Blend air problem and amplifier

[Ron in SC]

Intermittent problem with blend air door sometimes not closing completely. Result is heat coming thru vents when temperature control turned to cold. Can not pull codes for this problem on this model, a 1999 ML 320, not automatic climate control. Problem seems to occur when vehicle is cold and knob is turned to allow heat to enter cabin. When I want to shut it off blend air motor will not fully shut door. I’ve installed new blend air motor; did not cure problem. Blend air door not clicking or popping, in other words I don’t think it’s sticking. I can observe linkage and watch it go thru full range of motion or partial range depending on whether it’s working right or not. I can put my finger on linkage and feel it go thru it’s range, either full or incomplete and it seems quiet, fast and smooth. I’m thinking maybe the blend is motor is not getting proper signals. I found an amplifier, MB part# 163-820-01-97 which has Denso # or TN277300-0471. I examined circuit board and could not find any obviously problems like burn marks or cracks. Can not get any information from dealer regarding any pattern failure, they are very tight lipped.

To remove this ad, register today or login if you already are registered!

[pjparks83]

I have the same problem. I replaced both the amp and the Blend motor. My blend motor had a broken gear and needed replaced anyway, but I still have the problem...

I am somewhat knowledgable in electronics and I think it is the temp sensor in the dash below the LOW RANGE button. There seems to be steady resistance that changes with temp, but when I unplug it and plug it back in a few times, the blend motor will all of sudden spring to life.

I have gone through the entire system with a digital Multi-meter and found everything is working corectly, IE have power, have resistance that changes, but still no cold air.

I like the car, but this might be the last straw in a laundry list or fixes...

ANYONE???

[bbearden]

Yeah, I know I'm resurrecting an old post! My wife's ML has had the clicking within the dash from the stripped stepper motor gear attached to the blend air door. Recently, it stopped clicking and the car blows on hot air out the vents. I'm planning to replace the blend air motor assembly as I feel this is the culprit.

I have seen the .pdf and the video on this site showing the repair, but does anyone have any secrets for removing the screws holding the stepper motor in place. Is there a particular tool that can assist in removal? I've heard the 3 screws are a real B**** to get to. I plan to do the job myself since I don't want to pay the stealership for the 6 hours of book labor they have listed.
Thanks in advance!