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Replacing an Externally Controlled Variable Displacement Compressor with a Fixed Displacement Compressor

Victor is very passionate about anything technical, from computers to automotive technology.

The purpose of this article is to present a method of replacing an externally controlled variable displacement compressor (ECVDC) with a fixed displacement compressor (FDC) and yet maintain the good features of the original EDVDC system. These good features include:

  1. Turning off the compressor when the engine is overloaded to lighten the engine load.
  2. Turning off the compressor when the engine’s temperature reaches a critical point above which the engine starts to overheat. Turning off the compressor allows the condenser to cool off, thus removing a certain amount of heat from the cooling system.

Before you move on to the rest of this article, I suggest that you read my other article for background information on ECVDC.

A major advantage of the ECVDC over the FDC is the smoothness of operation and fuel economy. In cars equipped with an FDC, you will be experiencing the annoying clicking sound accompanied by engine surge or engine bucking as the compressor engages or disengages. You will not be having this annoying experience in a car equipped with an ECVDC. However, the cost of a ECVDC is much higher compared to that of an FDC.

If you are using your cars for commercial purposes like taxi or any other business in which the car’s aircon is running most of the time you would certainly wish you could replace your ECVDC’s with FDC’s. If so, I hope this article would be of help.

Steps in Replacing an ECVDC With an FDC

Replacing an ECVDC with an FDC involves the following tasks:

  1. Fabrication of adaptor brackets to allow the mounting of the new FDC onto the engine block.
  2. Installation of the new compressor and leak testing and initial charging of the A/C system.
  3. Assembling of a compressor interface. This is an electronic module which converts the pulse-width modulated (PWM) signal intended for a ECVDC into a two-state signal (ON or OFF) suitable for an FDC.
  4. Installation of the compressor interface module and making a little wiring modification.
  5. Testing and calibration of the system.

Fabrication of Adaptor Brackets

Different cars require different adaptor brackets. Fabricating an adaptor bracket is a purely mechanical job and should not be difficult. How to fabricate a bracket will not be covered here.

Installation of the New Compressor

This involves the following tasks:

  1. Modification of the high-pressure and low-pressure hose fittings to match with the corresponding ports of the new compressor.
  2. Mounting the compressor.
  3. Putting in the right amount of oil, vacuuming, and initial charging of the A/C system.

Assembling of Compressor Interface

Reminder: A little knowledge of electronics is required here.

Figure 1 shows the circuit diagram of the compressor interface.

Figure 1: Schematic Diagram of the Compressor Interface

Figure 1: Schematic Diagram of the Compressor Interface

Figure 2 below shows the parts layout and printed circuit board (PCB) pattern you may use. This is the interface module as seen from above. The yellow-colored images are the electronic components on top of the circuit board (colored black). Those in blue are the copper trace at the back of the board, and the purple ones are the solder pads, also at the back of the board.

Figure 2-Parts Layout

Figure 2-Parts Layout

In the original design, the relay is mounted on the circuit board, but it is eventually placed outside of the compressor interface module because of the heat it generates. The power supply is also oversized because of the same purpose of minimizing the heating of the interface module. See Figure 3.

Figure 3-Original Compressor Interface Module (relay taken out)

Figure 3-Original Compressor Interface Module (relay taken out)

Parts List


Installing the Compressor Interface

After the assembly of the compressor interface has been completed, it is now time to make the necessary wiring modification. Here are the steps:

Step 1

Remove the electric control valve (ECV) from the old compressor and mount it on a location that is dry and where the connector of the control cable can still be inserted into the ECV’s connector (see Figure 4).

Figure 4-Possible Location of ECV

Figure 4-Possible Location of ECV

The ECV will then serve as a dummy load to the control unit. This is done to make the car’s engine electronic control unit (EECU) and the A/C electronic control unit (A/C ECU) think that the original compressor (ECVDC) is still in place and thus continue to perform, among others, their important coordinated functions like

  1. Shutting off of the compressor when the engine is overloaded to free up engine capacity.
  2. Shutting off of the compressor when the engine temperature reaches a critical point, above which the engine starts to overheat (shutting off the compressor cools down the condenser, thus removing some heat from the radiator tank).

