I've been searching and looking at the manual to try and diagnose this problem, but before I order one new fan I just want to make sure there is nothing else I can check.

My problem is that the passenger side auxiliary fan never comes on. When I checked this yesterday, I actually found that it seemed as though the block heater power cord was kind of stuck in the bottom of the fan, so it didn't turn very easily (if at all). If anything, that probably burnt out that fan motor over the last long while. Anyway, it doesn't come on ever now. The driver side auxiliary fan works fine and comes on at the right times on both low and high speed depending on what it needs.

If it were easy to get at on this car, I would try supplying power directly to the passenger fan, but the plug for it is in behind the fan shroud, and I don't want to try and take that off unless I can actually fix the problem by replacing the fan at the same time. I found instructions on replacing a fan on this forum for this car, so I don't want to go through all that just to test it and find out that it is indeed burnt out.

So, my question is, if the driver side auxiliary fan comes on just as normal, is there any reason that the passenger side doesn't come on other than the plug being loose or the motor being burnt out?

From looking at the wiring diagrams it only splits off to two cables near the fans at the X64/4 2-pin plug connection. Is this correct?

Also, is it possible to get at the plugs for the fans (behind the fan shroud) without performing the whole procedure to get a fan out? Can you get the fan shroud out of the way enough to check the plugs and possible try and apply power to the non-working fan?

Thanks, Dave

__________________
1991 MB 300D 2.5 Turbo (207000km) (now gone)
1993 Audi 100 CS Quattro (300000km)
1999 VW Jetta TDI (290000km)
2002 Audi A6 3.0 Quattro (150000km)
2005 MB E320 CDI (34000km)

If I recall correctly.
The two electric fans come on separately, one is base engine cooling, and the other is aux A/C cooling.
All control is on the groundside of circuit.
You must apply direct power to the fan when confirming good/bad condition, unless the bearings are bad.
The E300 should have four sensors that control the fans:
#1. A/C 2 bar low pressure and 30 bar high pressure binary switch.
#2. A/C Temp. Switch; Located on receiver drier for auxiliary cooling fan, identified by 2 spade terminals on short pigtail with red base.
#3. A/C ACC Temp Sensor "Blue"; Air conditioning "OFF" emergency switch, located in intake manifold 130 degrees Celsius, "Blue".
#4. Engine Temp Sensor; 3 pole terminals: color code green: 105C/115C Degrees

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1991 Mercedes Benz 300D Turbo 2.5
Part: A/C ACC Temp Sensor
Note: ACC Temperature Sensor
Blue
Air conditioning "OFF" emergency switch: located in intake manifold 130 degrees Celsius: Blue
R1075-26553 A/C ACC Temp Sensor Behr IN STOCK $29.40

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1991 Mercedes Benz 300D Turbo 2.5
Part: A/C Temp. Switch
Note: @ receiver drier
Located on receiver drier for auxiliary cooling fan. Identified by 2 spade terminals on short pigtail with red base.
R1055-29376 A/C Temp. Switch ACM IN STOCK $33.59

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1991 Mercedes Benz 300D Turbo 2.5
Part: A/C Pressure Switch
Note:
2 bar low pressure and 30 bar high-pressure binary switch. This switch is designed to be compatible with R-12 and R134a refrigerants. Located on receiver-drier with 2 spade terminals. Used for compressor cutout.
R1054-31171 A/C Pressure Switch ACM IN STOCK $30.39
R1054-31171 A/C Pressure Switch Aftermarket IN STOCK $23.61

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1991 Mercedes Benz 300D Turbo 2.5
Part: Engine Temp. Sensor
Note: 105/115 degrees
3 pole terminals: color code green: 105/115 Degrees
G5045-12325 Engine Temp. Sensor Behr IN STOCK $35.28

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1983 300D
1984 190D

According to my 124 ETM the fans are wired in parallel, both should operate at the same time at either low speed (controlled by A/C condensor pressure) or high speed (controlled by engine temp sensor)

If one fan is turning and the other is not you either have a bad fan or a faulity wire feeding power to the inoperative fan. The fans are energized via a 12 volt feed - they are not supplied with a ground signal for activation.

From everything I read I understand that they both energize at the same time and don't operate separately. The information on the parts for the sensors is of course very handy to have. Thanks whunter for that.

