"W124 Cabin Sensor" - PeachParts Mercedes-Benz Forum

dablk1 · #1 03-30-2010, 11:59 AM

I believe that the blower that draws the air past my cabin sensor isn't working properly. The interior never maintains the correct temperature.
I was told yesterday to try the "paper test" whereas the fuctionality can be tested by tearing off a very small piece of plain paper, or even tissue paper and placing it over the vent in front of the sunroof switch above the rear view mirror and it should stay. Nevertheless the paper didn't stay.
Also on another note, while my car is idling without the climate control it maitains perfect cooling temperature. When I turn on the heat, not the a/c, the coolant temperature starts rising. Yesterday I believe that the high speed fans came on to cool the engine due to the fact that it hit 100 C. Does anyone have a clue as to what can be causing this?

dablk1 · #2 03-30-2010, 12:01 PM

Forgot to mention. It's a 1991 300D 2.5 turbo.

babymog · #3 03-30-2010, 02:52 PM

The cooling system symptom is an odd one. I do have a possibility though:

When the system is running A/C, the cooling fans should be running, which will help to keep the engine cool. With the heating running (no A/C on), no fans unless the engine gets to the high-temp cut-in for the fans (205*?). Perhaps your clutch fan is not engaging, allowing the engine temp to rise when the A/C and aux fans are not running.

As far as the temp-sensor blower, two recent threads on the same subject should point you to possible culprits: http://www.peachparts.com/shopforum/showthread.php?t=274468 and http://www.peachparts.com/shopforum/showthread.php?t=267966

dablk1 · #4 03-30-2010, 03:04 PM

Thanks for checking in. I sent you an email earlier. How do I check the clutch fan? I'll als take a look at those othere threads.

dablk1 · #5 03-30-2010, 03:25 PM

By the way Jeff as I was driving just a few minutes ago; hot air was blowing through the side vents and I didn't have the heat on.

babymog · #6 03-30-2010, 03:56 PM

When I started my car yesterday, it was ~45* out, car was parked in the sun and heat run when last parked. The cabin temp was high from the sun, so the system went into cooling mode (econ selected so no A/C compressor), and hot air came out of the dash center vents which do not dispense heater air.

The air in the system was hot, and it took a few seconds for the flaps to catch up. Then cold outside air.

Similar conditions will produce odd results.

I have had a few CCU failures (the pushbutton unit). Usually they fail with the A/C compressor coming on in all modes, pretty much at random, and other odd symptoms. There was a recall for solder failures in some of the Bosch units in MY1987, not on the '91s though AFAIK. A vacuum failure should have defaulted to the defrost position, which is the side vents and the windshield vents, and probably full-hot, do you have any significant vacuum leaks? Checking with a Mityvac or equivalent the input to the vacuum control unit will tell you if you have a failure of one or more of the vacuum pods under the dash. This is made easier if your car doesn't have the (optional) passenger airbag, as the glovebox liner can be removed fairly easily and the vacuum control module is right there, you can test the vacuum pods one at a time.

dablk1 · #7 03-30-2010, 04:14 PM

I figured out what happened. I was going to diagnose the low speed fan issue I'm having. When I disconnected the 4 wires held together by the 2 white connectors next to the drier(low speed) I got pulled away. When I came back, I must have connected them incorrectly. I just switched them back and the temperature is fine now. I won't be able to get back to the fan issue for a couple of hours.

dablk1 · #8 03-30-2010, 05:40 PM

Disregard my last post. It's still doing the same thing. The coolant level dropped and there are no apparent leaks anywhere. Does this sound like a waterpump issue?

ds190 · #9 03-30-2010, 11:28 PM

Issue 1:

The car failed the 1cm x 1cm piece of paper held up to temp sensor vent.

A hose connects that vent to the aspirator blower (or ventilation blower or sampler blower motor) in the dash just under the right air vent.

Replace it.

dablk1 · #10 03-31-2010, 02:41 AM

Am I replacing the hose or the aspirator?

babymog · #11 03-31-2010, 08:42 AM

Neither until you find the source of your symptom, see post #3 and the links contained.

dablk1 · #12 03-31-2010, 09:02 AM

Well Jeff I was waiting for a forum reply about the clutch fan.(see post #4) If there is a clutch fan, that means that my car has 4 fans. I only see 3.

babymog · #13 03-31-2010, 09:22 AM

And I was responding to your question whether you are replacing the blower or hose.

The clutch fan is the engine-driven fan, it has a viscous-clutch that is temperature controlled and RPM limited. If the fan spins freely on its shaft with the engine off, the clutch is bad. If the fan does not turn on the shaft, the clutch is bad. If it turns by hand with some resistance but you can't spin it a full rotation by hand, it might be good.

The FSM procedure, to the best of my recollection, is to run the engine until 195* on the gauge, A/C off, the fan should engage and rev with the engine (an audible roar as well as substantial air movement).

Oh, the clutch is also typically engaged when the engine is first started cold, a quick rev will indicate so with a roar of the fan and then it drops out.

dablk1 · #14 03-31-2010, 09:45 AM

Thanks Jeff and I appreciate all of your help. I think that I've been asking to many different questions and trying too many different things.
Nevertheless the fan checks out. It wasn't frozen and it didn't spin without resistance.

Jeremy5848 · #15 03-31-2010, 12:23 PM

The little blower for the temp sensor is located behind the passenger side under-dash panel. In your car the blower should have a bracket that is screwed to the cross-cabin brace. It's a very inexpensive blower and may simply need to be taken apart, cleaned, lubricated, and put back together.

The hose that leads from the blower up through the A-pillar to the temp sensor in the dome light module might have become disconnected. It is a black plastic hose about 1/2 inch in diameter.

The temp sensor itself has two wires that connect it to the CCU. If the wires become disconnected, the CCU won't work correctly.