Update on AC center vents repair (no air from center vents) - Benzworld.org

Goatman · 06-13-2005, 10:58 PM

I like to share my findings on how to troubleshoot and repair the Air Flaps on my 95 S600.The symptoms was that the AC did not blow any air from the center dash vents. This seems to be a default mode when the whole Flap assembly does not receive Vacuum to operate. Vacuum motors operate all flaps. The individual motors are connected via vacum lines to a solenoid block assembly (mounted behind the glove compartment) The vacuum solenoid assembly electrically connects to the AC controller (where you set the temperature etc.) Depending on mode (defrost, AC, heating) the controller tells the Solenoid block to open a valve to steer vacuum to the assigned flap motors. The controller has a test mode to test all individual flaps. To enter the test mode turn the ignition on, press the auto button and set both temperature wheels to 73F. Then press and hold the rest and C/F key simultaneously for several seconds until both displays change from temp reading to blank. At this time release the buttons. Now the left display shows a 0 and the right display shows Hi. Each individual number from 0 to 16 depending on model will test a flap. Left auto button selects the flap number and the right auto button toggles between Hi and Lo meaning flap on or off. Alldata has the number assignment for all flaps, but this is not necessary for a simple test. If the air flow changes when you run the test, it means that you have vacuum. If nothing changes as in my case, you have no vacuum. I consulted Alldata to find information about the vacuum source, but the information is not detailed or useful for troubleshooting. Usually the vehicle has two sources of vacuum. Vacuum from the engine and vacuum from the Central lock pump.
I removed the rear seat bench to check the Pump that generates accessory vacuum and accessory pressure. I found that someone had taken the P+ line off and plugged the pump outlet. I took to plug off and connected a combine Vacuum & Pressure gage( $20 from Kragen) Turning the ignition on activated the pump and I had a pressure reading of 70 PSI from the P+ pump port. When I connected the disconnected pressure line pressure dropped and the pump could not keep up pumping and eventually switched off. So there must be a leak on the pressure line. I plugged the P+ port again and disconnected the P- port to check for vacuum present. I had to reset the pump by unplugging the electrical connectors. Vacuum checked ok on the P- port. I connected the vacuum line via a T connector and vacuum was ok with the accessories connected. Vacuum from the pump could not be the problem for the AC flaps. That made me thinking to follow the pressure line. I followed the line to the passenger side inside wheel well by removing the inside carpet cladding. The pressure line connects to a diverter block that has 8 plug- n ports. 6 ports are used. I unplugged the line coming from the pump. A 7 mm open-end wrench will help to unplug the lines. I connected my trusted gage to the unplugged line. Connected the Pump P+ line and turned the ignition on. The pump started and I had pressure without a leak on that line. I reconnected the disconnected line to the diverter block and the pressure escaped. I could actually hear a slight hiss coming from the diverter block. Disconnecting the other lines from the block did not solve the leak. I assumed that the block must have a leak. Also connected to the block is the gray vacuum line coming from the engine and two green lines going to the AC solenoid block. I connected the gray vacuum line coming from the engine via a tee and two hoses to the green lines. I started the engine to see what happens. To my surprise the AC started to blow from the center vents. I run the flap test program and all worked ok. That means the diverter block that has vacuum and pressure connected to it must be defective. I purchased a new one from my dealer ($32) Part 129 800 1478 and reconnected all lines.
Pressure was ok and vacuum feed to the AC. The Vacuum Block or Diverter had a leak. I donâ€™t know the exact function of this device. I took the old one apart and it has two diaphragms inside. It seems like that it needs the pressure ort the vacuum to regulate something I could not figure it out. Alldata has no information about this part. Long story short, you need to have vacuum and pressure to operate the AC Flaps. It was not difficult to troubleshoot the pressure/vacuum lines. The nifty gage was the only test instrument used.
Does anybody has some info about the function of the replaced device?

pcmaher · 06-14-2005, 06:41 AM

Nice write-up Goatman! Thanks.

Brett San Diego · 06-14-2005, 11:31 PM

You should repost in the knowledge base.

So if I understand you correctly, the central locking pump also has a time out shut-down mechanism like the closing assist pump. Is this correct? You said you had to electrically disconnect and reconnect the pump to reactivate it.

Brett

Goatman · 06-15-2005, 11:04 AM

Thanks PCMAHER and Brett.
Yes I can transfer the info with pictures to the Knowledgebase.

To answer Brett, yes the central lock pump has a time out. I did not waste time to find the fuse to reset the pump so I just disconnected it. If the pump runs too long it will time out.

By the way, vacuum for the AC is coming from the Engine. The Pump is only supplying pressure to this magical diverter block. I will research this block further and post some pictures of it.
My next task is to find out why both orthopedic seats don't inflate. I have pressure and vacuum but I find only vacuum at the seats. Alldata has very limited information on this.

Benz-Friend · 06-15-2005, 02:32 PM

Goatman: Excellent write-up. This is the sort of thing which makes this forum so very valuable.[:)]

Peter Guenther · 06-16-2005, 05:42 AM

THe locking pump provides pressure and vacuun. It provides pressure for the lumbar, and vacuum for the HVAC when the engine is not running, or during acceleration when there is none. There is always pressure available for the lumbars via some sort of resovoir, and the same goes for the vacuum, The pump times out if it runs for more than 60 seconds in secondary consumers, and 10 seconds for lock/unlock mode. It maintains 600 mbar pressure for the lumbars by cycling on and off
It has a memory that can be read with an impulse counter at the test connector under the hood. Of course we all have one of those in our tool box.

93 BENZ S400SEL · 09-19-2005, 11:53 PM

Is that the switchover valve block. Located on the floor by passengers side foot space ?
PGP

Goatman · 09-21-2005, 05:11 PM

Yes, Its the black valve block down there. I don't know its function.

Werner

93 BENZ S400SEL · 09-21-2005, 05:22 PM

Well I just replaced my evaporator temp sensor but MB guy said to change switchover valve block first. I will see how mine holds up . Maybe I change your part next if it does not work. But you had no function. Mine is intermittent
PGP

ronan95d2 · 01-27-2006, 05:08 AM

The knowledge available on this forum is just amazing. With a little bit of common sense combined with the knowledge on the forum, I have just fixed my A/C airflow issue for â‚¬27 by replacing the switchover block, as described by Goatman.

Symptoms were as described by other forum members....little or no airflow from centre vents combined with slow demist. Fan and regulator were easy to check. Diagnostic for flaps was simple. Vacuum pump functioning and no leaks in lines. Next suspect...the switchover block.

Without the forum, I'd have had to sell the w140 long ago.

Incidentally, when removing the footwell carpet and footwell side panel, I was a little put off to see a multi-lingual airbag warning printed on the metal footwell plate under the floorwell carpet. Are the connections behind the footplate connected to the SRS system? There is one large connector with a bright red insulating hood....a bit worrying for the amateur mechanic!

Keep up the great postings!
Ronan