Step 2

Run a wire from the ECV’s signal wire to the compressor interface’s PWM input terminal. The following are possible ways of determining the ECV’s signal wire:

  1. Refer to your car’s wiring diagram if available.
  2. Oscilloscope method. Connect the ground probe of the oscilloscope to the car’s body (or any point that is connected to car’s ground), then probe on the ECV terminals or wires one at a time. The one with a pulse signal is the signal wire.
  3. Trial and error method. When all other tasks are done and just before the testing of the system, try connecting the wire from the PWM input terminal of the compressor interface to any one of the PWM wires at the ECV side, then test. If the system does not work, try the other wire. The compressor interface has a very high input impedance, so there is nothing to worry about causing damage to the ECU. Just be sure the two wires at the ECV are not shorted.

Step 3

Connect the positive power supply terminal of the compressor interface to a 12-volt supply that is hot only when the ignition is "ON", via a 250mA fuse. A good point to connect is the IG2 wire of the ignition switch. Connect the negative terminal to ground.

Step 4

Install the relay that will drive the new compressor’s clutch coil. Install the flywheel diode across the coil terminals of the relay. Connect the relay terminal, the one that has the negative terminal of the diode connected to it, to the positive terminal of the compressor interface output. The one with the positive diode terminal, to the negative terminal of the interface output. Connect one output terminal to a 12V supply that is hot only when the ignition is on. Connect the other terminal to the clutch coil of the new compressor.

Testing and Calibration of the System

  1. Review all the mechanical and electrical installations related to this project.
  2. Calibrate the compressor interface by doing the following:
    1. Run the engine and turn the A/C on.
    2. Measure the voltage at Pin 5 and Pin 6 of U2 (refer to Figure 1).
    3. Adjust trimpot R5 so that the voltage at Pin 6 is about 75% of the voltage at Pin 5.
    4. Complete the charging of the A/C system with refrigerant.
    5. Test the functionality of the computer interface by checking if the compressor shuts off when the engine is overloaded. There are different ways of overloading your engine, but this one, I think, is the easiest. Just push the accelerator abruptly. You should hear a clicking sound, indicating that the compressor clutch has disengaged. After a short delay, the compressor should turn back on. If this is so, the job is done. If not, decrease the reference voltage a little bit by adjusting trimpot R5 (as in item 3 above) then test again.

Circuit Description

Referring to Figure 1, there are circuit components that are enclosed in dotted frames. These are the sub-circuits that will be described in the lines that follow.

  1. Frame 1: This power supply is an overkill. We could have used a lower power voltage regulator, but this is intentionally done to minimize the heat from the regulator.
  2. Frame 2: This is the isolation circuit. The optocoupler PC817 isolates the interface circuit from the noisy PWM signal from the A/C electronic control Unit (the one we obtained from the electric control valve). The transistor 2N3904, being in an emitter-follower configuration, acts as a high impedance buffer.
  3. Frame 3: This is the low-pass filter that converts the PWM signal to analog.
  4. Frame 4: The first half U2 (LM358) serves as buffer amplifier for the low pass filter.
  5. Frame 5: The second half of U2 serves as a comparator which turns on the transistor 2N2222 when the voltage at Pin 5 of U2 is above the threshold voltage set at the trimpot R5.The combination of R7 and R8 is the hysteresis circuit which has a window of 1.5volts. When the A/C switch at the A/C console is turned off, the duty cycle of the PWM signal decreases gradually for a couple of seconds until its gone. However, the rate of decrease is not constant, but in my case fluctuates during this switching off period. This results to a corresponding fluctuation of the voltage output of Pin 1 of U2, which in turn results to the switching on and off of the transistor 2N2222 as the voltage at Pin 5 crosses the reference voltage at trimpot R5. This also causes the relay to switch on and off, which in turn energizes and deenergizes the compressor clutch. This is evidenced by the “click-clack” sound of the compressor. In my case, I observed that the maximum voltage fluctuation at Pin 5 during the switching off period is 0.8 volt. Setting the hysteresis window to 1.5 volts completely solved the problem.
  6. Frame 6: Trimpot R5 allows us to adjust the reference voltage at Pin 6 of the LM358. To make sure that the compressor is turned on only when the electronic control unit (the EECU or A/C ECU as the case maybe, depending on the car model) intends it to be "ON", I set the reference voltage from 75% down to to 65% of the voltage at Pin 1 when the A/C is on..

This compressor interface is very stable and reliable. I have been using this in my units for six years now without any problem at all. However, I still believe that the original A/C system is still the best because of its smooth operation and fuel economy. But if you have the same case as mine, where the cars are running most of the time and are experiencing A/C compressor break downs because of this prolonged operation, and where the cost of an ECVDC is three times more than that of an FDC, you might consider replacing your ECVDC with an FDC. Should it be so, I hope this article would be of help.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

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