AC blower in default mode - PeachParts Mercedes-Benz Forum

dieselmania · #1 09-13-2014, 12:05 PM

All the vacuum pods were replaced in July of 2006 and everything worked fine until about 3 years ago when all the airflow defaulted to the side and defrost vents. I did replace the vacuum pump last year with the updated version after reading about the old style grenading, vacuum seems good, door locks a tad sluggish but have always been that way. Finally having a bit of time to work on this car and want to figure this out. Where should I start? How long do the vacuum pods last? I have been reading about the switchover valves as well, are these easy to diagnose?

Thanks in advance

jay_bob · #2 09-14-2014, 02:53 PM

The vacuum to drive the climate control pods comes from the auxiliary tap on the vacuum pump. It is a green hose that goes back along the left side of the engine, past the brake booster, into a grommet in the firewall. This pops out behind the instrument cluster and goes to the right across the center console.

If you remove the glove box you will find a thing that looks like a harmonica. It has the green hose entering the end and seven multi colored hoses leaving along the length. This is the valve assembly that is driven by the CCU that switches the vacuum to the various pods.

There are a couple things that will cause "default" mode.
- Loss of vacuum to the "harmonica". All the pods are set up to default with no vacuum to be open or shut such that everything goes to defrost mode.
- Loss of electrical power to the CCU via Fuse 7 (or the failure of the CCU itself) will cause the harmonica valves to default everything to defrost mode. Also the fan control loses its reference so it goes to max air flow. And the mono valve loses its control voltage and falls open so you get max heat.
- The "harmonica" block has been known to fail. Jeremy5848 had this happen to him. He cracked it open and found cold solder joints. You can also find one at the junk yard, every 124 and 2nd gen (post 1986) 126 uses this block.

Chapter 83 in the FSM has the climate control troubleshooting procedure. It calls for a special breakout box to help test the sensors and harmonica valves, but if you have a copy of the wiring diagram you can read between the lines in the procedure and measure across the right pins in the harness with your meter.

First thing I would do is check fuse 7. Remove it and make sure there is not a micro crack in the element. That tripped me up about a year ago.

Next I would pop the glove box and put a MityVac on the green line coming from the engine, do you have vacuum? (No MityVac? It should suck hard enough you can feel it with your finger tip.)

dieselmania · #3 09-14-2014, 09:55 PM

Thanks Jay Bob, I am going to tackle that next weekend as the wife picked a different chore for me this weekend (making a couch table) which I just finished. Hopefully the center pod has not failed again. I'll start with the fuse as you suggest and work from there. I found Jeremy's write up on the harmonica looking part, and I do own a meter, an upright band saw and a pencil tip soldering iron so we will see about that. Actually sounds like fun. Last month I saved an 88 key Yamaha electric piano by re-flowing bad solder joints.

dieselmania · #4 09-21-2014, 02:59 PM

All fixed, if you have seen my other post about the main vacuum line you will see that after replacing numerous bad vacuum lines I now have cold air blasting me from the center vents, just in time for fall

. Well we really don't have fall here just two seasons, the warm weather then summer.

Here's a tip some might already know about, I noticed my main vacuum line rubbing against the strut tower so I slipped a piece of heater hose over it.

sixto · #5 09-21-2014, 03:09 PM

FWIW central locking has a dedicated electric pump under the right rear passenger seat. Vacuum to lock, pressure to unlock. If the locks are sluggish, there's a leaky pod or line in the system. The pump will run for less than 5 seconds with a tight system. It will time out after about 30 seconds if there's a major leak. You might might hear a hiss near the leak while the pump is running. You should find the leak before the trunk and fuel filler get stuck in the locked position.

Sixto
MB-less

dieselmania · #6 09-21-2014, 05:59 PM

Only one lock was sluggish, to the point of not working most of the time. I pulled the door panel and could see that the vacuum actuator was popping briskly, but it was struggling to overcome the linkage. I peeked up into the linkage and noticed it was dry and a bit rusted. Sprayed some white lithium grease into it and instantly the lock started working. It is now the fastest one. I'll try listening for the pump to see how long it runs and chase down leaks if need be. Not sure how fast the locks should work but they normally all work within one to two seconds.

Working on glueing the fabric back on the door pocket now where it rolls off at the top. It's annual Mercedes Benz Day at my house today.