My two center air vents have stopped blowing. They do not blow hot or cold air. Any suggestions on repairing them?

Ensure the center 'dial' underneath the vent is turned open. The center dial runs the center vent.

Ensure you're in AUTO mode. If not, I'd ensure the two buttons between the vents are not lite (they only function in Non-Auto mode).

Is there any fan running at all when you crank up the fan speed ? If not, I'd check the fuse first. Fan and then the A/C units.

 

My two center air vents have stopped blowing. They do not blow hot or cold air. Any suggestions on repairing them?

First diagnose the cause of the problem. Follow the attached procedure to put your climate control unit in a diagnostic mode and monitor control inputs and outputs. This is for cars 1997 and later. Refer to the table below (typed by gsm-decode in a thread a few months ago). In particular you are interested in items 63 and 64.

Nr. Description Unit

01 In-car temperature sensor °C
02 Outside temperature sensor °C
03 Left heat exchanger temp. sensor °C
05 Evaporator temperature sensor °C
06 Coolant temperature sensor °C
07 Refrigerant pressure bar
08 Refrigerant temperature sensor °C
09 Not assigned --
10 Blower control voltage Volt
20 Auxiliary fan control current mA
21 Engine Speed x100 RPM
22 Vehicle Speed km/h
23 Circuit 58d (% of batt. Voltage) %
24 Battery Voltage Volt
40 Software staus
41 Software staus
42 Variant Code 1st numerical value
43 Variant Code 2nd numerical value
50 Not assigned
51 Not assigned
52 Not assignd
60 Roof OPE/CLO
61 Left side air outlet, knurled wheel
potentiometer Volts
62 Vacuum actuator 46, feedback
potentiometer Volts
63 Center outlet, knurled wheel
potentiometer Volts
64 Vacuum actuator 47, feedback
potentiometer Volts
65 Right side air outlet, knurled wheel
potentiometer Volts
66 Vacuum actuator 48, feedback
potentiometer Volts

Item 63 is the input signal from the thumbwheel control for the center vent, and 64 is the feedback signal to the controller giving it the position of the vent flap. As you move the thumbwheel these values should continously vary from a minimum of roughly .7 to 4.5.

 

Hi, great info, supposed you get an ok reading on the 63 test but the 64 test (64 Vacuum actuator 47, feedback
potentiometer Volts) is static? Does that indicate a bad Vacuum actuator? Thanks, Scott

 

It could be a bad vacuum actuator. It could also be a leaking vacuum hose or a defect in the Y11 valve block that controls the actuator. The vacuum lines are #12 and #13 on Y11, and it is located behind the passenger airbag (LHD car).

 

MY 2000 SL500 climate control baseline readings

This is for cars 1997 and later.

FWIW, I decided to take some baseline readings by following the above Climate Control, etc. procedure for data collection. Here are the values I received after reading MERCEDES-BENZ: A/C Diagnostics.

Sensor# - Description - Unit Value

01 Inside temperature °F = 74
02 Outside temperature °F = 79
03 Heater core exchanger temp. °F = 58
05 Evaporator temperature °F = 47
06 Engine coolant temperature °F = 192
07 Refrigerant pressure Bar = 14
08 Refrigerant temperature °F = 127
09 Not used
10 Blower control voltage = 12.5
20 Auxiliary fan control current mA = 000
21 Engine Speed x10 RPM = 060
22 Vehicle Speed MPH = 000
23 Circuit 58d (% of batt. Voltage) % = 000
24 Battery Voltage = 13.9
40 A/C Controller Software Version = 03
41 A/C Controller Hardware Version = 03
42 Variant Code 1 = 11
43 Variant Code 2 = 6-C
50 Not used
51 Not used
52 Not used
60 Roof OPE/CLO = open
61 Left side air outlet(open), knurled wheel potentiometer Volts = 4.0
62 Vacuum actuator 46, feedback potentiometer Volts = 3.5
63 Center outlet(open), knurled wheel potentiometer Volts = 4.1
64 Vacuum actuator 47, feedback potentiometer Volts = 3.5
65 Right side air outlet(closed), knurled wheel potentiometer Volts = 0.8
66 Vacuum actuator 48, feedback potentiometer Volts = 0.9

 

FWIW, I decided to take some baseline readings by following the above Climate Control, etc. procedure for data collection. Here are the values I received on 8/5/07.

