. The three term switches are notorious for going bad (open connection) and there is a preferred alternative according to gsxr, that closes at a lower temp. Its the one used on later models and gassers, and is grey instead of the pale blue green colored stock switch. details here: http://www.w124performance.com/images/W124_stuff/Temp_switch_105-12x.jpg



also see this thread: 124 Acc

Attached Thumbnails

 

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

I suggest cheking the fan first and troubl shoot it backwards. If it turns freely by hand, disconnect it from the last harness before the fan and apply 12v to it. I used small guage wire jumpers from the battery and it would not turn. Using my battery charger made it turn. If it runs with direct 12v, see if the switch is getting power. no power at switch, check relay, etc...
Some electronic spray cleaner, emery cloth and dielectric grease is what cured mine.

Auxiliary Fans on a 124 300D (602)

__________________
1999 E300DT (131,800) 154,000 Black on Black SOLD

2006 CLK 500 coupe Capri Blue on Grey (zoom,zoom)
47,000mi

04 VW TDI Passat 80,000mi
(Techno)

How to eliminate oil dependency through market-driven approaches.
“We could cut oil use in half by 2025, and by 2040, oil use could be zero,”
The Sound of Diesel Speed
Ode to MB

I have a 100/110 in mine from a 190. 006 545 4224 I don't want my new 22 01 head to ever see 110.

Chris

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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

Now, that's brand new info.

That's the switch that I want.

Have you seen it engage at 100°C?

The one I bought the car with did not so I changed it out. Now when I'm going up a hill with the A/C on the fan comes on right at 100. It has never gone over 105.

Chris

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

My aux fan ended up working fine when powered directly. I put in a new switch, but can't get the temps hot enough to get it to turn on. My temps have always been in the middle, and will start to creep close to the 2/3's mark when climbing long hills, but it never gets there, so I can't get the fan to turn on to test the new switch.

It doesn't matter now anyway, after a few weeks after a evac and recharge, the a/c is no longer cool even when moving...I just hope I don't find any dye around the evaporator...

__________________
1970 280SE Cab
1983 300D Euro 4-speed
past MB's:
1971 280SE 3.5 s/r
1981 300D
1985 300TD
1987 300TD

1) The fan turns on low speed based on refrigerant pressure. The old (green) switch didn't trigger until 20 bar (290psi), which means the fan rarely ever came on. The new (red) switch triggers at 16 bar (230psi) and the fan will run much more often with the new red switch. Any time you open the A/C system, replace the green switch with red! This is a pressure switch only, not temperature. (The older W123 models used a temperature based switch.)

2) The high speed is triggered by the first stage of the 3-prong switch at 105°C. If your engine never reaches this temp, that is a GOOD thing. Every switch that I have seen over 15 years old has been bad... if your switch is old, or of unknown age, just replace it. Stock is 105/128 on the 1987 300D/TD but I replaced mine with the 105/120 switch from a 1987 300SDL.

3) The second stage of the 3-prong switch (the 120 or 128 rating) has NOTHING to do with the fan. This is the safety circuit which will turn off the A/C compressor to reduce the heat load on the engine. I would NOT want one turning off my A/C at only 110C.

4) The ECU does nothing, repeat nothing, to control the electric cooling fan.

__________________
Dave
Boise, ID
Check out my website photos, documents, and movies!

You can also trigger the temp sooner with one of Jim's little gems
Mine (CH92) is working like a charm no temps over 100C now using the A/C in stop & go traffic.

__________________
1999 E300DT (131,800) 154,000 Black on Black SOLD

2006 CLK 500 coupe Capri Blue on Grey (zoom,zoom)
47,000mi

04 VW TDI Passat 80,000mi
(Techno)

How to eliminate oil dependency through market-driven approaches.
“We could cut oil use in half by 2025, and by 2040, oil use could be zero,”
The Sound of Diesel Speed
Ode to MB

That trick doesn't work on the older cars with the 3-prong sensors shown above, which are simple on/off switches.

Jim's trick works on later models that use resistive sensors to tell the ECU when to turn on the fans. This works great, and I do have this installed on my E500. Doesn't apply to diesels, however - they don't use that setup.

Works like a Champ on my 92 OM602

__________________
1999 E300DT (131,800) 154,000 Black on Black SOLD

2006 CLK 500 coupe Capri Blue on Grey (zoom,zoom)
47,000mi

04 VW TDI Passat 80,000mi
(Techno)

How to eliminate oil dependency through market-driven approaches.
“We could cut oil use in half by 2025, and by 2040, oil use could be zero,”
The Sound of Diesel Speed
Ode to MB

Woops. Might be applicable from early 90's onward for the diesels, then? Definitely not up through 1989 however, and the original question in this thread was about a 1987 model.