w140 trouble with climate control (blower)

93 400sel, during longer trips with AC on automatic the blower stops working. It sounds like all is still working from behind the dash but no air coming from vents. It appears to be still cold at the vent so looks like evaporator working etc.. I can reset by pulling the fuse and things start to work again. Short trips don't seem to be an issue and once car has been off, on start-up back to normal??? Any ideas for a potential DIY fix?

[JimF]

Maybe it's the same malady:

A/C blower's output is low . . .

[mbdoc]

Many of the W140 chassis cars will allow the A/C evaporator to "freeze" on long trips. The problem is with the evaporator temp sensor. Have replaced many of them for that problem.
MB part number 210-830-07-72 with list price of $51.

Thanks MBDOC, is this something that I can try myself and if so could you point me to the area and briefly describe the process. It sounds like a relatively inexpensive attempt and something worth knocking out as a first step.

[Gilly]

There is one other possible solution, assuming the fan is actually still operating as you say it sounds like.
Under the hood in back, near where the blower motor is, you'll see either one or two vacuum operated valves (plastic vacuum line/lines run to them, the quantity depend on the year). What this/these operate is a fresh air flap on the cover assembly. If you put the car into recirculation mode, this flap closes.
What can happen is that the connectors on the end of the vacuum valve/s can break and allow the air the blower is pulling through to snap the door shut. If this happens, with the AC NOT in recirculate mode, you now have both the fresh air flap closed (due to the broken connector) and the recirculation flap closed (due to the mode that it's in).
Operate the AC like normal, and when it does this to you again, hit the recirculation button and see if you then get airflow.
You may even hear the fresh air flap drop back down after the pressure is relieved.
I have seen a few of these, the connectors are available seperately, cheap date.
Actually, if this is the cause, the reason cycling the key makes a difference is that you are shutting the blower off by doing that and the door opens itself up. You can leave the car running and shut the whole climate control off and achieve the same effect, it'll work again until the blower picks up speed and maybe a hit a little bump in the road and "WHUMP" no airflow again, except in recirc mode. MAN I was scratching my head awhile on the first one of these I saw! Blower motor noisier than heck, but sounded like it was really straining to work, but no airflow. Sound familiar?
Gilly

ps My experience with the evap temp sensor is that on longer trips, it will blow OK, but start blowing warm air. This really sounds like a blower related problems as noted above, a blower or more likely the regulator, except that the original poster makes it really sound like this flap problem because they can hear the blower running. The frozen evaporator? Well, maybe, except the poster says it's still drippping water, the dripping really starts to slow down when it's frozen. Could be though.

I had a very similar problem. It turned out to be the blower regulator which got overheated and then cut back the speed to minimum regardless of what I did to the controls. When it cooled off again it was fine, and the problem only occurred in summer. The regulator is mounted on the blower housing and is very easy to replace. I got a good price from George Murphy.

I have always been puzzled by the climate control since owning this vehicle due to never seeming to have the ability to have outside air blown through the vents via the blower as on any previously owned vehicles (non-mercedes). Maybe this has always been a malfunction and all related to my original posted problem---does this 93 400sel have the ability to simply allow outside air in through the vents? How do you set the climate control to have this happen?

[Gilly]

Shock:
It always uses outside air unless the recirculation button is used.
If you want "only" fresh air (not cooled AC air) then push the EC button, which deactivates the AC compressor (EC=economy mode)

Gilly