Adjusting a Variable-Displacement Compressor or Converting It to Function as a Fixed-Displacement Compressor
WARNING: This article is not for beginners.
A common complaint regarding the air-conditioning system on some newer cars is insufficient or no cooling at higher engine speeds—a problem I’ve never experienced with older cars.
During idle, the car’s cabin temperature is just right, and the low-side pressure is 40 psi, and the high-side pressure is 200 psi (as measured by a manifold gage). However when the engine is revved up, the low-side pressure rises to 60 psi, while the high-side pressure drops to 100 psi even if the clutch is still engaged, and cooling is lost. It is as if the clutch has disengaged.
I’ve found out that cars with this issue are using an Internally Controlled Variable Displacement Compressor. We’ve tried doing a number of things to solve the problem. Some worked, some did not. What I’m going to tell you about here are the ones that worked.
You can follow the procedures that I’m going to tell you without understanding why you should be doing each step, and still be able to solve the problem. However, I suggest that you read my other article titled “Variable Displacement Compressor – How It Works”. This will give you an in-depth understanding of how a Variable Displacement Compressor works.
- The techniques presented in this article apply to internally controlled Variable Displacement Compressors. Externally controlled Variable Displacement Compressors may require a different approach, although the same principle may apply.
- These techniques are only applicable to a compressor that still has a good pumping action. They do not work on a weak or defective compressor. There’s nothing we can do for a weak compressor but replace it.
Three Solutions to the Problem
If you have the problem I described above—lack of cooling at high engine speed—I suggest one of the following solutions.
Option 1. Adjust the displacement control valve, if it is adjustable. Displacement control valves in some compressors cannot be adjusted.
Option 2. Convert the variable-displacement compressor to run in fixed-displacement mode, if the displacement control valve is not adjustable. You just have to be ready to accept the slight change in performance that may result, particularly with regards to the smoothness of operation and fuel economy. A variable displacement compressor runs smoother and quieter, and cars equipped with this type of compressor are more fuel-efficient than those equipped with fixed-displacement compressors. To me this is not much of a concern, though. The only difference that our drivers have noticed after the conversion is the slight clicking sound that the magnetic clutch makes every time the ECU switches it on and off.
Warning! Convert the compressor only if it is equipped with a clutch (see below).
Option 3. Replace the compressor with a new one.
How to Locate the Displacement Control Valve
Fig. A shows the location of the displacement control valve (DCV). It is kept in place with a circlip. To remove it, just remove the circlip and pull the valve out. Fig. B shows the displacement control valve taken out.
Option 1. Adjusting the DCV (Displacement Control Valve), If Adjustable
NOTE: On some cars the displacement control valve is readily accessible and can be pulled out without removing the compressor. If yours is different, then you may have to pull out the compressor as well as the DCV.
1. Remove the circlip that locks the control valve in place (Fig. A.)
2. Remove the dirt around the top of the valve, apply a little oil, and pull it out gently. It may be a bit hard to pull the valve out because of the tight fit of the O-rings inside.
3. Remove the filter (Fig. B).
4. Using a hex key, tighten (turn clockwise) the lug screw for a couple of turns (Fig. C). I can’t specify the exact number of turns needed, but an important point to note is the more clockwise turns you make, the closer you get to making the compressor run as a Fixed Displacement Compressor (similar to older types of compressors). You do not want this to happen. All you want is to prevent the premature decrease of the compressor’s pumping action, which causes the loss of cooling inside the car’s cabin.
5. Reinstall the filter.
6. Replace the O-rings with ones that are of the same size as the old ones. O-rings are cheap and repairing a leak due to defective or wrong-sized O-rings can be expensive.
7. Install the Displacement Control Valve and lock it in with the circlip you removed earlier.
Reinstall the compressor, if you have pulled it out, and charge the system following the standard charging practice for variable displacement compressors.
8. Test the air-conditioning system. If ok, the problem is solved. If not—that is, there is still a premature decrease of the compressor’s pumping action that results in insufficient cooling—then re-adjust the control valve for a few more turns and test again.
If the compressor fails to adjust its displacement automatically, and the compressor is being switched on and off by the ECU via the compressor’s clutch, then you might have over-tightened the adjustment lug screw, thus making it run like a fixed displacement compressor. If this is the case, go back to Step 1.
Option 2: Converting the Compressor to Run as a Fixed-Displacement Type
WARNING: Convert the compressor only if it is equipped with a clutch. Some Variable Displacement Compressors have no clutch. Without a clutch, there’s no way a fixed displacement compressor can be turned off to prevent ice formation on the evaporator, except by turning the aircon off manually.
1. Remove the circlip that locks the control valve in place (Fig. A).
2. Remove the dirt around the top of the valve, put a little amount of oil and pull it out gently. It may be a bit hard to pull the valve out because of the tight fit of the O-rings inside.
3. Remove the filter and O-rings (Fig. B).
4. If the DCV is not adjustable, plug the hole at the tip of the control valve. One way of doing this is to fill the hole with solder, in which case you will have to clean that portion thoroughly to make sure the solder will adhere to the valve (Fig. D).
If the DCV is adjustable, just tighten the adjustment lug screw all the way. This is similar to plugging the hole. Take extra care so as not to damage the threads.
5. Install the filter back. This is not necessary though if the hole has been closed.
6. Install new O-rings of the same size as the old ones.
7. Install the DCV and lock it in with the circlip you removed earlier.
8. Reinstall the compressor, if you pulled it out, and charge the system following the standard charging practice.
WARNING!: Do not overcharge the aircon system. Serious damage to the compressor could result.
9. Test the air-conditioning system. Your compressor is now running in Fixed-Displacement mode. Every now and then you should be hearing the clicking sound of the compressor’s clutch as the engine’s ECU de-energizes and energizes the clutch to control the temperature.
If the clutch fails to disengage even if the temperature is very low, there is something wrong with the clutch control. This could be due to a defective or improperly installed thermostat at the evaporator, or an open electrical connection between the clutch’s coil and the relay or between the relay’s coil and the ECU.
Option 3. Replacing the Compressor
I am not going to elaborate here the procedure of replacing the compressor. As noted earlier, this hub is not for beginners. What I am going to talk about, instead, is the economics of replacing the compressor.
There are two ways you can replace the compressor. You can either replace the compressor with a new Variable Displacement Compressor of the same type as the original, or replace it with a Fixed-Displacement Compressor. The table in Fig. E below shows the advantages and the disadvantages of each type.
By following the above procedures, you should be able to solve the problem stated above. However, if you want to have a deeper understanding on the operation of a Variable Displacement Compressor, I suggest you read my other hub titled “Variable Displacement Compressor – How It Works”.
See you there.