ACtion Magazine - NAPA - Spring 2016 - (Page 18)

Externally controlled variable displacement compressors (evdc) Dave Hobbs We can cycle off the compressor by looking at EOAT to prevent freeze up while also complimenting the amount of cooling the customer wants (Series Reheat Reduction). But why not vary the displacement electronically to do the same job instead of cycling the clutch? That's what the eVDC's do. They don't only change the compressor's internal pressure to vary the swash plate for displacement / evaporator pressure control to prevent freeze up. Since they are electronically controlled with variable duty cycles, they can control evaporator temperature to compliment the HVAC temperature settings. This prevents the on/off clutch feel drivers sometimes notice / complain of. In fact, the clutch on some models with eVDC systems has been removed entirely. See Figure 1. Diagnostics: FRED controls the displacement! You may recall this tech writer's pet name for electronic modules of any nature as Frustrating Ridiculous Electronic Devices, or F.R.E.D. Well now that FRED controls the very physical internal displacement of the compressor, the typical plus and negative side of this changes. The negative side is the pressures you are used to seeing in the bay just may vary a bit all over the place as some of the conditions above occur to lower or even bottom out compressor displacement. Clutch and clutchless When checking for a no A/C complaint, remember these electronically controlled variable displacement compressors are made in both clutched and non-clutched models. I've only ran across those still using the electromechanical clutch on GM, but there may be more models still using a clutch. On the clutchless models, consult your OEM repair information to determine the construction of the breakaway feature required to prevent accessory drive belt breakage when a compressor locks up. The telltale signs this has occurred will vary by compressor manufacturer, and of course the pulley will still turn when this has occurred. Figure 1: The electronic DCV is controlled by the PCM. 1. Overly High Pressure > 464 psi 2. Non A/C Mode Selected 3. Low Evap Pressure < 29 PSI 4. Faulty Sensor Signal / DTC 5. Green / Econ Mode 6. Blower Not Running At All 7. OAT Very Low < 37°F 8. Engine Temperature Hot > 245°F 9. Pressure Sensor Fault 10. Evaporator Temperature Near Freezing 11. Low or High Vehicle Battery Voltage 12. Software Configuring / Programming Issues 13. Engine Just Started < 4 Seconds Ago 14. Engine Load High 15. RPM > 3000 for > 5 Sec 16. Vehicle Speed Very Low < 10mph 17. Very Low Blower Voltage < 3 Volts 18. Power Consumption of Compressor Too High 19. Temperature of EOAT and HVAC Head Setting Require Less A/C Cooling 20. Anti-Compressor Slugging Strategy 18 ACTION/NAPA * Spring 2016 Regarding scan tool PIDS, GM is one notable OEM using a PID called Compressor Control Valve Duty Cycle. It's an easy to understand PID, basically going hand in hand with compressor displacement. The higher the duty cycle, the higher the displacement command to the compressor. You can run your HVAC temp setting to the coldest and watch duty cycle go to 100%. High side pressures should go up as well as you feel or measure cooler duct temps and watch the low side pressure gauge drop down. Other OEMs such as Toyota use a PID for the current in the pressure control solenoids. Current draw on these compressors' electronic solenoid control valves can range from near zero (little to no displacement) to around 0.65 amps which would be the highest displacement (and lowest temperature set point). See Figures 2 & 3. Dave Hobbs An eVDC can be used to tailor the compressor's output from high to low for a multitude of conditions. Electronically controlled variable displacement compressor diagnostics Figure 2: These scan tool PIDs were from a 2013 Malibu E-Assist equipped with a belt driven eVDC. When the temp control knob was moved to 60°F, the Desired Left Duct Temp PID dropped into the 40's and the solenoid received a 100% duty cycle. The result was over 150 PSI.

Table of Contents for the Digital Edition of ACtion Magazine - NAPA - Spring 2016

ACtion Magazine - NAPA - Spring 2016
Watching yf
A/C Diagnostics and Troubleshooting
Staying visible in times of change
The compressor is not just a component
Restricted air flow
Externally Controlled variable displacement
A new Jeep
Increasing Facebook interactions
Mission possible
R-1234yf and other trends

ACtion Magazine - NAPA - Spring 2016