MACS Service Reports - 2016 - MAR5

Steve Schaeber

Figure 8: Snap-On's SolusPro scan tool displays four of the A/C parameter identifications (PIDs) right next to each other under the "EGR,
EVAP, ACC" section of the "Data Menu". Our initial test showed 0 PSI,
although the manifold gauge read differently, indicating a problem with
the sensor circuit.

"AC HI-SIDE (PSI)." This is where the PCM converts that
previous voltage reading into something we're more familiar with and that we can fully understand with just a
quick glance. If you don't have a scan tool handy at the
time, a DMM (digital multimeter) could work just as well
by back probing the three wire connector. As a general
rule of thumb, 1 volt is equal to about 100 PSI. Likewise,
2 volts would equal approximately 200 PSI, and so on.
Since these sensors top out around 5 volts, they'll read
close to 500 PSI at their upper limit.
Another important PID you should check is the Coolant Fan Relay, which may be hidden on another screen.
You can monitor the relay or with bidirectional control,
command the fans ON and OFF to double check their operation. See Figure 9.
To rule out the pressure sensor, and since they already
had a new one in house, I connected both sensors just to

Figure 10: Looking down into the business end of the sensor, only a tiny
port can be seen which allows refrigerant pressure to enter the sensor
body. In the center is a small pin, used to depress the Schrader valve in
the coupling. This design simplifies replacement, eliminating the need to
recover refrigerant when changing out a transducer.

Steve Schaeber

a wiring problem that needs to be diagnosed, which is
what we did next.
The best way to pinpoint wiring problems is to get an
accurate diagram that shows you how the system was
laid out to begin with. Wiring diagrams for most vehicles
can be found on either their OEM service information
websites, or can be obtained through several aftermarket information providers. In this case we used Alldata to
find the A/C Switches and Sensors diagram, printed out
a copy and took it out to the vehicle in the bay.
The diagram shows that the A/C Refrigerant Pressure
Sensor uses three wires which connect it back to the PCM
(Figure 11). In fact, these wires go directly back to the
PCM and don't go through any other connectors or splices. We first visually verified that Pin A on the sensor connects to Pin 51 of Connector C1 at the PCM with a black
wire, Pin B connects to Pin 23 of C1 with a grey wire, and
Pin C connects to Pin 37 of C1 with a red/black wire (this
car was built with the LN2 engine). See Figure 12. Now it
was time to verify continuity through those wires.
Before using our Fluke DMM to probe the wires, we
made sure the connectors were removed from both the
high pressure sensor and the PCM (Figure 13). This way
we would be testing only the individual wires themselves
without interference from any other parts of the circuit
or the PCM. Besides, it's never a good idea to test continuity of a wire while the PCM is still part of the circuit.
Things can and do go wrong, and damaging a PCM can
be a costly mistake, so it's best not to take any chances.
As it turns out, we found only one of the three wires

Figure 9: Don't forget to check this other very important A/C PID:
"CLNT FAN/RELAY" which shows the PCM controlled status of the
condenser cooling fan as either ON/OFF.

see if there was any difference, but you guessed it; a big
fat 0 PSI showed up on the display. That doesn't mean
that both sensors are necessarily good or bad, but rather
that the PCM is likely not receiving any signal back from
the sensor itself (Figure 10). It's more likely that there is

March 2016

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2016