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1999 ML430
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Discussion Starter #1
I'm new to the forum and already have found a lot of help in reading through older posts.

I've got the what seems to be common A/C blower stuck on either hot or cold, which generally seems to end up being the air blend motor. In my case, the A/C first stuck on hot, but then near the end of the trip on which this happened, it did finally flip over to cold, but even since then has been stuck on cold. Note that I have not noticed any clicking noises when moving the temperature control.

I first tried removing the glove box, but didn't like the idea of removing the air bag, and couldn't see the ABM with just the glove box removed. I read on some posts that the proper way is to remove the dash board to get access to the ABM, but since that looks fairly involved, I thought I would first try the A/C electrical tests on the A/C control module and N19 connector (as recommended by 43sqd) to try to diagnose the problem. I'm a bit of a novice at these kinds of repairs, and more comfortable with electrical stuff than mechanical. Anyway, I was able to remove the center section of the instrument panel, then remove the mounting screw from the A/C control module, such that I could let it drop down below the bottom of the dash where I could remove its cover and get access to the connector pins with the clips / probes from my multimeter leads. All tests were done properly, with ignition on or off as specified in each test, and with the N19 connector removed for the resistance tests.

Here's the tests that didn't meet nominal values:

3.0 In car temp sensor voltage: 1.03V (temp was probably between 20 - 25 C). Supposed to be 1.9V at 20C.

3.1 In car temp sensor resistance: 3.7K at N19, supposed to be 2.1K at 20C and 1.7K at 25C. Though I also measured this directly at the sensor and measured 1.7K there (I removed the module with the Low Range switch and temp sensor so I could access the leads of the sensor directly).

5.1 Activation voltage temp increase voltage: Was correctly > 3V in blue range (4.2V) but voltage dropped to 3.74V at red stop (supposed to be >4V).

6.0 / 7.0 Actuator motor blend air flap actuator motor voltage:

With temp selector at blue stop (max cold):

6.0 0.68V
7.0 11.35V

With temp selector at first click from blue stop:

6.0 0.37V
7.0 0.37V

With temp selector at 2nd click away from blue stop:

6.0 11.3V
7.0 0.68V

So the voltage to the AMB actuator does appear to be reversing, though just 2 clicks away from max cold. And blower still blows cold.

10.0 Outside temp sensor: Open circuit.

From these results, definitely looks like there is a problems with the outside temp sensor, though haven't yet found instructions on where that is exactly and how to replace it (could also be wiring harness to the sensor?). Help on that would be appreciated.

But that doesn't seem to explain the stuck on cold, since tests 6.0 and 7.0 seem to indicate that the ABM motor is getting proper voltage, with the voltage reversing as the control is turned, so probably the ABM does need replacing?

Maybe the ABM voltage reversal on just the 2nd click away from max cold is explained by the bad outside temp sensor? Though also not sure how to interpret the results for 3.0, 3.1 and 5.1?
 

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1999 MTrash 430, its a joke
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im curious to see where your diagnosis takes you. I have had mine do the exact same thing, but I am to lazy to pull my dash. It blows cold in the winter and usually hot in the summer. But lucky for us in socal, I just have to open the window and problem solved. Haha. :p
 

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Discussion Starter #5
Thanks very much 43sqd. Purchased the N19 module on eBay as you suggested, will be a few days before that arrives.

And posting the test results after re-running them this morning. I noted both results from yesterday, when temp was around 75 deg F, and this morning, temp around 60 deg F (SoCal winter!).

This time there was no reversing of the actuator motor drive voltage (tests 6.0 / 7.0). 3.0 and 3.1 still out of range, 5.1 second part still seems wrong, 8.1 is a fail (I'd missed that yesterday), and 10.0 the outside temp sensor open circuit.

I'm not sure where the B12 refrigerant pressure sensor (test 9.0) is?
 

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Thanks very much 43sqd. Purchased the N19 module on eBay as you suggested, will be a few days before that arrives.

And posting the test results after re-running them this morning. I noted both results from yesterday, when temp was around 75 deg F, and this morning, temp around 60 deg F (SoCal winter!).

This time there was no reversing of the actuator motor drive voltage (tests 6.0 / 7.0). 3.0 and 3.1 still out of range, 5.1 second part still seems wrong, 8.1 is a fail (I'd missed that yesterday), and 10.0 the outside temp sensor open circuit.

I'm not sure where the B12 refrigerant pressure sensor (test 9.0) is?

1. The Refrig. Pressure Sensor has nothing to do with whether or not the Blend Door moves from c9old to hot. A faulty sensor will only prevent the A/C compressor from coming on.

2. 98 & 99 ML's do not have an Outside Temp. Sensor. It was born with the 2000 models, so disregard any readings.

