124 HVAC expertise needed - PeachParts Mercedes-Benz Forum

lrg · #1 01-01-2010, 11:59 PM

First the car details:
1987 300D Turbo
All vacuum pods replaced 2 years ago (full dash out job) and verified working
CCU unit seems OK - modulates temp and fan speeds as it should
aux water pump working

Now the problem: I've had trouble getting cold air to blow through my center vents. It gets cold enough but there is no air volume as much of it goes through the defroster. I pulled the glovebox liner and tested the pods - all good and all vac lines connected as they should be. The input vacuum is strong without leaks. I tested by setting to max cold and full fan and discovered the diverter right behind the center vents is not opening (fortunately it's right next to the glovebox so it's easy to see and access). According to the service manual line #5 coming from the vacuum "tree" behind the glovebox should pull a vacuum to open the center diverter but it isn't. (Yes I know #7 does as well but only after #5 gets it 90% open) I cleaned the electrical contacts on the CCU and the "tree" but no improvement so it kind of has to be either the CCU has partially failed or the vacuum "tree" has failed. My thinking is the "tree" probably never fails (anyone had a problem before?) so my guess is the CCU. Unfortunately I don't have access to a spare to swap so I think I'll by a used one on fleabay. Anyone have any other thoughts? Am I missing anything? Thanks.

jbach36 · #2 01-02-2010, 11:48 AM

Mercedes has never owned up to the fact that their a/c units were crap. If you call Mercedes, their first response is denial ("Oh, we've never heard of any problems before with our air conditioning systems"), and their 2nd response it to push the blame on you (have you had your Mercedes in for servicing at a dealership?).

I don't know if this suggestion will help, but when you run the heat, the center vents stop, and the heat is diverted to the floor vents. When you go back to cool air, it takes it about 20-30 seconds for the vacuum pods to then
re-divert the air back to the center vents.

I'm wondering if there's something on yours that is set wrong, to divert it up, not in the middle, or not to the floor. Maybe try playing with the heat for a while, then cold, to see if the floor gets any air flow as it should.

Good luck.

lrg · #3 01-02-2010, 12:05 PM

Yeah it's a weak system for sure. I did fool around with the temp and cooling settings hoping to get it to work intermittantly but no luck. The center vent diverter just doesn't open. If I pull a vacuum directly on the pod with my Mityvac and it opens right up so it's not the pod. The service manual says to open the center pod/vent the setting is min temp with the econ or A/C buttons pressed. If I do that the vent is not open and I get cold air mainly just up the defrost vents.

jbach36 · #4 02-16-2010, 07:51 AM

I'm sure you've finished this job by now, and I'm just guessing anyways, but could it be the foam? I heard the foam around the defroster vents disintigrates over time, and allows air to leak through the defroster, and less through the center vents.

lrg · #5 02-21-2010, 02:25 PM

Haven't finished it yet. Thanks for the idea but I'm quite sure it's not the insulation around the sampling tube - mine is in good shape and putting the CCU in Max cold mode should override that anyway.

I did try subbing in another CCU and it made no difference - the cener vent still won't open fully, so I think I've eliminated a bad CCU. I need to try a new vacuum "tree" but haven't yet completely checked out the one I have. This being Winter I'm not missing the A/C.

jbach36 · #6 02-21-2010, 10:08 PM

I just replaced the blower motor in my car. Not a fun job, but it is relatively simple, once you've done it before. You can contact me by phone if you want to get to that area, I can walk you through it, and I previously posted how to change the blower fan. You can also YouTube cleaning an evaporator coil. Even though it might show it for a house a/c unit, you'll get the drift.

Behind the blower fan is the evaporator coil, sort of a radiator fin looking thing. There was foam from a piece above that, that has deteriorated over the years, and drops into that area, and clogs the fins up. Mine was clogged only on the driver's side, the passenger side was perfectly clean.

Without hurting the fins, vacuum up the heavy stuff. Then clean the fins with computer terminal cleaner. The stuff is sold in a can with a little spray straw on it so you can blast away with a nice burst of spray. It cleans, and won't hurt anything, and evaporates very quickly. When done, hit that area with a very small blast of water with a garden hose. Don't hurt the fins or anything behind that, the lines carry coolant, you sure don't want to puncture anything.

I think the coils help the a/c expel the heat, so I'm hoping that by cleaning mine, it will help get me cooler air.

Good luck.

lrg · #7 02-22-2010, 12:59 PM

I did the blower motor replacement about a year ago and it was a bit tedious, although not too hard. I found the intake to the evaporator about 50% blocked and vacuumed it out but unfortunately it didn't make a huge difference in the cooling. I vacuumed it on the other side as well when I had the dash off for pod replacement but it looked pretty clean on the inside. In hindsight I think I would have been better to blow some compressed air through from inside rather than vacuum.

My current problem is definitely related to the vacuum controls not sending a signal to open the center vent pod all the way. The pods are all new and good (I tested them twice), I'm getting a good vacuum from the main vacuum line at the top of the "tree" that controls the vacuum to the individual pods, I'm just not getting a vacuum at the tree outlet that opens the center pod (# 5 or 6 I think) so I think the "tree" controller is bad. When I get a chance to check it out further I'll report in.