AC Part 4: Changing the Expansion valve in w123

Reading Time: 4 minutes

Changing the AC expansion valve

An old and defect expansion valve will decrease performance in your AC system. That can result in either the commonly too warm air, or in fact too cold air causing frost on the vents! The air can also alternate between these two modes. The more expensive consequence is that it can also wreak havoc on the AC compressor since coolant will always flow (unless the compressor has a shut-off clutch).

In the final part of this “AC 101” series we will look at changing the expansion valve. If you need to read upon how the expansion valve works in an AC system look at Air Condition Systems 101.

The Ac expansion valve is quite accessible under the passenger side dashboard, but you will need some large spanners to loosen the AC hoses.

Preparation 

Before you do anything, make sure the AC system is completely emptied of refrigerant at a workshop. I repeat what I have written earlier in this series; The AC refrigerant is a highly volatile greenhouse gas. And if you have an old R12 system, the system might contain old freon gas which will deplete the ozone layer as well as being a volatile greenhouse gas.

So do that and then go shopping.

Parts:
  • New Expansion valve
  • New AC gaskets – You should change these to avoid leaks.
Tools:
  • Philips Screwdriver
  • Little flat screwdriver
  • Large open ended spanners

Procedure

Removal

Start by removing the passenger side lower dashboard cover. It is held in place with three screws at the top under some brittle plastic covers and a plastic screw on the lower right hand side. Remember when dealing with old interiors you have to take extreme care to not breaking any parts or loosing them. Replacement parts are long out of production by now and are not available except used ones (if you can find them) at often very high prices.

Passenger side dashboard ccover MB W123

Use a narrow and thin flat head screwdriver to carefully pry off the three small plastic covers.

Removing lower passenger dashboard cover MB w123

Then  unscrew the three philips screws on top and the lower right plastic screw. The left side and back side of the dash cover is held up by tabs in the floor and wall construction.

Removing lower dashboard cover w123
Weird plastic screw location

With the cover off. You can immediately locate the AC expansion valve. You should detach one end of the ventilation hose so you can access it. Detaching it from the left side is much easier and you don’t end up breaking stuff.

AC expansion valve w123
The Expansion Valve was covered in some very old foam that is disintegrating.

The expansion valve is covered by some foam that is long past its lifespan. Touch it and it disintegrates. So be prepared to get some styrofoam replacement (which you have to fabricate yourself). The only practical function of this foam is to protect the other plastic from the very cold valve.

Start by unscrewing the hoses, they might site quite hard and getting to them is a bit of a grind. Start by removing those who you find easiest and remove the other ones as you go.

AC expansion valve w123

Note the orientation of the old valve before removing.

AC expansion valve removed w123
AC expansion valve is removed

After the expansion valve is removed you should change the old gaskets with new ones before you install the new expansion valve.

The new expansion valve also says R134 on the side so it can in fact work fine with newer AC systems. The old one was for the obsolete R12 system.

AC Expansion valves Mercedes w123
New vs old AC Expansion valve
Installation

Remember to change the  gaskets and then start to screw in the hoses making really sure not to cross thread them. Take your time here. The hoses should go on quite tight to make a tight seal under the pressurized gas.

AC Expansion valve R134 w123
New expansion valve installed. It says R134 on it so it will work better with the R134 systems since R12 is obsolete. Notice the styrofoam placed on the backside.

Then install the dashboard cover again and your job is done, for now at least.

I highly recommend to perform pressure testing of the AC system before filling it with new gas, since many workshops do not even do this before they fill. You basically have to demand it before they fill. An AC leak is quite nasty since you will get a dirty cleanup job, not mentioning the greenhouse gas effects…

Cheers, Robs out!

AC Part1: Air Condition Systems 101

Reading Time: 4 minutes

Why you should get in love with thermodynamics

Here comes the 4 part series you all have been waiting for. The ultimate on how to fix the air condition system in your already too hot car. Bored of driving sweating and always going around with the windows open? Now it’s the time, you have been letting this job off for far too long. Brace yourselves, global warming is coming!