Sensor# - Description - Unit Value

01 Inside temperature °F = 74
02 Outside temperature °F = 79 (sitting still, engine running)
03 Heater core exchanger temp. °F = 58
05 Evaporator temperature °F = 47
06 Engine coolant temperature °F = 192
07 Refrigerant pressure Bar = 14
08 Refrigerant temperature °F = 127
10 Blower control voltage = 12.5
20 Auxiliary fan control current mA = 00
21 Engine Speed x10 RPM = 0600
22 Vehicle Speed MPH = 000
23 Circuit 58d (% of batt. Voltage) % = 000
24 Battery Voltage = 13.9
40 A/C Controller Software Version = 03
41 A/C Controller Hardware Version = 03
42 Variant Code 1 = 11
43 Variant Code 2 = 6-C
60 Roof OPE/CLO = open
61 Left side air outlet(open), knurled wheel potentiometer Volts = 4.0
62 Vacuum actuator 46, feedback potentiometer Volts = 3.5
63 Center outlet(open), knurled wheel potentiometer Volts = 4.1
64 Vacuum actuator 47, feedback potentiometer Volts = 3.5
65 Right side air outlet(closed), knurled wheel potentiometer Volts = 0.8
66 Vacuum actuator 48, feedback potentiometer Volts = 0.9

I decided to take some more readings by following the same Climate Control procedure for data collection. Here are the values I received today, 2/10/10.

Sensor# - Description - Unit Value

01 Inside temperature °F = 75
02 Outside temperature °F = 72 (sitting still, engine running)
Note: the outside thermometer read 59 (sitting still) and the climate Control dropped from 72 to 59 after taking a short drive.
03 Heater core exchanger temp. °F = 74
05 Evaporator temperature °F = 39
06 Engine coolant temperature °F = 195
07 Refrigerant pressure Bar = 13
08 Refrigerant temperature °F = 109
10 Blower control voltage = 8.0
20 Auxiliary fan control current mA = 00
21 Engine Speed x10 RPM = 0600
22 Vehicle Speed MPH = 000
23 Circuit 58d (% of batt. Voltage) % = 000
24 Battery Voltage = 13.9
40 A/C Controller Software Version = 03
41 A/C Controller Hardware Version = 03
42 Variant Code 1 = 11
43 Variant Code 2 = 6-C
60 Roof OPE/CLO = closed
61 Left side air outlet(open), knurled wheel potentiometer Volts = 4.0
62 Vacuum actuator 46, feedback potentiometer Volts = 3.8
63 Center outlet(open), knurled wheel potentiometer Volts = 4.1
64 Vacuum actuator 47, feedback potentiometer Volts = 3.8
65 Right side air outlet(opened), knurled wheel potentiometer Volts = 4.0
66 Vacuum actuator 48, feedback potentiometer Volts = 3.8

 

Center vent problems

Hi, I have just fixed an identical problem on my 92 R129.
In my case it turned out to be a popped out connection on the distribution block under the bonnet. This had disabled the center vents, and the side vents worked, as they default to on, but were not swithing off.
Heaps easier to get at than Y11 too!
The distribution of vacuum starts under the hood in the computer modules box. Vacuum from the manifold goes into a 7 way distribution block, via some isolation valves to the transmission and others. The Red-gray one is the feed to the vacuum reservoir for the A/con. My line was a little tight and had pulled out, possibly when some work had been done on the car a while back.
Hope this helps. The diagrams in the R129 Classic DVD are very useful...
GregE.

 

Hi, I have just fixed an identical problem on my 92 R129.
In my case it turned out to be a popped out connection on the distribution block under the bonnet. This had disabled the center vents, and the side vents worked, as they default to on, but were not swithing off.
Heaps easier to get at than Y11 too!
The distribution of vacuum starts under the hood in the computer modules box. Vacuum from the manifold goes into a 7 way distribution block, via some isolation valves to the transmission and others. The Red-gray one is the feed to the vacuum reservoir for the A/con. My line was a little tight and had pulled out, possibly when some work had been done on the car a while back.
Hope this helps. The diagrams in the R129 Classic DVD are very useful...
GregE.

