Is My Indy Right About This?? - PeachParts Mercedes-Benz Forum

Cal Learner · #1 12-25-2008, 09:52 AM

When I recently had some A/C system repairs (receiver/drier/compressor) done at my usual, reliable indy, he pointed out that my defrost vacuum element needed replacement too because cold air was coming out the defrost with the A/C engaged. I did not do his recommended replacement of the defrost element. However, a couple of weeks later, I had him replace the transmission oil pan/gasket to correct a slow but persistent ATF leak. Immediately thereafter, driving with A/C on, the CCU functioned as it should, with cold air only at the center and side vents, not out the footwell OR defrost vents. For the past month or so since the transmission pan r&r, the CCU has worked perfectly, defrost vents have not let cold air out.

I theorized that the transmission pan leak had affected the CCU vacuum to such an extent that it was leaving the defrost element in its normal open state. Then, when the trans pan work was done, there was no more vacuum leak to affect the climate system so the defrost element functioned as it should by closing those vents with A/C engaged. My indy dismissed my suggestion as impossible. Obviously, I don't know enough about how transmission vacuum and CCU vacuum are related, if at all. What do you think? Is my indy right, as usual?

vanwiek · #2 12-25-2008, 11:58 AM

I think your indy is correct. You had a fluid leak at the tranmission, not a vacuum leak. I don't think engine vacuum is in any way connected to the inside of the transmission. It's only present at the transmission controls. If vacuum were applied internally to the tranny I would think tranny fluid would be sucked into the engine.

I could be completely wrong so someone else please chime in.

Arthur Dalton · #3 12-25-2008, 12:54 PM

He may have found a leaking modulator vac. line at the tranny and cut it back..common when working on a trans to see that vac line split or even off............you just cut an inch off and slip it back on.

Cal Learner · #4 12-26-2008, 12:19 PM

OK, thanks. I thought it was a long shot that there would be any connection between the trans pan and climate control vacuum, but it was so noticeble immediately after the work that I wondered if that could be it. I can see how Arthur's idea could have been involved. Anyway, for the moment, I'm going to enjoy my small good fortune that I don't have to have the tech tear the dash apart. I might also use the occasion to try to find a good primer on MBz vacuum systems, so that I can really understand this stuff. Anyone have a suggestion for a good, beginning write-up on the subject?

stevebfl · #5 12-26-2008, 12:41 PM

If Arthurs suggestion for the relationship was the case, you would have had distinctly harsh shifts that would have significantly changed after the repair.

I find it unlikely that a complete vacuum leak at the modulator could affect the defrost control unless the defrost was marginally capable in the first place.

More likely the change in weather has altered a small leak. The diaphrams in those elements can often work with a leak if they move fast enough. After movement the vacuum can actually seal the leak. This happens with the door lock actuators quite often.

Arthur Dalton · #6 12-26-2008, 08:01 PM

Yeah ..I was going to ask if the shift changed, but that was just an off the wall, long-shot guess..............trying to justify a remedy with a tranny being looked at and evidence of the remedy thereafter....we will scap dat one.........

johnjzjz · #7 12-27-2008, 09:41 AM

More likely the change in weather has altered a small leak. The diaphrams in those elements can often work with a leak if they move fast enough. After movement the vacuum can actually seal the leak. This happens with the door lock actuators quite often.

I am in for this idea - its very possible the vacumn control was stuck in the open / or closed and weather cold hot even rain dew entered the vacumn pots control door and un stuck it - or as was said while refilling the trans the tech seen a vacumn line off or cracked and just re did the connection ????????? -- jz

Cal Learner · #8 12-27-2008, 10:56 AM

Quote:

Originally Posted by stevebfl

If Arthurs suggestion for the relationship was the case, you would have had distinctly harsh shifts that would have significantly changed after the repair.

I find it unlikely that a complete vacuum leak at the modulator could affect the defrost control unless the defrost was marginally capable in the first place.

More likely the change in weather has altered a small leak. The diaphrams in those elements can often work with a leak if they move fast enough. After movement the vacuum can actually seal the leak. This happens with the door lock actuators quite often.

Thanks for your insights. No, there were no harsh shifts before the r&r on the pan (and none since). The trans has always had very nice smooth shifts under all accelerations. I guess what you're telling me is that I shouldn't be too surprised if/when the defrost issue returns, as the diaphragm may be beginning to fail even now. Until it does, I'm delighted to avoid the $800 labor cost of tearing the dash apart. Thanks.