Air only comes out of the defrost - PeachParts Mercedes-Benz Forum

Stuiie · #1 07-13-2013, 07:10 PM

After I reinstalled the belt tensioner and belt the air only blows out of the defrost vent.

compu_85 · #2 07-13-2013, 07:15 PM

You knocked a vacuum line off, perhaps at the pump.

Sent from my iPhone using Tapatalk

Stuiie · #3 07-13-2013, 07:40 PM

I checked the only line that I thought I knocked off but I will check it again

jay_bob · #4 07-13-2013, 08:47 PM

My OM603 has a secondary fitting on its vacuum pump that has a rubber vacuum distributor on it. One of those lines drives the climate control vacuum feeder. The factory hose color for this is green.

If you lose your vacuum the system goes to safe mode and the defrost door fails open and all the other doors fail shut.

Stuiie · #5 07-13-2013, 11:43 PM

Ok so I thought I fixed all the vacuum leaks under the hood but I still don't have air blowing out of the dash vents.

jay_bob · #6 07-14-2013, 02:30 PM

Under the passenger side dash is the 7 way vacuum valve.

Unfortunately you have a 2nd gen 124 with the pax side air bag. (On 1st gens, it's much easier, you can remove the glove box liner to access the valve.) I believe the lower dash cover on the pax side of a 2nd gen 124 can be removed, this hopefully gives you access to the 7 way valve. It will be close to the center console.

On the end of the valve assembly is a green hose, this is the vacuum supply from the vacuum pump. Pull this hose off and see if you have vacuum there with the engine running. If you don't have a mity vac put your finger on the end of the hose it should be obvious if vacuum is present.

If you have a mity vac, check for vacuum on the line, and then connect your mity vac to the inlet of the 7 way valve, pump it down, and see if your flaps start working.

If you have no vacuum at the end of the green hose, check at the joint in the green vacuum hose near the master cylinder.

The hoses running over the engine get brittle over time especially where they run in proximity to the hot engine parts. I have had to re-do the hoses on my 124 under the hood after several cracked after being bumped while doing other work under the hood. Many of the hoses are grouped under black tubing and I had some that had cracked under the covering.

Pelican sells the hose but unfortunately only in white/clear.

Stuiie · #7 07-15-2013, 09:30 AM

Is it possible that this problem maybe related the auto temp controller in the dash and not a vacuum problem?

jay_bob · #8 07-15-2013, 10:18 AM

Yes it is possible, if you have no power going to the climate control PBU, that would force everything to safe mode also.

The PBU fuse in 1st gen 124s is F7, it may be the same in a second gen but I am not certain.

Stuiie · #9 07-15-2013, 03:36 PM

Ok so this is what it does right now I can change the fan speed from low to high and high to low.
I can change the temp from cold to hot and hot to cold.
I can turn it off and on.
I will not change where the air blows out of, it is currently blowing air out of the vents next to the doors and on the windshield, if I push any of the buttons to change the direction of air flow such as to get it to blow air on the floor or through the middle dash vent it will not do anything.
I highly doubt I have a vacuum problem but I that also needs to be checked.
A few days ago I was able to play with the temp knob and that got the air to blow out of all the dash vents but now that doesnt even work.

jay_bob · #10 07-15-2013, 04:22 PM

If you can influence the air speed that tells me your PBU is getting power. If the PBU is not powered up then no reference signal is being sent to the regulator (porcupine) from the PBU. The regulator defaults to max blower speed when it loses its reference signal. The regulator power feed for the electricity that drives the blower motor is on a separate fuse than the PBU control fuse in the fuse box.

Is the air discharged at the windshield hot or cool (assuming the engine is warmed up)? If the monovalve loses its control power (regulated by the PBU), then it defaults to max flow. If your air is not hot that tells me the PBU has control of the monovalve.

I still think you have a lack of vacuum to the 7-way valve, that is how the air gets redirected.

The air flow flaps are rigged so that under a no vaccum condition, all the flaps close, except the defrost flap, which goes wide open.

The reason for these behaviors, is so that in the event of a system failure on a cold winter day, the windshield can be kept clear. That is a safety issue, and the remaining functions of the climate control are comfort based.

Please, verify you have vacuum at the 7-way valve under the passenger side dash. I had the same thing happen to me and it was due to the "green" vacuum hose being loose under the hood. This hose starts at the auxiliary port on the vacuum pump, continues past the engine to the firewall near the brake booster, goes into the dash behind the instrument cluster, and ends up on the 7 way valve.

Stuiie · #11 07-16-2013, 02:12 AM

My car has the air bag in place of the glove box so I will have to work around that but Im sure its not too terribly difficult to access.

Stuiie · #12 07-17-2013, 04:13 PM

jay bob
You were right.
I found a vacuum tee under the intake upper plenuim was disconnected, I reconnected it and it is working like it should now.
Thanks for the help.