'87 300D Water Vapor from Cneter Vents

[sublettm]

Well, teh AC works after the evaporator core replacement, but now I have another strange problem. After running the AC for about 30 minutes, what appears to be steam comes out of the center vents. If I shut down the AC for about five minutes, this problem goes away but comes back after about 30 minutes. If someone can either help with this problem, or direct me to a good link, it would be appreciated.

On another note, my father and I did a podectomy on his 81 300D about a month ago. His newest quirk is that at low speeds, the AC works like a champ, but once you get above 50 mph, warm air blows out of the left center vent, and the left vent by the driver's door. Could this be a monovalve problem? We swapped out the CCU this past weekend with no solution to the problem.

Mike

[dannym]

Maybe your condensate drain is clogged??

Danny

[Cateaux]

Maybe the evaporator is icing up. When the compressor shuts off and the evaporator warms, the ice begins to melt and is blown out the vents in the form of vapor.

[brabus]

I can state this as a fact for your particular car, but on nearly all A/C systems there is a temp switch to turn off the A/C compressor once the evaporator goes below freezing temperatures. This is to prevent excessive condensation and the collection of ice. When this switch fails it is very common that water vapor will blow from the vents.

[sublettm]

Where might this switch be located on my car?

Mike

[Cateaux]

Originally Posted by sublettm

Where might this switch be located on my car?

Mike

A temperature sensor should be cuddling with the evaporator on the side where the air blows out, and may be a part of the expansion valve assembly. The sensor will connect to the switch. I have no direct experience with W124s, so I can't be specific.

[jcyuhn]

I am not convinced there is a problem here. Under the right conditions, air conditioners sometimes have a bit of visible condensate in the output air. It's not a big deal. I've seen it happen many times in many different vehicles under mildly warm, humid conditions. Sounds like Houston this time of year, eh?

To test if the evaporator is icing up, let the a/c continue running. The ice blocks airflow; eventually you'll notice less and less air coming out the vents despite more and more fan noise. This car does have a temp sensor for the evaporator, it's supposed to shut off the compressor when the air coming off the evap falls below ~38 degrees. The temp sensor is buried in the HVAC box, the electrical connection for it plugs in kind of near the throttle pedal.

Try flipping it into 100% recirc mode when it begins misting. I bet that stops it right away. The coming warmer weather will also take care of the problem...

- JimY

[jcyuhn]

No, afraid not. I used to think that was the evap too. But having been there, done that, it's actually the heater core. The evap is hidden behind the heater core, closer to the firewall. Makes sense if you think about it. Have to cool the air before heating it if you want to dehumidify it.

- JimY

[sublettm]

I know where the sensor is, I remember disconnecting it when I pulled the box out of the car. Do the wires run up to a switch somewhere that shuts the compressor down? I am guessing that I am just now seeing this problem as it has just started getting REALLY hot the past week or so. I did not own this car last summer and just did the evaporator core about a month ago. I would really like to resolve this pretty soon since I will be making a trip to Florida in this car in about a month and it is really annoying having to cycle the air on and off every half hour to let the ice thaw.

Mike

[jcyuhn]

Don't have the wiring diagram handy. One of the wires goes to the PBU. The other I think goes to ground. The PBU measures voltage, which varies with the resistance of the evap temp sensor, which varies with the temperature of the evap. Higher temp equals higher resistance. PBU decides whether to switch the compressor on & off based on this input.

Yours must be an open circuit - something isn't plugged in correctly, or the sensor was damaged. Infinite resistance corresponds with a very warm evap. You can test it by pulling the PBU using an inexpensive digital volt meter. Just have to know which pin on which of the two plugs it is. I don't know, and my shop manual isn't handy. Try searching the archives, maybe you'll find it.

- JimY

[estod]

My 240 does that -- and it has been the condensate drain clogged so far -- the water needs some time to build up, then sprays out. I'd check that out first -- pretty simple -- before I progressed to more complicated possibilities ...

[Cateaux]

Originally Posted by sublettm

it is really annoying having to cycle the air on and off every half hour to let the ice thaw.

Mike

When I was a teenager I had a '68 Barracuda with an ancient "Frigiking" underdash unit. I had a toggle switch to control the compressor, so if I left it on the evaporator would ice up. I found that cycling the compressor off before the evaporator became encased in ice made things much easier. About every ten minutes I would shut the compressor off for a minute and all would be fine. If I forgot to shut off the compressor for fifteen or twenty minutes, the unit would start blowing water vapor. By this point there would be a considerable amount of ice on the evaporator and it would take much longer for it to thaw. The moral of the story? Try switching to economy briefly before the evaporator freezes, and you should be able to avoid shutting off the air entirely. Not an actual fix, but it should make things more bearable in the meantime.