AC Recirc Mod - Hissing Noise? - PeachParts Mercedes-Benz Forum

jdh122 · #1 06-21-2010, 10:33 AM

Just over a year ago I performed the "full-time recirculated air" modification on my '83 300D. It seemed to work reasonably well, but I wasn't convinced that it was closing both the main air flap and the outside air flap in the cowl because when driving through an odorous area (say, next to someone mowing their lawn) right away I could smell inside the cabin whatever odor was present (e.g. fresh-cut grass).

Yesterday I checked my work and decided that the piece of vacuum tube I'd used in the mod (the piece leading to the T joint in which you plug the two lines on the far right) was a little too large, resulting in a less-than-snug fit over the second nipple from the right. I replaced it with smaller gauge tubing, turned on the AC, watched the two flaps close properly and then put everything back together. So all I did was change out one short piece of tubing.

This morning the AC seemed to work better than usual, and in auto mode the fan never really had to go past level two or three (and this in Houston, mind you). Also I didn't smell grass when driving past a couple of yard crews in my neighborhood. However, after a few minutes when the cabin reached the desired temperature, the compressor clicked off as usual and then a new "hissing" sound began to emanate from general area of the CCU. It isn't particularly loud, but definitely noticeable. At the same time, I could tell that more humid, outside air was entering the car.

After a few minutes of this I lowered the temperature on the thermostat until it clicked, at which point the hissing stopped and the air became colder and less humid. During this hissing period, the car didn't really warm up THAT much, but it definitely became more humid.

My question is, is the hissing a normal by-product of the recirc mod, and it is appearing just now because I didn't do the mod correctly the first time? Or is something else at work here?

Thanks.

vstech · #2 06-21-2010, 11:32 AM

any time the compressor clicks off and the fan continues humidity will rise rapidly.
sorry, I can't offer any help on the hissing normality.

Cr from Texas · #3 06-21-2010, 11:42 AM

I'd suspect a vacuum leak. Any chance you damaged a plastic nipple it replacing the tube?

JHZR2 · #4 06-21-2010, 11:46 PM

Hissing is normal in AC systems in general, IMO. It has to do with the expansion valve, IIRC.

jdh122 · #5 06-22-2010, 08:41 AM

Ok, I fixed this hiss last night. Cr from Texas nailed it: it was a vacuum leak. The replacement tubing I used just wasn't sealing tightly around the nipple, nor could I get it to seal right no matter what I tried. So, I replaced it with yet another section of tubing with an even smaller inside diameter, and that seems to have fixed the problem.

The strange thing is, the leaking tubing had a smaller inside diamter than the tubing from my original mod a year ago (which never leaked, or at least not audibly), so theoretically should have been a tighter fit and therefore less prone to leaks.

leathermang · #6 06-22-2010, 08:46 AM

While you may have had a hissing from a leak..

JHZR2 is correct about there being lots of normal hissing from a car AC system...We just don't normally hear it due to other noises...

ashedd · #7 06-22-2010, 12:02 PM

Semi hijack...

What all needs to close for the air to be recirculated? I think my TD's closing one of the flaps but the one on top of the blower motor will not close.... does that one need to close?

leathermang · #8 06-22-2010, 04:33 PM

ASHEDD !!!! UNHIJACK this thread !!!
really, post that question so people can see it and reply with good information.

ashedd · #9 06-22-2010, 04:47 PM

Sorry

leathermang · #10 06-22-2010, 05:16 PM

Not a problem..
except you want to attract the best answers...
Your cars are way more complicated than mine... so I do not know anything about your vent system ... my books are all 123...