Defrost Flap Actuator Access - Mercedes-Benz Forum
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post #1 of 18 (permalink) Old 12-20-2018, 03:24 PM Thread Starter
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Defrost Flap Actuator Access

How is the best way to access the defroster vacuum actuator? The bottom line holds vacuum, but the side one does not. I need to check to see if this line is attached, has a bad fitting, or if I have a bad center vent actuator. it appears that these lines are teed together.

It looks like I might could access it by removing the radio and/or the instrument cluster?
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post #2 of 18 (permalink) Old 12-20-2018, 04:13 PM
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Is your defrost actuator leaking vacuum? Keep in mind that it defaults to open and only closes with vacuum applied. If that circuit is bleeding off vacuum, you could always block it off as opposed to replacing it. IIRC, the dash assembly needs to be removed for the defrost actuator replacement. The center dash element can be replaced without doing so although it is difficult.


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post #3 of 18 (permalink) Old 12-20-2018, 06:08 PM Thread Starter
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All of the vacuum lines that I can get to through the glove box opening except the red/blue & red/white one holds vacuum. I can verify the fresh air vent flaps are opening (dark red), and I think I have the legroom flaps opening (red/yellow), although I cannot see them. The supply line (red/green) to the switchover valve holds vacuum, but the check valve at the supply is not holding the vacuum (on order). The vacuum storage cube holds vacuum, but it is useless because of the bad check valve.

My diagram shows that the red/white line is default open, and should close the defrost flaps when vacuum is applied at the switchover. It takes a little vacuum and then it leaks off. The red/blue line takes no vacuum, but should open the center vent and supply vacuum to the flap side of the defrost actuator (not sure why - possibly throttling the defrost flaps?). I think there is a tee close to the defrost actuator and the line goes on to the center vent actuator.

So from what I can gather my defrost actuator is leaking or the connectors are leaking (likely). I had to replace most of the rubber connectors in the glove box hole, because the rubber had stretched at the connections and was leaking. That may be the same thing with the defrost actuator, but I was not sure how to access it. If the connectors are leaking I may be able to change them. I need to take the instrument cluster out to change a light bulb, so I may do that next. The only other way would be thru the radio hole? I would hate to have to take out the dash, but I guess I could do it? It has some cracks that I could try to fix.....
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post #4 of 18 (permalink) Old 12-21-2018, 11:10 PM
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The defroster flap actuator is unique in that it is a double diaphragm unit which allows it to be controlled to partially closed via the red/blue circuit or fully closed by the red/white circuit. If you can pull a slight vacuum on the red/white but none on the red/blue circuit then likely the inner diaphragm is torn as the two circuits operate on either side of that diaphragm.
If you were to block off the red/white line going to the defrost actuator (on the actuator side, not the switchover valve side) and then apply vacuum to the red/blue you may find that you will get the partial closing of the defrost flap and full opening of then center vent flap.
Iíll dig further into my resources and see if I can locate details on how to access the defrost actuator.
The diaphragms can be purchased separately from
http://www.perfanalysis.com/climate-...acc-version-i/
but they are a challenge to take apart. Ask me how I know.


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post #5 of 18 (permalink) Old 12-22-2018, 04:53 AM
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The link to member Ears home page is buried in the Welcome Sticky The 107SL Source Page He covers the center vent and heater box.
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post #6 of 18 (permalink) Old 12-22-2018, 06:18 AM
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Originally Posted by rowdie View Post
The link to member Ears home page is buried in the Welcome Sticky The 107SL Source Page He covers the center vent and heater box.


Excellent write up! Thank you, rowdie.


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post #7 of 18 (permalink) Old 12-22-2018, 11:54 AM Thread Starter
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Thanks Rowdie, that answers all my questions. I need to remove my instrument cluster and radio and then should be able to check all of lines going to defrost actuator. I should then also be able to test the center vent and defrost actuators. Since all of the others are ok, I am hopeful that the connectors are all leaking like they were on the glovebox side.
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post #8 of 18 (permalink) Old 12-26-2018, 10:28 AM Thread Starter
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As a follow up - I removed the radio and was able to reach the defroster flap actuator connections. Unfortunately, both the defroster and the center vent actuator are leaking (not responding to the mity-vac), so it looks like I will have to remove the dash to replace or repair them.

What is the best source for a procedure to remove the dash?
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post #9 of 18 (permalink) Old 12-26-2018, 02:44 PM
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This one is pretty informative:


How hard can it be?
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post #10 of 18 (permalink) Old 12-26-2018, 05:08 PM
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Couldn't you just remove the transmission surround to get at the front of the box? I took out everything but can't recall how much access there was after the removal of that hump.
Dash removal is a major pita.
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