I will cover a standard fix of AC system in four parts. Before even beginning you need to know the basics and the theory behind the technology, this will be covered here in part 1. If you are an expert on AC systems then just head over to part 2.

Part 2 will cover how to change the AC compressor. Part 3 covers the changing of the receiver/drier and the last and fourth part covers how you change the expansion valve. These three components are usually the ones you need to care about if your AC system is not working

Automotive AC Theory 

Air conditions are based on the thermodynamic heat pump cycle and if you are really into thermodynamics you can dig into this topic really geeky and deep, but this is not the purpose here. The automotive AC works just pretty much like any refrigerator, this technology is old and proven, no rocket science really. A heat pump moves heat from one source at a lower temperature to another location with a higher temperature with using mechanical work.

Either used for heating or refrigerating the operational principle is the same, but here we are interested in the refrigerating part.

Mechanical work is needed to shift the heat to flow from a colder location to a warmer one since this is opposite to the natural heat cycle in the universe. The mechanical work in a car is made by the car engine.

The vapor-compression cycle is most common for automotive uses. Here it uses a recirculating liquid called a refrigerant that undergoes phase changes to absorb and remove the heat. Many of you have heard about Freon, but this is just a brand name for some types of refrigerant that was common in the olden days, now it is prohibited due to the ozone depletion of such CFC gases. Newer refrigerants do not deplete the ozone layer, but have high global warming potential, so it’s important that you don’t leak these refrigerants out into the atmosphere.

Vapor-compression cycles all have 4 key components: a compressor, a condenser, a thermal expansion valve and an evaporator. For most uses you will also find a fifth component which is the receiver/drier, this is always found in automotive AC.

Schema over AC system
Vapor-Compression system
The compressor

Since the system is closed and recirculating it doesn’t really have a start, but lets start with the refrigerant entering the compressor. The compressor is the part that circulates the refrigerant and is driven by the engine through a tension belt. The refrigerant enters in vapor form at low pressure, but gets compressed by the compressor and it will form a high pressure vapor with higher temperature, known as superheated vapor. It then enters the condenser.

The Condenser

The condenser is a radiator where the superheated vapor runs through tubes where a fan or water cools down the vapor so it condenses into a liquid. Here the refrigerant removes away the heat to the flowing air or water. This is the part where the heat pump can be used as an oven, but in the car refrigeration is wanted and the condenser is just radiating heat into the atmosphere. The colder high pressure liquid is then routed to the receiver/drier as a saturated liquid.

The Receiver/drier

It is located usually before the expansion valve in the high pressure part of the system. They serve three important functions:

  • Temporary storage container for refrigerant when system is not in use.
  • Filtering debris inside system.
  • Removes moisture that can have gotten into the system which can create corrosion and destroy the compressor.

The liquid then goes to the expansion valve where the liquid can drop a lot in temperature.

The Expansion valve (metering device)

The saturated liquid goes through the expansion valve where the pressure suddenly drops, the sudden drop in pressure lowers the temperature of the now liquid and vapor mixture to a much colder temperature than the temperature of the space which need to be refrigerated, like the inside of a car. The cold mixture of refrigerant then is routed to the evaporator.

The Evaporator

Inside the evaporator, which is also like a radiator where tubes crisscross over metal fins to expand the possible area of which heat can be absorbed. A fan blower pushes warm air inside the car through the evaporator where the heat gets absorbed by the cold refrigerant and cold air comes out on the other side. This you will feel as the cold air conditioned air coming out of the vents. The liquid inside the evaporator tubes gets, you guessed it; evaporated into a vapor form due to the heating and carrying this heat back to the compressor completing the cycle.

I hope this was informative and covered the basics, so now you have the understanding of your car’s not so complicated AC system.

Cheers, Robs out