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Old 06-17-2014, 10:47 PM
Registered User
Join Date: Jun 2008
Location: Rochester, NY
Posts: 180
Arghhh...yet another A/C question (default mode)

Happy Summer,

First a word of thanks to VSTECH's efforts to teach us the right way of charging our A/C systems

And to DMORRISON for his tutorials on changing vacuum pods, although my hands are still healing from that center pod I changed without pulling the dash. It's possible but painful to do.

My current challenge:

My car is a 1985 300D-T
A/C is running cold but flaps are in default mode (blowing air to defrost and side vents only.

What are the causes of this? Maybe in priority of likely solutions?

I have reflowed the solder joints on one ACC pushbutton unit - no joy same default condition

I swapped in another ACC unit (unknown condition) - no joy same default condition

I've spent many hours searching online without clarity of what is to be addressed next.

Would a faulty pod cause a default mode? Switchover Valves? Temperature controller?

It's kinda fun driving around with my dash disassembled but the wife would really like her seat back and the footwell free of parts!

1985 300D 172,110mi and goin' strong
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Old 06-17-2014, 11:19 PM
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Join Date: Sep 2010
Location: tallmadge ohio
Posts: 80
Hi.I have same problem.It was switchover valves.My car is 1991 350 sd.I dont know if they are same.Good luck.
English second language.
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Old 06-18-2014, 07:56 AM
jay_bob's Avatar
Control Freak
Join Date: Oct 2011
Location: Columbia, SC
Posts: 3,926
Big D has a second gen 126, this is the same design as the system as the 124s. These use a "harmonica" block for the vacuum valves. It is accessible by removing the glove box (or passenger air bag).

1st gen 126s and 123s use the older design that has individual valves on a rack behind the center console. Big pain to get to these.

If you lose vacuum or control power to the valves, all the pods are set up so they default into defrost position.

The "green" hose coming from the engine bay is the climate control feed. I would start there.
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1998 E300DT sold to TimFreeh
1987 300TD sold to vstech
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Old 06-18-2014, 12:29 PM
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1979 & 1985 300D's
Join Date: Oct 2012
Location: Tucson, AZ
Posts: 1,097
The leading cause of this is a lack of vacuum supply to the system. As mentioned make sure the green vacuum line in the engine compartment is hooked up.

Do your other vacuum systems work OK? For example, your door locks?
1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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Old 06-18-2014, 02:11 PM
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Join Date: Jun 2008
Location: Rochester, NY
Posts: 180
Default mode

I'll jump into the switchover valve set today and let you know the findings. Thanks for the info.

The vac system is in very good shape, all locks work, boost is good and main vac line from vac pump to brake booster yields -21 psi
1985 300D 172,110mi and goin' strong
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Old 06-18-2014, 04:17 PM
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Join Date: Jun 2008
Location: Rochester, NY
Posts: 180
Good to know that MB manufacturing is so consistent... 4 out of 5 valves were bad..waiting on new ones and hopefully that'll fix things.

1985 300D 172,110mi and goin' strong
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