AC Part 4: Changing the Expansion valve in w123

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!

AC Part 3: Changing the Receiver/Drier in W123

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!

AC Part2: How to change the AC compressor (York R4 type on M110 engine)

How to change the compressor
A DIY tale

Is your compressor crappy? Dried out for the 15 last years and making squeaky sounds and for sure not giving any refrigerations at all? Well in this second part of the AC system we will embark on the journey to change the broken compressor. A critical part and the one to often fail due to the mechanical nature of operation and lack of maintenance.

This guide will only cover the York type compressor in an M110 engine.


The AC compressor pressurizes the refrigerant from a vapor state into a liquid state and at the same time increasing the temperature of the liquid by drawing mechanical energy from the engine via a drive belt from the crank shaft.

Some compressors are lubricated by the refrigerant itself, but the York R4 compressor has it’s own lubrication by oil inside the crank. Lack of lubricating in the compressor crank in combination of little or old refrigerant is the most common reason for these to fail.


To change the compressor you need to make a lot of preparation even though changing the compressor is not such a big job in itself. This will include emptying the AC system of refrigerant at a professional workshop. You cannot do this at home because the refrigerant can actually deplete the ozone layer (if it’s an old type of refrigerant, this is common in old cars) and they are all extremely potent green house gases that should be disposed of properly. Also a big preparation is getting hold of new spare parts, not only the compressor, but also a new receiver/drier, new gaskets for hoses/connections and compressor oil. You probably need a new pulley for the drive belt as well. You will also need some special tools to finish the job.

Parts and Preparation:

  • Empty refrigerant at a workshop!
  • Get a refurbished or new compressor, can be quite expensive
  • Get new gaskets, they are like 2$ only…
  • Compressor mineral oil compatible
  • New Reciver/drier (must always be changed when changing the compressor)
  • New pulley bearing for drive belt
  • New AC compressor serpentine drive belt


  • Big selection of long and short spanners, also ones with ratcheting mechanism to reduce time
  • Socket tool set
  • 3/8 inches spanner
  • Clutch/bearing remover (to remove the clutch from the compressor shaft)
  • Small funnel
  • Brain: Some ingenuity to loosen the clutch (not easy since the compressor shaft and the clutch will rotate together)


Getting access to the various bolts and nuts around the compressor is not easy. They are awkwardly placed and there is not much space to get leverage for the spanners. Be prepared test your patience! This job is not super difficult, but will save you tons of money compared taking it to a workshops, since they will use a few hours just to remove and assemble the compressor, unless they know the car really well. And lets face it, no one knows the car like you do yourself.


WARNING: Do not proceed unless the AC is emptied of refrigerant!

Now that is out of the way, let’s proceed to the fun stuff. Disconnect your battery to avoid shorts! Locate your compressor, it’s the big thing hanging off the passenger side of your engine. The compressor probably has a serpentine drive belt if the AC has been in use before. This must be loosened first by using the adjusting screw on the AC pulley, which located on the upper part at the front of the engine. If you are not using it after, then just cut it off with a knife to save time, the compressor should have a good belt otherwise it might just slip. There is very little room between the pulley and the fan housing, but you do not need to disassemble anything when replacing the pulley.

Start by checking the pressure in the AC system, it should be empty, but just to be sure use a pin and press on the Schrader valves sticking out on top of the  AC hoses. There might be a slight fizz due to a small pressure building up in the empty system.

R¤ Compressor mounted on engine
Compressor fitted on engine, notice the location of first bolt and wires on the right side of the compressor. (This is a new compressor f.y.i. and the AC hoses should always be disconnected, not shown here!)

Then you can start unbolting  the compressor mounts, there should be 5 bolts holding the compressor in place. On the right side there is a supporting rod holding it on the corner, start by unbolting this. You must also disconnect the wires to the clutch actuator.

AC Compressor R4 Compressor M110 engine
Location of bolt on the front side. There is one like it under the clutch as well (not pictured here)

Two are located on the front left side, one above and one below the compressor. Until now these were the easy bolts, now the hard part starts. There are two more bolts located on the back left side of the compressor, and these are pretty hard to gain access to. Also you will have little room for the spanners, so if you have spanners with a ratcheting mechanism you will save tons of time here.

AC compressor bolt location R4 York on M110 engine
Location of 2 last compressor bolt location. Tricky location.

The bolt that is lowest actually is a long long with a cylinder, that comes off going all the way through the exhaust manifold. Don’t loose this cylinder part, and be prepared to save all shims and remember the bolts’ locations, since they are not all the same, use pictures and notes if necessary.

AC compressor bolt R4 York M110 engine

Notice the long compressor bolt and its’ cylinder.Just before the last bolt gets loosened completely you can detach the compressor and stop if from falling, it’s quite heavy so don’t crush your hand when it might fall unexpectedly.

AC compressor and manifold bold M110 engine
Don’t loose the bolt or any of the shims! They are there for a reason….

