Gatortpk

I have a 2000 ML320 and only Hot air blows out of the vents. I noticed this problem when the cold air didn't seem very cold (when turning the knob all the way to cold after having the knob on the white/middle temperature setting), then I tried to adjust it by moving the temperature knob to hot then back to cold, then it just stayed hot.

I concluded that the actuator motor to move the hot/cold air flap is malfunctioning. The pipes under the hood on the low pressure side are cold and the compressor runs. My concern is that when I adjust the temperature knob, it stays hot, but I can hear it "open" up; get louder; sound less muffled, when I switch it all the way to cold, the last knob click or two to the cold side. This tells me that something is moving, so part of the duct work is changing/moving. Does this mean that my diagnosis may be incorrect?

I have ordered plastic pry tools: http://www.ebay.com/itm/181033477622?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT and separate keys for removing the radio http://www.ebay.com/itm/360623824146?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT. I also ordered a actuator motor for A/C flap http://www.ebay.com/itm/221245754936?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
I found this link to describe how to get to and replace the motor: DIY To properly replace the Blend Door Actuator/Motor

Does anyone have any more information or a better way of replacing this motor? (I have seen the way through the passenger airbag that would require breaking the cover clips)

Is is possible that my diagnosis is incorrect because I can hear a change in the duct work sound (sounds louder; less muffled; more open) when I turn the knob all the way to cold, (but the air stays hot even though the compressor is working fine)

Thanks for any assistance in a better way of fixing this or other possible diagnosis.

Maj. Dundee

If you hear movement then in all probability the Blend Door Motor needs replacing because of a broken gear.

The link you have posted is the proper way to remove and install this motor.

Gatortpk

Thank you very much for the info Maj. Dundee. I finally received actuator motor today. I'm starting the repair. How much time does such a repair take? I know the time varies greatly, I would give myself twice as much time as what a professional would take. I figure the second time I do the repair it would take half the time.

Maj. Dundee

Normally about 2 hrs.

Gatortpk

I have followed all the instructions in: DIY To properly replace the Blend Door Actuator/Motor. I noticed some of the PDFs included in that post aren't for my particular 2000 ML320, though most of it applies.

I am stuck at the last part, I can't remove the upper dash completely. Looking from the front though the windshield, I see (three) screws that attach the dash to the defrost vent ductwork. It appears that the dash is supposed to come free leaving the defrost ductwork in place, while the side and center vent ductwork do come free attached to the dash. I don't see anyway I can get to the screws under the dash behind the defrost vents facing toward the windshield.

What am I missing? I see no mention of these particular screws in the DIY post. Here's a photo though the windshield (I had better angles, but they didn't show surroundings for reference, sorry it's taken at night):


Here's my actuator motor:

Even though I can see the actuator motor at this point, I just can't reach inside to replace it. When I needed to roll up the windows for the night, I did reconnect the battery and try the temperature adjustment. I saw no movement in the motor. It seems to be stuck at about 30° counter-clockwise, is that the correct position for heat? When I turned the temperature all the way to cold, I heard the same change in sound, it sounds like the blower motor spinning faster as if it went to max-cool. So the change in sound I heard before had nothing to do with the actuator motor, but was still present. The fact that the actuator motor had no movement at all was somewhat reassuring for my original diagnosis, however is there a chance that the problem could be electrical (outside of the motor) and that the motor is getting no power? I have checked the fuses briefly and didn't see any blown.

Again, thanks for your help.

Maj. Dundee

What screws can't you find or remove? Please use the diagram instead of just words.

Gatortpk

Thanks again for your help. If the screws are in the diagram, it would have to be #45, but it's not clear. They are not labeled in any color on the diagram.

Is it posible to replace the motor by only lifting the dashboard up, and not removing it completely? I can lift it up, but not much. The directions say "gently pull away the top portion of the instrument panel," I took that as to mean to remove the entire top dashboard. (I do not have 6,7, and 9 to remove)

I can see the screws that seem to be holding the back of the dashboard in place, the first photo in my last post shows two of the screws that would be easy to access if the windshield were removed. Obviously that shouldn't be necessary. (The top of the photo shows the back of my review mirror for context, my annotations on the photo weren't very clear so posted the original photo hoping it would be clear, should I repost with annotations?) The screws I seem to need to remove are not on the diagram, unless #45 is pointing to screws behind the defrost vents underneath the top back, attaching the defrost ductwork to the top dashboard. #45 is not indicated in RED. If #45 does indicate screws visible only through the windshield, how do I access them?

I'm surprised I was able to pull the dashboard up as much as I did because it doesn't appear that the defrost ductwork is moving at all, it appears to be made of a much harder stiffer plastic than the other ductwork that easily comes off with the top dashboard.

Also, are there instances that the actuator motor fails completely and there is no movement even when everything else is fine? I'm trying to determine if a fuse that I'm not seeing could be the culprit, and causing the motor to have no power.

Maj. Dundee

You have written a very confusing post. Forget about everything else in the diagram, the only screws that matter are the ones that are colored.

Gatortpk

Yes, many different possibilities and "ifs" in my last post, my apologies.