See the attached wiring diagram I've been looking at to try and figure this out.

Is it possible to get the fan shroud off to get at the plugs for the two fans without going through the whole procedure to take the fans out completely (to replace)?

Attached Thumbnails

 

__________________
1991 MB 300D 2.5 Turbo (207000km) (now gone)
1993 Audi 100 CS Quattro (300000km)
1999 VW Jetta TDI (290000km)
2002 Audi A6 3.0 Quattro (150000km)
2005 MB E320 CDI (34000km)

I pull the fans and bench test, when there is any doubt.
I have tested them in the vehicle, but do not recall how, this is why I like writing DIY's, I may need them later.
Just went through catastrophic hard drive loss on my two prime computers.
Rebuilding data files will take a month or more.
My total data loss was roughly 400-500GIG.

I hope you had a back up? Reminds me, I haven't done that in a while. I write a compressed copy of the entire HD to a couple of CD's as well as burning the Word files to a separate CD (not compressed)
As for the thermo switch, the green one (looks aqua to me). These tend to go bad because they are carrying the full fan current. You could short across the contacts and see if the fan comes on, probably easier on the knuckles than poking around trying to get at the fan plug.
Also, there is a grey switch that comes on at a lower temp (from the '95 300D), Dave gsxr recommends it be used in place of the green thermo sw. I've got one ready to install. FYI my green sw is only a year old in case somebody wants it cheap
I just realized the 602 IS in a later model car than the one gsxr made reference to, but since the wiring diagram is impossible for me to read I'll just make the above comments anyhow, not based on experience with this car. Still, thermo switches seem to have common failures in all cars, always suspect one if its involved!

__________________
'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 have taken out the fans and replaced the one. I followed the procedure in Neil Chandler's write up found in this thread:

124 aux fan replacement

I have taken some pictures of some of the stuff I had a tough time with and will add those pictures here later.

__________________
1991 MB 300D 2.5 Turbo (207000km) (now gone)
1993 Audi 100 CS Quattro (300000km)
1999 VW Jetta TDI (290000km)
2002 Audi A6 3.0 Quattro (150000km)
2005 MB E320 CDI (34000km)

Well, I did replace the one fan that wasn't working, but now neither appear to work under the car's power.

If I power the fans at the X64/4 connector (near the two lines going to and from the condenser), they both run fine.

But I can't get the fans to power up even if I disconnect the coolant temperature sensor on the top of the engine. When I did this yesterday, the one fan powered up just fine. Now, today, neither do. I didn't do anything in the engine compartment so I can't see how anything I did affected this. All I did was swap out the one fan. Very strange. Though, it is kind of cold out today (15 degrees C), but I don't see how that would affect anything, especially not unplugging the coolant temp sensor.

How can I figure out what is going on? The aux fan fuse is fine, it's #7 and D in my fuse box. They are both fine. I took the two aux fan relays out and put them back in, still nothing.

I just don't understand how yesterday night I could get the one that wasn't broken to spin, but now neither of them spin (unless I just give them power through that X64/4 connector).

What the heck!

Now it works. I unplug the coolant temp sensor and they both come on just fine now. So there must be some other thing that wasn't letting them turn on. The car has been sitting for a little while now and is in a warm garage, plus the car is heating up the garage. So I thought it was worth another check. It looks like it's all OK now.

Does anyone know what the extra thing is that wasn't making it come on is, though? Is it another temp sensor?

__________________
1991 MB 300D 2.5 Turbo (207000km) (now gone)
1993 Audi 100 CS Quattro (300000km)
1999 VW Jetta TDI (290000km)
2002 Audi A6 3.0 Quattro (150000km)
2005 MB E320 CDI (34000km)

In the attached photos, you can see the bolts that you need to undo for the bumper to come off (I pointed to them with red arrows).

You can also see the four clips to take off the outer headlight lens. The bottom two are MUCH easier to get at once you take the bumper off. To take the turnsignal lamp out, there is a little push clip inside the engine compartment near the air filter box that you need to push to release it. Then it just slides out forwards.

There are a couple pictures just showing the removal of the shroud and the fan, and a picture of only the shroud once it was out.

For those of us who like pictures, I hope this helps.

Attached Thumbnails