3. Just wait till you get N19 and see what happens when you plug it in.


Instead of removing the center console to R&I N19 follow the below steps as a work around:

1. remove the radio, keys needed, radio code needed on reinstall.

2. remove the Center Section face plate.

3. remove the A/C Pushbutton Control Module.

4. remove the cover under the driver's side instrument panel.

5. with a long Phillips screwdriver remove the screw holding N19 in place, through the opening of A/C model. At the same time have your left hand in the space now made possible by removing the cover in step four. This is so you can catch the screw and hold N19 in one hand and remove the connector with the other. Hope you follow this but you will see exactly what it is all about when you have everything removed.
 

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Discussion Starter #7
Instead of removing the center console to R&I N19 follow the below steps as a work around:

1. remove the radio, keys needed, radio code needed on reinstall.

2. remove the Center Section face plate.

3. remove the A/C Pushbutton Control Module.

4. remove the cover under the driver's side instrument panel.

5. with a long Phillips screwdriver remove the screw holding N19 in place, through the opening of A/C model. At the same time have your left hand in the space now made possible by removing the cover in step four. This is so you can catch the screw and hold N19 in one hand and remove the connector with the other. Hope you follow this but you will see exactly what it is all about when you have everything removed.
Yes, these are the steps I did yesterday to get access to the N19 module, though I somehow managed this without step #4. See pic with everything removed and N19 module dropped below dash to the right with cover removed and probes attached.
 

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Discussion Starter #8
2. 98 & 99 ML's do not have an In- car Temp. Sensor. It was born with the 2000 models, so disregard any readings.
Not understanding this. The in-car temp sensor is there underneath the Low Range switch (photo attached).

Perhaps there is no outside temp sensor, since I'm getting an open circuit on that (test 10.0)?
 

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Not understanding this. The in-car temp sensor is there underneath the Low Range switch (photo attached).

Perhaps there is no outside temp sensor, since I'm getting an open circuit on that (test 10.0)?
That's correct. I was referring to the Outside Temp. Sensor. I have edited my post.

Thanks for catching that.
 

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Discussion Starter #10
3. Just wait till you get N19 and see what happens when you plug it in.
Received the N19 module, plugged it in, no change from original module. Ran through the tests again and results were basically the same as posted in "AC Heater Tests -results", 61 deg F column.

I was trying to figure out what would cause the in-car temp sensor resistance (test 3.1) to be high. I see from the schematic I found in an earlier post (though for 2000-2001) that the temp sensor is in series with a resistance inside the air blend motor. Is that resistance inside the ABM supposed to vary with position of the ABM flaps as a position sensor? In Test 3.1 it is adding about 2K of series resistance - I measured it by putting one probe on the lead of the temp sensor connected to the ABM and the other probe on pin 2 of the N19 connector. By being stuck at 2K is that an indication that the ABM is stuck in the wrong position, compared to where it should be with ignition off? In other words, the ABM is faulty?

I also experimented by shorting out the temperature sensor so that just the series resistance in the ABM is presented to the N19 module. With the short in place I observed voltage change from 0.7V to 13.4V in test 6.0 and from 13.4V to 0.7V in test 7.0 to be in the middle range of the temp control, which would seem to indicate correct actuation voltages for the ABM motor, and I twisted the control back and forth a few times to see if the ABM was actuating. But still no hot air and I couldn't hear the ABM doing anything.

I was debating whether to attempt to remove the upper dash to check the ABM, but don't really want to attempt this unless I am fairly sure it is the ABM, and also have a new ABM in hand. I have to drive out to Vegas on Tuesday (Consumer Electronics Show...), and don't have time to do any lengthy repairs beforehand. I could potentially get an ABM from the dealer on Monday if they have one in stock, but don't really want to be rushed doing the repair. I can live with making sure I have warm clothes and gloves for the colder nights in Vegas!
 

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Yes, these are the steps I did yesterday to get access to the N19 module, though I somehow managed this without step #4. See pic with everything removed and N19 module dropped below dash to the right with cover removed and probes attached.
LOL I did the exact same with my N19 and stuck in behind the middle compartment (just enough space). The heat/cold knob started working again after a few weeks for no apparent reason.
 

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Sorry the N19 did not work out but it was the easiest way out.

I probably should have made you do this test before telling you to order N19, but put the center section back in place with the white tube connected. Turn the key to pos. #2 and hold a small piece of paper at the grille and see if it stays in place.
 

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kstrong said:
3.1 In car temp sensor resistance: 3.7K at N19, supposed to be 2.1K at 20C and 1.7K at 25C. Though I also measured this directly at the sensor and measured 1.7K there (I removed the module with the Low Range switch and temp sensor so I could access the leads of the sensor directly).
I see from the schematic I found in an earlier post (though for 2000-2001) that the temp sensor is in series with a resistance inside the air blend motor.
Is that resistance inside the ABM supposed to vary with position of the ABM flaps as a position sensor? In Test 3.1 it is adding about 2K of series resistance - I measured it by putting one probe on the lead of the temp sensor connected to the ABM and the other probe on pin 2 of the N19 connector. By being stuck at 2K is that an indication that the ABM is stuck in the wrong position, compared to where it should be with ignition off? In other words, the ABM is faulty?