I ended up replacing the motor without removing the entire dashboard. However, I couldn't get the dash back down, I had pulled the center ductwork pretty hard it got stuck and I ended up removing two screws that are above where the back of the radio would be. Those may have been the screws marked in green? It was then that everything came loose. I then took everything apart and repaired some minor damage.

When I had replaced the motor and turned the ignition on, the motor turned to the exact same position that the old motor was. It was then that I realized that something else was at fault, the circuit was telling the motor to go all the way to the "hot side" no matter the position of the temperature knob. I think it may be a faulty temperature sensor? I haven't researched that yet.

I took apart the old motor and moved the main gear to the "cold" position and reinstalled the motor. But I didn't plug the electrical connection, therefore the motor will keep the air blend door on the "cold" side (it tends to blow to the "hot" side with the blower on). Hopefully when I repair the faulty temperature circuit component, I can reach in and plug the motor back in and it'll behave normally. Hopefully it can self calibrate?

Thanks again for your help!

Maj. Dundee

This is why I asked you which screws you can't remove. It was in the diagram that the two BLUE screw had to be removed.

Again, in you original post you stated that you heard movement every time you turned the temp. dial. This is an indication that the motor is faulty.

Being that this was not the case, now you will have to change the AAC controller module which should take no more than 20 mins.

Go to ebay and type the TNxxxxxxxxxxxx# in the search box and purchase one. When you receive it, remove both left and right covers under the instrument panel.

https://mbworld.org/forums/5039586-post13.html

Then look through the opening of the Center Console and you will see this black box attached by one Phillips screw. Remove the screw and then remove the connector and then install the new controller and see how things work.

Gatortpk

Thanks again. The DIY instructions threw me off a little when it said that some screws may or may not be there. I didn't see any screws marked in blue up front, but after careful examination it should have been obvious that the blue marks indicated screws that were further back.

I plan on getting the car back together this afternoon, I've been keeping it at another location and borrowing another car since the problem started. I mentioned that I reattached the old motor after moving the gear to the "cold side". I have left it unplugged so it won't move.

When I replace the AAC controller, and then plug in the motor, will it self calibrate and move properly from the cold to hot sides? It wasn't clear to me how it "knows" what position it is in, and it has a very simple DC motor inside. I'll try to return the new motor if the old motor will still work even though I have manually changed its position.

Thanks again.

Maj. Dundee

It's plug and play.

Gatortpk

Thanks, that's great to know. I ordered the the AAC controller and it should be here by Saturday. I'll replace the controller and then plug the electrical connection to the motor. I suppose somehow it'll know what position I put it in (Cold) and know where to move from there.

Maj. Dundee

Wait a minute here. I thought you already replaced the blend door motor with new.

Secondly those motors are indexed meaning. The position of the gears must be in a certain position for the entire motor to operate.

Gatortpk

Oh, ok, that makes much more sense. It didn't seem obvious how the motor would know its position, therefore, by moving it manually, the old motor is no longer indexed correctly. I haven't returned the new motor yet. I was hoping all I had to do was stick my hand in there and plug the electrical connector to the old motor currently installed (and moved to the "cold" position).

I didn't want to take apart the new motor, and I'm using the old motor to "hold" the air blend door to the "cold side". (If I had kept the new motor installed, it would still be stuck on heat.)

How do I index the motor correctly? This will obviously mean I have to remove the motor entirely again and take it apart. I'll receive the new AAC Controller tomorrow. (much sooner than expected!)

Gatortpk

I have now replaced the Air Blend Actuator Motor and the AAC Temperature Controller. The motor still stays on the "hot side". (Initially when I plugged in the new motor with the old AAC Controller, it moved to the "hot side" from center and stayed there telling me that the old motor is likely fine and that it has power.)

I have figured out why the motor sounds louder when I turn temperature knob all the way to the cold position, a recirculation flap opens exposing the motor to the interior. Because of this, I know that the knob is electrically connected properly (at least to that servo).

If the AAC Controller and the Actuator Motor are fine, what else could be the problem?

Gatortpk

I have read that if one of the voltage loops goes open as in disconnected/bad connection, a temperature sensor, or broken temperature knob then the system interprets this as a hot setting and air blend actuator moves to the hot side. If one of these issues is the culprit, what and how should I test for first?

I have also read that the system was analog until November, 1999. My ML320 is a 2000 model, however it was likely manufactured before November, 1999. Which is it? It certainly doesn't seem like a digital system because of variable voltages.

I replaced my AAC Controller model #TN277300-0790 (from 2000) with a model #TN277300-0471 (from 1998). I have read that both of these will work, is that true? If not, could that be the problem? Though, I don't see how anything could be interchangeable from a analog system to a digital system.

Maj. Dundee

It is my belief that the Denso-0790 is for 2000-2001, but will work for any ML 1998-2001. But the old Denso-0471 will not work in newer Ml's 2000-2001.

This is the reason why I specifically told you to purchase the Denso-0790.

