AC Part 4: Changing the Expansion valve in w123

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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.


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.

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



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

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!