ACTION NAPA AUTO Parts - 12
By Pete Meier
A
ll cars and light trucks today rely on a computer to control the A/C compressor. And
the computer in charge is usually the Powertrain Control Module (PCM). That's because the A/C compressor has an impact on
fuel efficiency and emissions, and that is the realm of the
PCM. Other computers can be involved in the decision,
as well as controlling other HVAC functions like blower
speed and mode door positioning. But the PCM has the
final say when it comes to the operation of the compressor.
Control unit logic
Diagnosing computer controlled circuits is not hard if
you consider how it all works. A computer can only function according to its programming. Before it can make any
sort of decision or take any kind of action, it needs information. That information is supplied by input devices,
like a system pressure sensor or an evaporator temperature sensor. The information collected is plugged into the
programming, and the resulting decision is carried out
through an output device, like the compressor clutch coil
relay.
12
ACTION/NAPA * Spring 2015
The control module also needs to know that the decision was carried out successfully, and it determines this
through feedback. Feedback can be a dedicated sensor or
switch located in the actuated component itself, or it can
be provided by an existing input device that the control
module monitors for change. An example of the latter is
the change in A/C system pressure seen by the PCM when
the compressor engages.
Now, the control module has a relatively simple mind.
It can only understand ones and zeros, yes/no, high/low or
on/off. The information it receives has to be in this digital format. If an input signal doesn't reach its full minimum or maximum level, the computer won't understand
the message. Some input signals are varying voltages, like
those from a potentiometer, but the computer doesn't use
them until they pass through an analog/digital converter
(A/D converter) internal to the control module.
Output signals work the same way. The control module
can turn a circuit on by supplying power, or by completing
the path to ground through an internal electronic switch.
Power is supplied by a device called a high side driver,
and a ground is supplied by a low side driver. The circuit
can be on or off, or it can be cycled on/off many times per
ACTION NAPA AUTO Parts
Table of Contents for the Digital Edition of ACTION NAPA AUTO Parts
Engine Cooling Systems: Electric cooling fan operating strategies
The next 609 is here
Computer controls for air conditioning compressors
Getting legal
Mechanical fan clutch diagnosis
Properly mixing coolant
Selling the right things
Ford 6.4L Powerstroke
A big bad leak, but where
Recovering the entire charge
Blower Motor Mystery
Case Study: 1999 Jetta A/C Fuse
System Charge Determination
ACTION NAPA AUTO Parts - Cover1
ACTION NAPA AUTO Parts - Cover2
ACTION NAPA AUTO Parts - 3
ACTION NAPA AUTO Parts - Engine Cooling Systems: Electric cooling fan operating strategies
ACTION NAPA AUTO Parts - 5
ACTION NAPA AUTO Parts - 6
ACTION NAPA AUTO Parts - 7
ACTION NAPA AUTO Parts - The next 609 is here
ACTION NAPA AUTO Parts - 9
ACTION NAPA AUTO Parts - 10
ACTION NAPA AUTO Parts - 11
ACTION NAPA AUTO Parts - Computer controls for air conditioning compressors
ACTION NAPA AUTO Parts - 13
ACTION NAPA AUTO Parts - 14
ACTION NAPA AUTO Parts - 15
ACTION NAPA AUTO Parts - Getting legal
ACTION NAPA AUTO Parts - 17
ACTION NAPA AUTO Parts - Mechanical fan clutch diagnosis
ACTION NAPA AUTO Parts - Properly mixing coolant
