ACTION NAPA AUTO CARE - 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 CARE
Table of Contents for the Digital Edition of ACTION NAPA AUTO CARE
ACTION NAPA AUTO CARE - Cover1
ACTION NAPA AUTO CARE - Cover2
ACTION NAPA AUTO CARE - 3
ACTION NAPA AUTO CARE - 4
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ACTION NAPA AUTO CARE - Cover3
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