...I also experimented by shorting out the temperature sensor so that just the series resistance in the ABM is presented to the N19 module. With the short in place I observed voltage change from 0.7V to 13.4V in test 6.0 and from 13.4V to 0.7V in test 7.0 to be in the middle range of the temp control, which would seem to indicate correct actuation voltages for the ABM motor, and I twisted the control back and forth a few times to see if the ABM was actuating. But still no hot air and I couldn't hear the ABM doing anything.
The ABM is controlled by the difference in voltages between the in car temp sensor/ABM loop and the temp control loop.

http://www.benzworld.org/forums/w163-m-class/1495807-ml320-blend-air-motor-controls.html#post3943965


Here's "my" thread for what it's worth.

http://www.benzworld.org/forums/w163-m-class/1550441-ml320-air-blend-motor-i-need-2.html#post4799204
 

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Discussion Starter #14
Update from today: I was about to put everything back for now since I need to drive the car, and to do the in-car temp sensor test as suggested by 43sqd, but then noticed that I could see the white levers attached to the ABM through the hole in the center section of the instrument panel. I then removed the glove box to enhance the view.

Tried turning the temp control, while also probing the drive voltage for the ABM at the N19 module. I could see the polarity reversing, but the levers did not move at all. They are stuck in a position furthest away from the front of the dash compared to this pic from several other posts http://www.benzworld.org/forums/attachments/w163-m-class/286094d1263986845-ml320-blend-air-motor-controls-blend-door-motor-rods.jpg.

I was able to get a pair of long nosed pliers through the hole and carefully pulled (and pushed) on the vertical piece of plastic that is on one of the levers to see if it would budge the levers at all. I tried this both with ignition on, and the drive voltage polarity in the hot position, and also with ignition off. Also tried the same thing with the metal rod that connects the levers. And also stuck a screwdriver in there and carefully pushed on the levers.

The levers are firmly stuck. Not sure if this is normal or not, though seems to me that it should be possible to move the levers at least when the ignition is off. Broken cogs inside the motor causing the mechanism to jam, jammed motor, or mechanical obstruction inside the flaps?
 

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Discussion Starter #15

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Discussion Starter #16
Sorry the N19 did not work out but it was the easiest way out.

I probably should have made you do this test before telling you to order N19, but put the center section back in place with the white tube connected. Turn the key to pos. #23 and hold a small piece of paper at the grille and see if it stays in place.
Finally tried this last week, the piece of paper stays in place.

During my Vegas trip the blend remained stuck in the cold position for the whole trip (more than 9 hours total driving). But then last week on a couple of short trips around town it suddenly started working for a while (ambient temperature was about 85 deg F during heat wave last week), but finally ended getting stuck again, but this time in the hot position. Has remained there since.
 

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Discussion Starter #18
Maybe it is time you considered replacing the Blend Door Motor. Look at the repair instructions in the below link.

It might look like a headache to do but it really isn't. Just 15 or so screws hold the top skin (very light) on and everything is exposed.

Also, if you intend on doing it, I would suggest buy the OE motor as you do not want to do this again in 6 mos.

http://www.benzworld.org/forums/w163-m-class/1417771-diy-thread-collection-5.html#post4763314
Yep, seems like I need to bite the bullet on this.

I see from one of your other threads the motor part number is #163 820 01 08 and that parts.com is a reputable online store for OEM parts? Searching on parts.com led me to this:

Auto Parts - Parts.com, For Every Part of Your Life

Also, in your instructions for the upper dash removal, I assume the "green screws" are the 2 that show up as a turquoise color in the diagram? When I had the center section of the instrument panel out earlier, I had actually looked for those screws in case I eventually needed to remove the upper dash, but couldn't find them. I guess I didn't look hard enough.
 

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Yep, seems like I need to bite the bullet on this.

I see from one of your other threads the motor part number is #163 820 01 08 and that parts.com is a reputable online store for OEM parts? Searching on parts.com led me to this:

Auto Parts - Parts.com, For Every Part of Your Life

Also, in your instructions for the upper dash removal, I assume the "green screws" are the 2 that show up as a turquoise color in the diagram? When I had the center section of the instrument panel out earlier, I had actually looked for those screws in case I eventually needed to remove the upper dash, but couldn't find them. I guess I didn't look hard enough.
Yes, the part # above is the correct part number for your year and model.

And yes the turquoise screws are the GREEN screws. They are recessed further back but you will be able to see them or feel them.

Any other questions I will be happy to answer.
 
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