When the A/C is on I don't see any air bubbles exactly but it does appear white inside. Is freon white or is this just millions of tiny air bubbles?

Hi there,
A white foamy look in the sight glass can mean several things - for instance it could mean that water has entered the system and is turning the mineral oil a milky color, a very bad situation indeed. It could also mean that the system has been converted to R-134 and the original oil wasn't completely removed, and the ester oil has mixed with it to make a milky looking lubricant, a not desireable but usually not fatal problem, at least for awhile. A good conversion process requires removing the oil from the compressor and flushing the components to remove any remaining mineral oil, which is not compatible with the ester oil used with R-134, regardless of what the 39.95 box says on the side. Since you didn't describe the refrigerant and history of the A/C, we cannot say what your situation might be.
Richard Wooldridge
'82 300D/4.3L V6

Brother of The Benz, fredsegal
Freon is colorless and odor free.
You may be seeing bubbles that have the appearance white.
The little glass window on your A/C drier reciever unit is to determine whiether your A/C is full of refrigarant or in need of filling.
When the glass shows nothing, the A/C is either full or low and should be serviced.
A simple test is to have the A/C at minimum cold setting, the fan on high(it goes to high speed with minimum temp setting), doors and windows up and closed.
Allow the unit to come to operating condition.
With someone in the front seat, have them to press the economy button while you watch the SIGHT GLASS and see what happens. If the unit is full, you won't see anything except the fluid dropping. Have the helper to press A/C button and watch again; fluid will rise meaning the unit is full or fall away showing bubbles or in your condition very low and should be service post hast to prevent damage to the compressor.
Hope this helps
Happy Trails Beep Beep from The Spiderman in Houston!!!

I don't know if the system was converted to 134 but it would fit the profile of the previous owner to have done the conversion. We have used the A/C alot over the last 4 months and I would say it blows about 40 degrees at the vent, if there was water in the system would it continue to perform in this manner?

BTW I changed the monovalve plunger/diaphram today because the heater was until now, a nonstarter.

If moisture is in the system, the system will be cold at the beginning but the moisture (ice) starts clogging up the expansion valve and the AC stops cooling. Low side pressure will be low or negative. The moisture usually is in the drier but for some reasons (possibly due to saturation and/or temperature changes) the moisture comes out of the drier.

The white stuff you see may be desiccant powder that comes from breaking down of the desiccant inside the drier.

Without seeing and touching the system myself, it is hard to say exactly what the white stuff you see from the sigh glass is. They may be just bubbles.

For R-12 system only, if the system is full, you see no bubbles except during acceleration, deceleration, and clutch kicking in/out.

To my knowledge, the mineral oil inside the system after conversion is just sitting there doing nothing.

David

Brother of The Benz, fredsegal
You've been given all of the information about what has or will happen.
I'm not as knowledgabile as others on the subject, but I've never seen moisture visible in the sight glass. The drier, if it is performing will contain it in the desiccant or the molecular sieve. Beyond that it freezes at the expansion valve orific, and there would be no cooling.
I would suggest you go to a service station and have it check out.
If you are concerned if it has beenh converted to R-134A, look at the service ports for retro fit fittings, one red and the other blue.
Happy Trails Beep Beep from The Spiderman in Houston!!!