MACS Service Reports - 2016 - MAR4

Steve Schaeber

MACS Staff

Figure 7: It might sound simple, but we've all been burned by this one
before. It doesn't take long, and a quick check of the fuses and relays could
save you quite a bit of diagnostic time. You'd surely think so if you started
out tracing circuits only to find that a bad relay or burned out fuse ended
up being the problem!

Figure 5: The high side manifold block of this 2016 RAV4 is similar to
that which can be found on most other Toyota products. Shown is the
three-wire high side pressure sensor (transducer) and high side service
port.

Steve Schaeber

that I knew something was up; they had no answer except that, "for some reason they couldn't figure it out."
When I stopped by the shop, the car was already in a
bay and connected to their R/R/R machine. The gauges
were reading around 100 psi (it was hot that day), and
it appeared as though all the components were properly
installed and connected. So I followed up by using their
Snap-On scanner to check the A/C PIDs.
With the scanner connected to the OBD-II port, I first
wanted to see whether or not the PCM was even getting
an input signal from the head unit to turn on the air conditioning. It's pretty easy to do, as most of the available
OEM or aftermarket scan tools are capable of displaying
this somewhat basic information. The nice thing about
looking at a scan tool's live data stream is that it not only
shows YES / NO for A/C REQUEST, but also ON/OFF
for A/C RELAY (which is what operates the compressor
clutch). See Figure 8. That way, you know if the PCM is
receiving a request signal from the operator, and also if
the PCM is commanding the clutch on or not. In this case
I was seeing YES for the request, but OFF for the relay
command. For some reason, the PCM was not allowing
the compressor clutch to engage.
Looking further, I was not surprised to see 0 on the
A/C HI-SIDE (PSI) display, since that would explain
why the compressor was not turning on. Now I had a
starting point, because without this primary input there
is no way the A/C will work if the PCM doesn't know
how much pressure is in the system. In fact, when trying to command the compressor clutch relay to ON with
the scan tool, I was not able to make it work because the
PCM would not allow compressor operation as long as
the signal from the high side transducer was missing.
Therefore, this had to be fixed first.
Just above the REQUEST - RELAY line on the display
is where the scanner shows the values for the pressure
transducer. On the left side it says "AC HI-SIDE (V)"
which is the measurement in volts that the PCM is receiving back from the transducer. On the right it says

Figure 6: That little green A/C light doesn't mean the A/C system is actually
operating. Rather, it's an indicator to the driver that the A/C button has been
pressed (turned on), requesting A/C function from the PCM.

point they thought there was just one possibility left to
try: that high side pressure sensor.
You can probably tell where this is going, and that after
they replaced the sensor with a new one, the compressor
still would not engage. At this point they're tired, have
quite a bit of time and money invested in the job, and
are getting more frustrated with each passing day. Not to
mention the customer's not too happy either, considering this job was well on its way into week two and still
not finished yet.
That's where I got involved, as it didn't take long for
the shop's owner to stop by and ask me for help. He explained all the work they had done, and it immediately
sounded to me like there was a control system issue going on that they just hadn't quite nailed down yet. Maybe
the dash controls were not working, a connector in the relay/fuse panel was burned, or some wires got damaged
somehow (Figure 7). But with all the new parts they had
installed, it just didn't seem likely that any of the refrigerant system hard parts could be the culprit at this point.
I asked them if they had used a scan tool to check any
of the A/C PIDs (Parameter Identification), and was assured they had done so a few times. But that didn't quite
make sense either, because they should have been able to
get pressure sensor readings from that high side transducer while also being able to command the compressor
clutch on and off. It was when I asked about their results

March 2016

MACS Service Reports

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