The AC receiver/drier should always be replaced when servicing the AC system.
Luckily the receiver/drier is the most easily accessible and easiest component to change when servicing the AC system. It is located behind the front headlight in the engine compartment of the w123. If you were following my Part 2 of the AC system series, you might already have read that you always need to replace the receiver/drier when changing the compressor. The failure of the drier might have been the cause of failure of the compressor in the first place.
You can read about the drier/receiver function in AC Part1: Air Condition Systems 101
For preparation you need not to do much else than emptying the AC refrigerant at a professional workshop, get a new receiver/drier and have some basic tools ready.
- New Receiver/Drier (it comes with the necessary gaskets)
- New high pressure AC hoses if they are cracked/worn/old.
- Socket tools
If you haven’t emptied the AC refrigerant, take the car to a workshop and get them to empty and properly dispose your old refrigerant. AC refrigerants contains highly volatile greenhouse gases and needs to be disposed properly!
When back at your own garage. Start with opening the hood and disconnect the battery since you will be disconnecting some wires. Locate the receiver/drier behind the right headlight. It is the small canister with two hoses connected to it. Start by disconnecting the two high pressure AC hoses. Again this will mark the importance of emptying the refrigerant before, since we are dealing with high pressure systems and volatile gases. You might not be able to access both of them while the drier is connected to the chassis, then loosen the drier until you can access both hoses.
As a tip the holes to the bolts going through the thin metal in the chassis might have started to rust. To avoid holes in the future you might want to get rid of the rust and paint over the area.
Disconnect the electrical wires connected to the receiver/drier, but remember how they were connected by marking the wires or taking pictures before. One is a pressure sensor for the compressor and another is a temperature sensor which controls the auxiliary fan.
The sensors do not come with the new receiver/drier, so these sensors have to be taken off the old one and put back on the new one before proceeding further. Be careful not to break off the brittle electrical connector pins, since they can be quite fragile. If you break it you need to find new replacements components since the AC system will not work without them. Also the pressure sensor will need a tight seal with the receiver/drier otherwise refrigerant will leak out. The pressure sensor has a small O-ring, replace the old one with a new O-ring that comes with the new receiver/drier as seen in the picture under.
The temperature sensor sits at the top of the canister and can be removed with a 19mm spanner. It does not require any gasket. The pressure sensor can be removed with a 27mm spanner. Remember to replace the gasket before assembling it with the new receiver/drier.
After the the sensors are transferred you can install the new canister. Start by replacing the gaskets for the two hoses and tighten them onto the canister.
Then finally secure the canister to the chassis with the two bolts to the chassis. If you have other AC components to change this is a good time to do it. For example you can replace the expansion valve which also is a common wear item on the AC system. Although the receiver/drier are more common to change.
At a workshop they will pressure test and fill up the the AC system and find any leaks if they are present.
Cheers! Robs out.