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.


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!

How to do a fuel filter change on w123 – Petrol engine

Reading Time: 4 minutes

How to change the Fuel filter

So how do you change the fuel filter? It is in theory simple, but in practice it can be quite messy and also tricky to fully access in the rear end fuel delivery system on the w123 chassis. You also need to make sure you have as low fuel as possible or drain the fuel tank dry.

There is a high explosion and fire risk when dealing with petrol fumes, so don’t light up your cigarette and avoid sparks!


Make sure you inspect the fuel delivery system, since a lot of parts there might need to be changed along with the fuel filter, and since you are already draining fuel you don’t want to do this again in 2 months. See the older article on How to re-haul the rear fuel delivery system on w123 (gasoline)

Part requirements:

  • New Fuel filter
  • Optional parts:
    • Fuel delivery assembly rubber bushings – I highly recommend to change these
    • Fuel pump
    • Fuel accumulator
    • Fuel hoses
      • Main tank hose
      • High pressure hose
      • Low pressure hoses
    • Fuel hard line and fittings


  • Spanners
  • Sockets
  • Hose clamp


You need to access the rear end fuel delivery system on the rear right end of the car. It consists of the fuel pump, fuel filter and the fuel accumulator. They are suspended from 4 rubber bushings secured in a weird metal clamp. It is partly hidden behind a plastic cover near the right rear wheel.


Start by removing the battery, trust me, you dont’ want sparks while working under the car with petrol fumes! And don’t smoke!

Then jack up the rear end of the car up on jack stands and block the front wheels from rolling. As extra safety measure I always leave the jack in raised position on the same side as I will be working.

Remove the rear right wheel and you have better access to the fuel delivery system. Locate the plastic cover which is held by a few screws, use a long thin socket extension to remove these.

Fuel delivery assembly w123

Now you have to drain the fuel from the tank to avoid petrol spills and excessive fumes. This can either be done with using a vacuum pump with a  hose down into the filler neck, or by disconnecting the main hose to the fuel pump. It will also leak fuel from the delivery line to the engine, and this can be blocked from continuously dripping with a hose clamp on the high pressure hose. Make sure the clamp is of high quality with no sharp edges, since a new high pressure hose is quite expensive.

Fuel filter removed
Fuel filter removed. Notice the high pressure hose is clamped.

When the fuel is drained. You can start by loosening the 2 lines that connects to the fuel filter. The left side (from rear) is the high pressure line and the right is the hard line. Notice the position of the two copper washers on the hard line fitting.  A lot more fuel will now drain out from the filter so take care. It is possible to remove the filter from the cage without removing the whole assembly.

Rubber bishings fuel pump w123
Installing new rubber bushings for fuel pump assembly.

If you need to also change the rubber bushings, then change them one by one and you don’t have to take the whole assembly down. Alternatively to get better access you can secure the assembly with zip ties while changing them.

Fuel delivery assembly w123
New bushings installed


Installation is pretty much reverse of removal, but you have to remember to use the new copper washers that followed with the new filter. Also I found that you need to torque down the screw between the hard line and the filter for it to not leak.

After everything is put together and you have put some fuel in the tank, try to start the car, it should not take long for the engine to fire up after you crank it. The fuel pump is pretty quick pushing fuel through. When the car is running, go back and inspect carefully if fuel is leaking and check for any wetness.

New fuel filter installed on w123
New fuel filter installed.

If it’s dripping or you can see wetness, shut off the car. Tighten the bolts, wipe off the fuel so it’s completely dry and try again. I had to redo this step twice before it was properly sealed, and the leak was indeed between the new copper washers and the hard line. The copper washers will actually start to seal better when exposed to moisture and some small corrosion will start forming in the gaps.

Be sure to check for any wetness under the rear end of your car after you parked it the first times, since leaking fuel is bad for your wallet and is a potential fire hazard.

Cheers, Robs out! 

AC Part 3: Changing the Receiver/Drier in W123

Reading Time: 4 minutes

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
  • Spanners


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.

AC drier w123
Old AC Drier

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.

AC drier
The Pressure sensor inlet has a protective cap which has to be removed.

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.

AC drier pressure sensor gasket
The pressure sensor gasket

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.

AC drier Temperature sensor
Temperature sensor placement

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.

Replace the high pressure hose gaskets

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.

AC drier w123
New AC drier installed

Cheers! Robs out. 

Happy New Year and may 2018 be filled with classic cars

Reading Time: 1 minute

2018 and things to come..

2017 has been a fantastic year for this website, almost 100.000 visitors have found it’s way to this site. That is absolutely amazing and I thank you all for the support by reading. Lets hope 2018 will be even better and with more high quality articles.

I will continue to publish DIY posts and other classic car related posts in this year to come, so stay tuned.

What new can you expect from 2018?
DIY for the W140 S600 as something new and exiting. I hardly find any information for these cars on the internet.
Some long awaited videos will come up around Q2-Q3 where we will take a spin in some Mercedes classics.

Mercedes w123 driving in winter