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

AC R4 York Compressor

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

Author: Robs

Passionate about vintage Mercedes Benz where driving, maintaining and learning new things and teaching others on the black arts of car maintenance.

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