When recharging R134a do I need to replace drier?
 

1988 300 SEL
It was converted to R134a before I came along. A/C has been gradually getting warmer over the past two years and I figure it's time for a recharge. Did a lot of reading here and other sites about AC systems but still have a couple of questions.
1. If I am just recharging is that considered an "open system", in which case everyone seems to recommend replacing the drier. Mercedes I believe recommends replacing the drier when the system is open and the drier is more than 5 years old. Conversion on this car was done in '99 so I assume it has not been replaced since then. In sum, if I recharge only, should I replace the drier?

2. If I do replace the drier, is that something I can do without vacuuming out the system? As a corollary question, do you need to vacuum out the system every time you recharge

My plan was to replace the drier, since it is 9 years old, and to then recharge with R134a. I just don't want to blow myself up or kill baby seals or whatever badness happens when this stuff gets released. Thanks.

If you are going to open the system its a best practice to replace the drier. If you are going to just pump it down and add more 134 or just adding 134 I wouldn't bother.

If the dryer is original, you definitely need a new one. The old desiccant is not compatible with 134a. The new models work for either R12 or 134a.

If you replace the dryer, you should definitely pull a deep vacuum before recharging. Remove the old dryer, and try to extract as much oil as possible so you can measure it. Put in the new dryer the larger quantity of what you got out, and what the FSM says for the dryer replacement, then quickly attach the dryer and start the vacuum pump.

You do not need to vacuum the system for a top-off of refrigerant, but you should definitely vacuum it any time the system is open to the atmosphere, no matter how short of a time it is exposed. If you're not flushing, try to keep the time that the system is open to an absolute minimum. The oil absorbs water and turns acidic.

I think the system still has some charge to it. If I start out by disconnecting the drier will that release r134 into the atmosphere? Do I need to take any precautions when disconnecting the drier?
The order then would be, disconnect drier, install new drier, vaccum out system, and then refill with r134.

The system is under pressure. It will freeze your hands by simply disconnecting the drier!! So, remove freon with a recovery machine, then remove/replace drier. Then vacuum system down, then add 134. If 134 is low, it has a leak somewhere. When you add the 134 add leak detection dye to the system and check for a leak in 2 weeks.

+1

Simply solution.

Bring it to my shop, where I charge $49.95 service fee plus the cost of the freon added.

We recover, measure whats in there, check for leaks, test the operation of the system, and ad some dye for finding future leaks.

Most customers get out of there for about $80.

No offense, but let a pro fix it.

Unless you have the right training, tools and equipment you could hurt yourself easily.

Not to mention it's highly illegal to vent R134 or R12.

Leave it alone. I wish R134a wasn't sold over the counter, what is going to happen when somebody who doesn't know what's going on hooks a can of R134 to the high side of the system?

Unless you want a serious case of frostbite, don't touch it. You don't go around disconnecting a/c lines without the proper equipment.

This is why R12 got banned, and R134 is going to be banned soon. People who don't know what they are doing are venting gas into the atmosphere and getting themselves hurt.