What really does cause induction contamination? - TAT - The ...

Air conditioning and serial data
BUS communication by Jack Stepanian and Sam Nazarian
A
VX Holden Commodore
presented itself to the TaT
research workshop with
inoperative air conditioning along with
a note that all it blew was ‘hot air’.
It was revealed, during history taking,
that a third party had attempted to
diagnose the cause and told the
customer that the system had enough
gas and it should be OK, even though it
wasn’t.
We decided on a text book approach
to diagnosis. This story will reveal how
the dashboard air conditioning switches
alert the multifunction control unit, often
referred to as body control module, and
how it communicates with the power train
control module via a serial data BUS line,
to enable activation of the compressor
clutch. First, the fuses and the high and
low pressure hoses were checked and
we even had a play with the dials on
the dash, with no result (pic 1). Manifold
gauges were connected and both high
and low pressures measured. Yes,
compressor on and off via a relay
based on two parameters a) input from
air conditioning pressure sensor and
b) serial data BUS request message
received from the multifunction control
unit. The multifunction control unit in
turn initiates a request message to
the power train control module based
on two parameters, the activation of
air conditioning master switch and the
heater blower relay.
So where do we start? Perhaps
communication between the two control
units.
Serial data BUS
communication and block
diagram
While there are many ways of checking
whether there is communication between
the two control units, including using an
oscilloscope, we opted to use the scan
tool and check if communication was
present (pic 2).
Multifunction control unit
and circuit diagram
The circuitry of a multifunction control
module is arranged such that the body
control module needs to see two active
inputs.
One is the grounding signal of zero volts
produced by the blower inhibitor relay
which in turn enables the operation of
the blower motor via the blower motor
resistor pack and the other is a 12 volt
supply signal generated by the master
a/c switch.
Appropriate terminals were checked and
both were OK.
Power train control module
and circuit diagram
Since the power train control module is
responsible for turning the compressor
on and off via a relay, the controlling
relay was identified and bridged to check
compressor clutch operation and all were
OK (pic 3).
1.
there was enough gas for the system
to operate but the compressor was not
engaging.
A scan tool was connected to the on
board diagnostic connector and we
looked for any trouble codes in every
control module. While there were a few
unrelated codes, these were promptly
noted and erased and the process
repeated, again with no result. A quick
look at data lists of some of the control
modules proved to be indecisive since
we were not sure of the process. It was
time to profile the chain of command to
facilitate a defined prognosis.
Chain of command
A quick glance at the block diagram
revealed that the power train control
module is responsible for turning the
The Automotive Technician 24
2.
Communication channel was not isolated
(it was normal) and the search continued
elsewhere.
Power train interface module
(PIM)
In the block diagram it was also noted
that the multifunction control unit at
times communicates with the power train
control module via an interface module.
This interface module is only used for V8
engines where PIM translates universal
asynchronies receive transmit (UART)
messages into CLASS II.
By the way, a V6 engine uses UART as
a form of protocol to communicate with
other control units and since this car was
a V6 it was one less item on our agenda
to check.
3.
Closer examination of the circuit diagram
revealed that while the compressor
clutch solenoid is controlled by a relay,
there seemed to be a ‘spy’ or a form
of feedback wire from the compressor
solenoid winding side back to the control
unit (being pulled up to supply voltage).
A quick look at the data list revealed
no such data information available and
a physical measurement of the wire to
and from the power train control module
proved to be OK.
Prognosis
Now we had a set of dashboard
switches that were OK, a serial data
BUS communication that was OK and
the compressor relay circuit which was
OK. So our attention turned to the air
conditioning pressure sensor (pic 4).