Skip to main content
Updated date:

How to Replace an Externally Controlled Variable-Displacement A/C Compressor with a Fixed-Displacement Compressor

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

This article shows you a method for replacing an externally-controlled variable-displacement compressor (ECVDC) in a car's air-conditioning system with a fixed-displacement compressor (FDC) while maintaining some 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.

If Your AC is Heavily Used, You May Wish to Replace the ECVDC with an FDC

The original A/C system is often the best because of its smooth operation and fuel economy. In cars equipped with an FDC, as opposed to the original ECVDC, you will be experiencing an annoying clicking sound accompanied by engine surge or engine bucking when the compressor engages or disengages.

But if you have a situation like the one I had, for example a fleet of cars used for taxi service or other commercial purposes, where cars and their air conditioning systems were running most of the time and experiencing A/C compressor breakdowns because of this prolonged operation, you might consider replacing your ECVDC with an FDC. Furthermore, a FDC is only one/third the cost of an ECVDC.

If you decide to go this route, I hope this article will 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 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.

Step 1: Fabricating 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.

Step 2: Installing the New Compressor

Installing the new compressor 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.

Step 3: Assembling the 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

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 1.5vvolts. 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 in a corresponding fluctuation of the voltage output of Pin 1 of U2, which in turn results in 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 de-energizes 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.

Parts Layout and Circuit Board

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 was mounted on the circuit board, but it was 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

replacing-an-externally-controlled-variable-displacement-compressor-with-a-fixed-displacement-compressor

Step 4: Installing the Compressor Interface With a Wiring Modification

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 4A

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 4B

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 4C

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 250-mA fuse. A good point to connect is the IG2 wire of the ignition switch. Connect the negative terminal to ground.

Step 4D

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.

Step 5: Testing and Calibrating 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.

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.

Related Articles