Now that the compressor is off the engine, you can begin to remove the bracket that is attached on the right side of the compressor. This is pretty straight forward and requires little explanation, it is just attached by a few bolts. There is also a bracket still on the engine block, it is not necessary to remove this, but you can if you want to clean the area around it etc.

PROTIP: Since the compressor is removed, now you have the perfect opportunity to replace the lower radiator cooling hose, which is nearly impossible to replace when the compressor is mounted.

AC Compressor Bracket R4 York M110 engine
AC Compressor bracket attached to engine block. Not necessary to remove this.
R4 York AC Compressor Bracket
Bracket hold in place by 4 bolts (the bolts are removed in this picture).

Since your new or rebuilt AC compressor does not come with a clutch and the clutch magnetic actuator,  we have to transfer the old components over to the new compressor. Removing the clutch is the most difficult part of this job, since the compressor crank shaft will rotate when you try to loosen the clutch center bolt. You need a way to keep the compressor shaft from rotating while loosening the clutch bold. Easier said than done.

AC compressor Clutch R4 York
This will not work… Trying to loosen the clutch bolt…everything rotates…duh!

I ended up placing the compressor housing in a large wise, then using some large welding pliers (adjustable) to grab onto the inner clutch plate and while holding it still then also unbolting the clutch bolt. It was not the most elegant solution and I recommend you to find a better one, since in my case the outer plate got slightly scuffed by the pliers.

When the center bolt is loosened, the clutch is still pressed on the crank shaft. You need to find your clutch/bearing removal tool, it looks like some spider joint out of a horror movie. Unfortunately it’s a tool that seldom gets used, so don’t put too much money into a tool like this, borrow from a friend if you can. I think I only used it once earlier when I removed the center drive shaft bearing. Place the outer spiders around the clutch wheel and start tightening the center screw. This time you can use a long rod between the spider legs to act as a lever while you tighten the spline, you will hear a loud pop when it comes off. It has probably rusted in place after so many years.

Removing AC Compressor clutch from R4 York
Pulling off the clutch from the compressor drive shaft. Prepare for the big pop when it comes off.

The last part to disassembly is the magnetic clutch actuator. It is held by 4 bolts on a bracket, but be warned, they are Imperial sized! This is the first time I have come along imperial sized bolts on my Mercedes, but the York compressor is not made by Mercedes and this is just how it came from the original manufacturer in the United States. You actually need a 3/8 inches spanner, if you don’t have imperial sized tools, go out to get only this one spanner. I almost rounded out one bolt at first since I was dumb enough to try a metric spanner which didn’t perfectly fit.

R4 York Compressor clutch actuator fittings
3/8 spanners required, notice the almost rounded out bolt since I used metric spanners.

After the clutch assembly is off, you can start by assembling the new compressor.

R4 York Compressor
Everything disassembled from old compressor. The old compressor you can deliver in for a rebuild and get a few dollars actually. Don’t throw it!

WARNING: Do not forget to refill the York Compressor with mineral compressor oil before assembly, otherwise it will break!

Forgetting the above step is pretty serious. So before anything else, let’s go through how we refill and measure the right quantity of oil. Also keep the protective dust caps on the compressor fitted at all time until the very end.

Make your own dipstick from a piece of steel wire, make a marking at 57mm and 41mm from the end.

york compressor dipstick

On each sides of the compressor is one small bolt in the middle, these bolts are the filler holes for the oil. They are quite small so a small funnel needs to be used. You need at least 355ml of fluid (12 oz) which s corresponds to around 57mm on the dipstick and it should never be below 177ml (6 oz), 41mm on the dipstick. Use normal mineral compressor oil,  some even use engine oil, but do not use ATF.

The compressor comes with a small quantity of oil in it already, pour this out before pouring in the new oil, it is not important to get all out. Put the dipstick in over the crank case at 45deg as shown in the image to proper measure the fluid level, the crank key should be facing up, there is a small nob on the shaft. It should not be hard to crank it by hand.


When filled up to the right level, start assembling the compressor. Now in opposite order of the disassembly. Remember to keep the protective intake and outtake protective caps on until you are ready to connect the AC hoses back on.

There is no need to over tighten the clutch center bolt since it will also tighten itself over time when the clutch is actuated.

When the compressor is mounted on the car, remove the protective dust caps and make sure there are fresh plastic gaskets on the inlet and outlet holes on the top of the compressor. Tighten the hoses firmly, but do not over-tighten them, the workshop will make sure the gaskets are not leaking when filling your AC system with refrigerant. Over-tightened gaskets will need to be replaced.

Next step

You cannot change the compressor without also changing to a new receiver/drier, otherwise your new compressor might fail too early and all the job you just have done might be in vain. This is the next part of the AC series, so stay tuned for part 3.

Back ground story

To be honest my AC system has not been operational in the last 8 years or so due to a broken compressor. I just didn’t get to the job of fixing the AC system until now, because you know… (Norway is cold most of the time and you really only need AC like 3 months of the year, but then it’s really nice to have on those few really hot days)

Cheers! Robs out

AC Part1: Air Condition Systems 101

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!