BRO,
SORRY TO TROUBLE U . I'VE GOT THE SAME PROBLEM , CAN U PUT SOME PICS AS TO WHERE N HOW U SOLVE THE PROBLEM . WILL REALLY APPRECIATE.
THANKS

 

Fixed the problem. It was a vacuum hose leak .Thanks Ray

 

Vacuum fixes for vents / photos

Sorry Robin, thought I'd taken some photos, but cant see any, I'll pop the cover off and take a couple over the weekend..
Greg

 

I am having a problem with my climate control on my 95 SL320. First my fan does not change speeds when the bottoms are pushed. When the bottoms are pushed only hear a slight buzz that change it pitch. To top that now when I turn on the air, I get hot air coming through the left and right air outlets, and cool air through the center outlets. Vents are open, when closed the heat moves to the center vent. Checked fuses in two locations under the hood and all fuses look OK. Tried to diagnose the system but it did not work (I done it before). Any help or suggestions?

 

Tried to diagnose the system but it did not work (I done it before).

If the climate control unit can not perform diagnostics, then it seems logical to me that it can't perform some of its basic control functions as well. I would look for a used replacement module.

 

Aircond vacumm distribution

Finally got to post some pics- here they are:

See the diagnostic connector just outside the plastic covered area exposed here to show the vacuum distributor. In my case the green one was a bit tight, just pulled out of its push fit socket..

Way easier to get at than the other bits inside the car, worth a look first.

Cheers Greg

 

Thanks, was hoping it was something simple. Seem that once I tranfer Benz from California to Jacksonville, Florida I start having problems like the air and the SRS light staying on. I need to find me a mechanic other than the MB dealer.

 

I didn't have a problem with my A/C controls, but I was intrigued by the data that this diagnostic would provide, so I tried the procedure and accessed the information.

Unfortunately, my blower motor remains running even after I turn off the engine and remove the key. It's as if the REST button is stuck or remains on for some reason.

So that my battery wouldn't drain or my blower burn up while parked, I removed the big 30 amp blower fuse under the hood by the master cylinder/brake booster (American driver's side).

Any idea on how to correct this?

 

heater vents

I am also having a problem with my footwell vents opening,only the upper vents would blow heat!What is the diagnostic check on my 1999 sl500?Thanks speedmaster 2.

 

I am also having a problem with my footwell vents opening,only the upper vents would blow heat!What is the diagnostic check on my 1999 sl500?Thanks speedmaster 2.

Read the bottom of the above post #3.

 

I would start looking for a shorted wire, bad relay, or ignition switch.

Regards.
aam.

 

heat system diagnostic

I tried the tissue for the interior temp sensor at the upper console,and it stayed,I cut the tissue slightly larger than the vent,I found that it is a very light air pressure pulling the tissue in,The draw of air is mostly on the right side of the vent.I got my diagnostic numbers and some seem to be way out,compared to Lynns numbers. so Im going to check again,and will post,maybe You guys can give me an idea as to what problem I need to fix.Regards speedmaster2

 

I tried the tissue for the interior temp sensor at the upper console,and it stayed,I cut the tissue slightly larger than the vent,I found that it is a very light air pressure pulling the tissue in,The draw of air is mostly on the right side of the vent.

Thanks for the feedback this may help me to find out if my mni fan that draws air into the vents actually works.

Please see if the following helps you at http://www.benzworld.org/forums/r12...rg/forums/r129-sl-class/1501006-9-95-climate-control-cabin-air.html#post3988202

 

I tried to follow the attached procedure & do the diognose but i couldn't, i pressed the Reset SW for more than 6sec & nothing happined, set the temp to 72F (22C) coz my car is golf vergin same of the europain vergin. So what you think the prob is....

 

Fixed mine....whew

I just got my 2000 SL500 3 weeks ago, and have been experiencing the same center vent problem. I've read every R129 post on Benzword and this one seems the most applicable.

I did the diagnostic, and all vent wheels adjust the readings on #61, 63, and 65 between .8 and 4.2, but 64 always shows .8, while 62 and 64 read 4.2.

I live in Phoenix Arizona, so solving this problem was a high priority for me.

I ended up finding the proper vacuum hose for the center vents (white one for mine), and it had disconnected from the black outer hose between the wiper motor block area and the engine compartment. Took a little work to get the white one back to where it was supposed to be, but once I got them reconnected all my vents work properly, and I'm in heaven.

I am attaching some pictures of what I found and where I found it. Hope this helps

The first picture show the location of the next 2 pics.