Gatortpk

Thanks for the info. I did read info that they were interchangeable. Sorry, I wasn't aware you mentioned the Denso-0790, I can't find that post. You posted an image of the Denso-0471. Maybe that's when I looked into it and read that the two models were interchangeable.

Hopefully I can return that part!

Maj. Dundee

My apologies, you are correct. I posted the wrong photo. I should have posted the one you removed from your truck.

I have corrected my post on the subject and have found a brand new one on ebay for $39. Again, I am truly sorry.

http://www.ebay.com/itm/Mercedes-Benz-Air-Control-ECU-2002-2003-/251013631295?pt=Motors_Car_Truck_Parts_Accessories&hash=item3a71940d3f&vxp=mtr

Gatortpk

Don't worry about it, you've been very helpful to me. That's exactly the part I found on eBay a few days ago, great deal. I am expecting this new one for $39 tomorrow. It's $31 less than the used one I purchased! It's unusual for me to purchase something so much more expensive, I must have gotten it because of quick shipping.

This older part doesn't seem to do anything; acts just like the possible faulty part driving the air blend motor to full heat. I'm hoping this is expected. How likely is the problem remedied by a good AAC controller? What else could be wrong? I've read that an open circuit could cause this same behavior. Do these wire harnesses go bad often enough that it's a possibility? As I've said, I have great A/C, but that's because I disconnected the motor after putting the blend door to the cold position.

Maj. Dundee

It's probably not a wiring problem. Just install the new programmer and see what happens.

The only other problem is a code is locked into the Climate control system and must be cleared, but let's see what happens.

gguruusa

I recently had my A/C serviced, and it was blowing cold nicely.
A few days later, I noticed it was only blowing 100% hot, regardless of knob positions
The only thing I did between the two events: removed the Low Range/Aux switches to left of radio.
A/C compressor lines are cold, so A/C definitely should be working.

Still haven't put switches back in the vehicle, but I suspect this to be the problem as there is a thermistor between the two switches. As that thermistor is obviously not present (it's attached to the switches), I suspect the "open" caused by its absence to be the problem. Will post again when I put switches back in (a few days)

Gatortpk

Thanks for the info about a wiring problem being unlikely! (I'll let you know what happens after this post when I finally finish this project.)

I have installed the new AAC Controller so I could return the older (1998) unit. (I've just received the refund.) However, I haven't removed the dashboard again for the third time to plug the motor back in. I still have the air blend door locked to the cold side with the original motor unplugged and manually adjusted to the cold side. I know you said that the motor is indexed, it appears that the orange gear inside the actuator (the gear before the final/actual servo gear) has a sensor for its positioning feedback. If I reposition this final servo gear in the actuator to the original "hot" side where it was before I unplugged it, how likely is it going to be properly indexed; that I can still use it? (Then I can sell/return the brand new motor I haven't used and/or disassembled.)

The current setup (having no electronic air blend control) is actually nicer in the summer because the controller has no control of the air blend door and won't add heat once the interior starts to cool down. (The instrument panel's external thermometer briefly hit 101°F today while moving! A personal record.) I see the temperature of the interior air coming out of the vents varying from 26°F to 39°F every 15-20 seconds. I suppose this is intentional to avoid freezing the evaporator on the inside by keeping it above 32°F periodically. Is this done by cycling the compressor clutch on and off?

This experience confirms what I have suspected for a long time. The temperature knob does not control the position of the air blend door directly, it tells the controller what temperature I like it, then it adjusts the blend door accordingly. It seems to work the same way in my S430 except the ML320 doesn't have a digital temperature display. Honestly, I would rather have an option of full control of the air blend door position, and to keep the air from automatically recirculating when the temperature knob is turned to the last notch to the cold setting.

Gatortpk

I think you are correct, in that the missing switches are the problem to your air blend door position moving all the way to the "hot side". I have read, and I believe I posted this in this thread earlier, that any open loop/circuit will be interpreted as infinite heat is needed, and disconnecting the switches will likely open (one or two) circuits to the climate control. The air blend door actuator motor will move completely to the "Hot side." Though, the thermistor is not likely in the switches themselves, but at the other end of the white hose attaching to the wooden center frame (near those switches). Notice the small vents on the left side of the wooden panel? This is to allow air to flow through the white hose to the thermistor at the other end so the AAC controller can decide how cold (or hot) the air coming out of the vents should be depending on the interior cabin air temperature. (I am making an somewhat educated speculation on that this is how it actually works)

I have taken that center wooden panel out several times now (after removing the radio each time also). I originally did this to get access to a pair of the many screws holding the dashboard in place. As I mentioned before, I noticed that the white hose used to carry the interior cabin air to a temperature sensor, and a blue plug connecting the Low Range/Aux switches on the left side of the center wooden panel. (Ironically, after 14 years of driving this vehicle, I had never tried that "Low Range" switch until yesterday! Lo and behold, it still works and it switched to a very low ratio gearing! It seems to be about 2.5:1 step-down. The engine spins at 1K RPM for every 10 MPH I drive. I never went over 40 MPH before I switched it back to "Normal Range".)

May I ask why you disconnected the Low Range/Aux switches in the first place?