ACTION NAPA AUTO Parts - 20
ACTION NAPA AUTO Parts - 21
ACTION NAPA AUTO Parts - Selling the right things
ACTION NAPA AUTO Parts - 23
ACTION NAPA AUTO Parts - 24
ACTION NAPA AUTO Parts - 25
ACTION NAPA AUTO Parts - Ford 6.4L Powerstroke
ACTION NAPA AUTO Parts - 27
ACTION NAPA AUTO Parts - A big bad leak, but where
ACTION NAPA AUTO Parts - 29
ACTION NAPA AUTO Parts - 30
ACTION NAPA AUTO Parts - Recovering the entire charge
ACTION NAPA AUTO Parts - Blower Motor Mystery
ACTION NAPA AUTO Parts - 33
ACTION NAPA AUTO Parts - 34
ACTION NAPA AUTO Parts - Case Study: 1999 Jetta A/C Fuse
ACTION NAPA AUTO Parts - 36
ACTION NAPA AUTO Parts - System Charge Determination
ACTION NAPA AUTO Parts - 38
ACTION NAPA AUTO Parts - Cover3
ACTION NAPA AUTO Parts - Cover4
https://www.nxtbook.com/nxtbooks/macs/action_20170708
https://www.nxtbook.com/nxtbooks/macs/action_20176
https://www.nxtbook.com/nxtbooks/macs/action_201706
https://www.nxtbook.com/nxtbooks/macs/action_201705
https://www.nxtbook.com/nxtbooks/macs/action_201704
https://www.nxtbook.com/nxtbooks/macs/action_201703
https://www.nxtbook.com/nxtbooks/macs/action_20170102
https://www.nxtbook.com/nxtbooks/macs/action_20161112
https://www.nxtbook.com/nxtbooks/macs/action_20160910_v2
https://www.nxtbook.com/nxtbooks/macs/action_20160910
https://www.nxtbook.com/nxtbooks/macs/action_20160708
https://www.nxtbook.com/nxtbooks/macs/action_napa_2016spring
https://www.nxtbook.com/nxtbooks/macs/action_201606
https://www.nxtbook.com/nxtbooks/macs/action_201605
https://www.nxtbook.com/nxtbooks/macs/action_201604
https://www.nxtbook.com/nxtbooks/macs/action_201603
https://www.nxtbook.com/nxtbooks/macs/action_20160102
https://www.nxtbook.com/nxtbooks/macs/action_20151112
https://www.nxtbook.com/nxtbooks/macs/action_20150910
https://www.nxtbook.com/nxtbooks/macs/action_20150708
https://www.nxtbook.com/nxtbooks/macs/action_napa_2015spring
https://www.nxtbook.com/nxtbooks/macs/action_napa_2015spring_v2
https://www.nxtbook.com/nxtbooks/macs/action_201506
https://www.nxtbook.com/nxtbooks/macs/action_201505
https://www.nxtbook.com/nxtbooks/macs/action_201504
https://www.nxtbook.com/nxtbooks/macs/action_201503
https://www.nxtbook.com/nxtbooks/macs/action_20150102
https://www.nxtbook.com/nxtbooks/macs/action_20141112
https://www.nxtbook.com/nxtbooks/macs/action_201409
https://www.nxtbook.com/nxtbooks/macs/action_20140708
https://www.nxtbook.com/nxtbooks/macs/action_201406
https://www.nxtbook.com/nxtbooks/macs/action_201405
https://www.nxtbook.com/nxtbooks/macs/action_201404
https://www.nxtbook.com/nxtbooks/macs/action_201403
https://www.nxtbook.com/nxtbooks/macs/action_20140102
https://www.nxtbook.com/nxtbooks/macs/action_20131112
https://www.nxtbook.com/nxtbooks/macs/action_20130910
https://www.nxtbook.com/nxtbooks/macs/action_201307
https://www.nxtbook.com/nxtbooks/macs/action_201306
https://www.nxtbook.com/nxtbooks/macs/action_201305
https://www.nxtbook.com/nxtbooks/macs/action_201304
https://www.nxtbook.com/nxtbooks/macs/action_201303
https://www.nxtbook.com/nxtbooks/macs/action_20130102
https://www.nxtbook.com/nxtbooks/macs/action_20121112
https://www.nxtbook.com/nxtbooks/macs/action_20120910
https://www.nxtbook.com/nxtbooks/macs/action_201207
https://www.nxtbook.com/nxtbooks/macs/action_201206
https://www.nxtbook.com/nxtbooks/macs/action_201205
https://www.nxtbook.com/nxtbooks/macs/action_20120304
https://www.nxtbook.com/nxtbooks/macs/action_20120102
https://www.nxtbook.com/nxtbooks/macs/action_20111112
https://www.nxtbook.com/nxtbooks/macs/action_20110910
https://www.nxtbookmedia.com