The 2nd picture is the block with all of the vacuum hoses for the vents, 2 red, 1 green, and 1 white. The white hose is for the center vents.

The 3rd picture shows the white hose coming out of the wiper motor wall into the engine compartment, and connecting to the black hose.

 

I just replaced the vacuum block P/N 129 800 14 78
(between the two firewalls on passenger side) and now I have vent control.

I do have the issue with no control over center vents.
I suspect a vacuum leak on the distributor/switchover valve under the passenger bolster.

In the mean time I kept the center vent partial opened by wedging a piece of vacuum hose in it

One of the lines will be inspected on the distributor/switchover.

I think that's were the problems lies.
http://www.benzworld.org/forums/r129-sl-class/1674401-r129-climate-control-vacuum-schematic.html

1 Switchover valve for main air flaps (large stroke)
2 Switchover valve for main air flaps (small stroke)
3 Switchover valve for recirculation flap
4 Switchover valve for defroster flaps (small stroke)
5 Switchover valve for defroster flaps (large stroke)
6 Switchover valve for bypass flap
7 Switchover valve for blend air flap
8 Switchover valve for footwell flap (large stroke)
9 Switchover valve for footwell flap (small stroke)
10/11 Switchover valve for left side outlet
12/13 Switchover valve for center air outlet
14/15 Switchover valve for right side air outlet
36 Vacuum distributor block
38 Vacuum element for defroster outlet flaps
39 Vacuum element for footwell flaps
41 Vacuum element for bypass flap
42 Vacuum element for recirculation flap
43 Vacuum element for left side main air flap
44 Vacuum element for right side main air flap
45 Vacuum element for blend air flap
46 Vacuum element for left side air outlet
47 Vacuum element for center air outlet
48 Vacuum element for right side air outlet

Color Code :
bl = blue
hbl = light blue
drt = dark red
ge = yellow
mgn = medium green
gr = gray
rt = red
ws = white
sw = black
dgn = dark green

 

Awesome thread and exactly what I was looking for.

So are we saying that without any vacuum (i.e. primary hose disconnected) the defroster and side vents are defaulted on? If so, this would explain my situation perfectly. Side vents blow cold but cold air is also blowing on the windshield.

 

Me too. Quick fix

During my recent oil change apparrently I disconnected the vacuum hose from the intake manifold. Put it back and presto, I have center vents blowing air again.

 

During my recent oil change apparrently I disconnected the vacuum hose from the intake manifold. Put it back and presto, I have center vents blowing air again.

you may also have a bad check valve in the vacuum hose between the engine and interior. That line should have a check valve that holds vacuum inside when the engine looses vacuum.

A check is to see if your vents quit working during acceleration when you loose engine vacuum. If it does, likely bad check valve.

 

Hi, I too have the central vents not blowing. Diagnostics show everything else as working. Both left central and right vents fluctuate between 0.8 and 4 when moving the circle buttons. However, the central vent shows 1v for 64 no matter what the position is.

63 Center outlet, knurled wheel potentiometer Volts - 0.8-4
64 Vacuum actuator 47, feedback potentiometer Volts - 1 V


When I got the car I found a disconnected vacuum hose (about 2in) at the back of the engine between the firewall. Reconnected that, but nothing has changed.

I have the vacuum diagram, but just want an idea where to start. Pics would be great. Maybe replace the vacuum distributor block? (129 800 31 78) Can I narrow the issue down?

 

I know... (where have I been.....)

Start with changing that cracked block there bud. It's the issue probably 75% of the time with the center vent issue. ;-)

Jason

 

Issue fixed by replacing the cracked vacuum block. Blows nice and cold :grin

 

Is there a special tool to remove the vacuum lines?

 

Yes a 7mm open wrench.

Put it in, bend....

Regards,
aam.

 

So has anyone had their center vents fail after the car has sat out in the sun? I had left my car outside and the temp got up to 3 figures and I had left my windshield sunshades at home. When I drove the car the center vents and driver vent worked sporadically. After the car cooled overnight all was fine again. Speculating my theory is that the actuators jam in high heat but I guess the vacuum lines could collapse too?

 

They have failed completely now. The vent unit is missing parts and I am kind of suspecting the rotary switches.. sourced another vent unit.. We will see if that is the problem and not the junction box.