heat from side vents on a 140 - PeachParts Mercedes-Benz Forum

126nut · #1 05-04-2004, 03:04 PM

My employers wife's 97 S500,

With the a/c on full blast it will intermittently send superheated air from both sides of the dash vents. Any ideas? I did a search and found a similar problem on some of the smaller Benz models, monovalve maybe? I am used to flaky climate controls on my 126 car but it appears the 140's have just as many problems.

Thanks for any help!!

KenP · #2 05-04-2004, 08:24 PM

The 97 W140's have a digital climate control head with some self diagnosis capabilities. It will also let you read various sensor reading in the ducts/evap/interior etc. Start there. If you do a search you'll find the steps.
The system is very complex but doesn't have that many parts...
If the problem is intermittent, my guess is that its probably in the control head itself or the vacuum control block behind the glove box.
Bad news is that either of these is big dollars to replace... don't guess... do the right diagnosis or you'll go broke trying to replace pieces. Good news is that the control head can be changed in 30 seconds while the vacuum block takes about 30 minutes.
Hope this helps.

126nut · #3 05-04-2004, 11:16 PM

Thanks Ken,

I will start looking around here. Using the self diagnosis feature would be a great place to begin. It would be a good idea to get a service manual (cd), right?

It is a very intermittent problem at this point so I have a bit of time to do some research.

sixto · #4 05-05-2004, 03:24 AM

It would be a very good idea to get a service CD. If only MB would make one available.

Sixto
95 S420
87 300SDL

Peter Guenther · #5 05-05-2004, 09:01 AM

MB claims a 2 CD manual for the 140 is weeks away $150, In the meantime look at this page, it shows how to do a diagnostic using the controls,
http://pages.prodigy.net/jforgione/MB_S500.html

126nut · #6 05-05-2004, 09:51 AM

Thnks for the link but correct me if I am wrong, from what I have read it appears the earlier 140's used an analog system?

They went digital in the mid 90's, right?

Not sure if this box would work on a 97.

KenP · #7 05-05-2004, 07:43 PM

You're correct, the W140's went from analog to digital in 1996 model year... earlier diagnostics are different.
If you can't find the diagnostic procedure anywhere, you can sign up on the ALLDATA site for the on line manual. They have the whole procedure.

joe p · #8 05-05-2004, 09:33 PM

Sounds like a Evap temp sensor going open to me. The 140 is the only one with a override for the center vent. (thats the blue and red buttons on the vent.) With the blue button pressed and ac set if the evap temp sensor goes open momentarly the SIDE vents will go full hot. The side vents perate the "dual zone" climate control by adjusting indepently of the center. Confused yet? It gets better. They never decided exactly where to put the sensor. 95% of them are in the side of the case below the expansion valve plumbing (which you cant see from inside but its over the gas pedal) and all but hidden from view by the wiring and various other odds and ends in there.

Joe