420sel A/C on Defrost Vents Only - PeachParts Mercedes-Benz Forum

RC2007 · #1 06-05-2007, 09:17 PM

My "money pit" 1988 420sel just developed another problem, this time with the climate control system. The air conditioning only seems to be pumping through the defrost vents on the dash and the vent for the rear on the center console. Nothing through the front center vents. It is ice cold but on 95 degree days like today is hot by the time in circulates. Any suggestions???

rsmartin · #2 06-05-2007, 09:54 PM

My thought would be the climate control push button module needs replaced. My 84 300SD was pumping hot air through all the vents even with A/C charged. Swapped out the button unit from my 85 300SD and cold air coming out of the correct vents.

I had to buy new unit for the 85 300SD after a bad auxiliary pump fried the unit so you might also want to check that if it turns out you need to replace the push button unit. I think I got my unit from www.***************

Not an expert though.

Rockman59 · #3 06-05-2007, 11:09 PM

It is possible some of your vacuum lines under the dash that control the vent positions are leaking. I had that happen on my '86.

brewtoo · #4 06-06-2007, 08:09 AM

I would also check the vacuum pods.

TX76513 · #5 06-06-2007, 08:29 AM

Like stated above it's a 95% odds of vacuum pods - easy to check easy to fix.

raymr · #6 06-06-2007, 08:45 AM

I fixed mine by replacing the burned out change-over valves. The condensation from the A/C blowing on the windshield was very annoying.

TX76513 · #7 06-06-2007, 08:51 AM

Quote:

Originally Posted by raymr

I fixed mine by replacing the burned out change-over valves. The condensation from the A/C blowing on the windshield was very annoying.

Those are pricey - did you autoposy old valve to see what failed?

david s poole · #8 06-06-2007, 09:19 AM

the default for the air when there is a significant vacuum leak in the a/c system is to blow air thru the defrost vents.strangely though the defrost vacuum element is often the one to go bad.take out the glovebox and you can get access to the most prevalent offenders,the two elements that control the fresh air door and the defrost.if you are in a hurry you can use tie wraps to close the defrost and the fresh air to provide temp relief.

raymr · #9 06-06-2007, 10:02 AM

Quote:

Originally Posted by TX76513

Those are pricey - did you autoposy old valve to see what failed?

I checked each one for current. 2 of them were bad. The ones that failed were the ones that are activated whenever the car is running. I guess its to be expected after 20-some years.

PS. I just checked: The '88 has a different change-over valve setup from mine, so my info won't apply here.

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