ACC Temperature Calibration: What Affects This? - PeachParts Mercedes-Benz Forum

shertex · #1 05-04-2009, 09:04 PM

The climate control on the 91 300D I just picked up works perfectly EXCEPT for the temperature calibration. In order to get significant heat, I have to turn the wheel almost all the way hot....in the high 80s. On my 92, I'm used to this control being dead-on accurate.

So what are the likely culprits?

babymog · #2 05-04-2009, 10:25 PM

It's a warm day, and there is some sun load to the roof.

The temperature sensor is in the roof, part of the courtesy light assembly. If the aspirator blower isn't working, or isn't connected (rubber hose not connected at temp sensor), it is possible that it is reading the temperature of the air in the roof area above the courtesy lamp assembly which will read warmer (typically). Also, a very good idea IMO to vacuum the little vent hole in the assembly behind which the sensor lives, I've found a great deal of dust there in most of my cars (which could also impede proper operation).

A good cheap start.

shertex · #3 10-09-2009, 11:09 AM

If air is being pulled through the opening by the light, can I at least assume that everthing is connected properly?

It passes the 1/2 inch paper test....held there by the suction.

brandlj · #4 10-09-2009, 11:57 AM

Is your aux water pump running when you have it turned towards heat?

shertex · #5 10-09-2009, 11:59 AM

Quote:

Originally Posted by brandlj

Is your aux water pump running when you have it turned towards heat?

Interesting question. I've been wondering about the aux water pump. Sometimes at idle I have good heat, sometimes not. So I'm suspecting it's on its way out.

What would be the connection between my temp calibration issue and the aux water pump?

brandlj · #6 10-09-2009, 01:28 PM

Quote:

Originally Posted by shertex

Interesting question. I've been wondering about the aux water pump. Sometimes at idle I have good heat, sometimes not. So I'm suspecting it's on its way out.

What would be the connection between my temp calibration issue and the aux water pump?

I believe that the aux water pump affects the amount of coolant entering the heater core at idle to stabilize the overall warmth of the car when it is cold outside. I know that my pump is disconnected at this time and my heat at idle is not as hot as when I am driving.

compu_85 · #7 10-09-2009, 01:17 PM

I find that in my SDL I have to adjust the temp 5* or so away from what I really want... 5* over on heat and 5* under on AC. I think the aspiration blower is working, but I've never really check it.

-Jason

babymog · #8 10-09-2009, 02:10 PM

Vacuum the temp sensor. If that doesn't help, you might try swapping it for another, easy to change and they're in every car.

shertex · #9 10-09-2009, 11:49 PM

Quote:

Originally Posted by babymog

Vacuum the temp sensor. If that doesn't help, you might try swapping it for another, easy to change and they're in every car.

Do I have to be careful about which attachment to use? Do I use narrow nozzle, one with brush at end, or what?

Thanks.

babymog · #10 10-10-2009, 12:01 AM

If it's in the car, anything that works. If you drop the dome-light assembly and disconnect the hose, you can simply blow into the tube and blow the dust out.

Be careful disconnecting the hose, the clips that hold the temp-sensor into the light/switch assembly are fairly fragile.

dablk1 · #11 03-31-2010, 11:14 AM

How is the dome ligt assembly removed? 1991 300D 2.5

compu_85 · #12 03-31-2010, 01:20 PM

Slide the whole assembly to the right, and pop the tabs down from the roof lining.

-J

dablk1 · #13 03-31-2010, 01:29 PM

Thanks guys, I have it diassembled rigt now. Te tube is still connected and te sensor wire seem to be ok also. I'm going to blow in the tube and see what happens. I'lll report back.

leathermang · #14 03-31-2010, 02:29 PM

Sometimes on things it is easier to change the label on the selector...LOL
Than to get an old piece of equipment to work like it came new....
As long as the range of selection still covers your needs...

dablk1 · #15 03-31-2010, 02:39 PM

Nevertheless I disconnected the blower(powerful little sucker) jumped it after I took it apart. The blower works?? I used a 14.4V power drill battery. I'll clean and lube it up and see what appens when I reinstall. By the way leathermang..all of my selectors work fine. My cabin temperature just won't remain constant; so I gave it a shot.