PeachParts Mercedes-Benz Forum - View Single Post

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.)