Getting A/C and heater out of default mode

[brian bray]

The A/C and heat in my 1991 300 SL remains in "default" mode after replacing the A/C controller which was tested and worked. In "default mode", the heated air or the A/C air comes out ONLY from the defrost and side outlets NOT the center dash vent. Since the heater core is NOT responding to the controller, is there a way to manually set the airflow to come out the center dash vents? Thanks.

 

[LostKiwi]

No there isn't.
What you have is a classic loss of vacuum symptom.
Behind the glovebox is the vacuum distribution control. This operates to route vacuum to the various actuators that operate the flaps. The vacuum feed comes from the inlet manifold via a vacuum distribution manifold in the module coffin in front of the passenger windscreen. This has a habit of cracking at the ball bearings used to seal each end. If cracked carefully fill the crack with epoxy resin. Other than that check for cracked or broken vacuum lines.

 

[brian bray]

No there isn't.
What you have is a classic loss of vacuum symptom.
Behind the glovebox is the vacuum distribution control. This operates to route vacuum to the various actuators that operate the flaps. The vacuum feed comes from the inlet manifold via a vacuum distribution manifold in the module coffin in front of the passenger windscreen. This has a habit of cracking at the ball bearings used to seal each end. If cracked carefully fill the crack with epoxy resin. Other than that check for cracked or broken vacuum lines.

Thanks for your quick response even though I was hoping for a simple solution. Brian Bray

 

[brian bray]

Kiwi,

Thanks for your quick response. I was hoping for a simple solution. Brian Bray

 

[LostKiwi]

Kiwi,

Thanks for your quick response. I was hoping for a simple solution. Brian Bray

That is a simple solution. Check the manifold in the module coffin. Very common for the vacuum manifold to crack and very simple to fix. Around 5 minutes to find, 15 to fix if it's the cause. All the tools needed are a crosshead screwdriver to access the module coffin, a 10mm spanner to remove the manifold (you can leave pipework attached) and a tube of epoxy.

 

[brian bray]

That is a simple solution. Check the manifold in the module coffin. Very common for the vacuum manifold to crack and very simple to fix. Around 5 minutes to find, 15 to fix if it's the cause. All the tools needed are a crosshead screwdriver to access the module coffin, a 10mm spanner to remove the manifold (you can leave pipework attached) and a tube of epoxy.

Kiwi,

Again, thanks for the quick and concise reply. Your explanation encourages me to attempt to repair.
Brian Bray

 

[LostKiwi]

Here's where it is:

Here's where they crack:



This is all shown removing the block but with care you can do it in situ

 

[brian bray]

Here's where it is:

Here's where they crack:

View attachment 47681

This is all shown removing the block but with care you can do it in situ

Kiwi,

Again, thanks for the quick and concise reply. Your explanation encourages me to attempt to repair.
Brian Bray

Here's where it is:

Here's where they crack:

View attachment 47681

This is all shown removing the block but with care you can do it in situ

Here's where it is:

Here's where they crack:

View attachment 47681

This is all shown removing the block but with care you can do it in situ

Here's where it is:

Here's where they crack:

View attachment 47681

This is all shown removing the block but with care you can do it in situ

Kiwi, I'm amazed! Not only did you provide an clear explanation but you also included pictures as a guide. With all of that help, I should be able to do this. Thanks again! Brian Bray

 

[brian bray]

Thanks to Kiwi my A/C distribution problem is solved as of today (the first opportunity to work on it). Kiwi sent detailed instructions and even pictures. His encouragement made even a mechanically challenged person like me fix the problem. I removed the air distribution block per his instructions, found and epoxied the crack and reinstalled. Now, the comes thru the center vents as intended. I also removed the cover over the cabin air filter (3 clips), cleaned out all of the dirt, and installed a new cabin air filter. Cabin air filters are approximately $19 online. I doubt if the cabin air filter had ever been changed judging by the amount of dirt that was present. I took the old filter outside and and could not see the sun thru the filter. The air flow coming thru increased dramatically. Many thanks again Kiwi.

Kiwi,

Again, thanks for the quick and concise reply. Your explanation encourages me to attempt to repair.
Brian Bray

my A/C

 

[LostKiwi]

Thanks to Kiwi my A/C distribution problem is solved as of today (the first opportunity to work on it). Kiwi sent detailed instructions and even pictures. His encouragement made even a mechanically challenged person like me fix the problem. I removed the air distribution block per his instructions, found and epoxied the crack and reinstalled. Now, the comes thru the center vents as intended. I also removed the cover over the cabin air filter (3 clips), cleaned out all of the dirt, and installed a new cabin air filter. Cabin air filters are approximately $19 online. I doubt if the cabin air filter had ever been changed judging by the amount of dirt that was present. I took the old filter outside and and could not see the sun thru the filter. The air flow coming thru increased dramatically. Many thanks again Kiwi.
my A/C

You're welcome. Good work puling it apart and fixing it.
Many times the fear of doing things is worse than actually doing it. Good to see you overcame your reluctance and fixed it