I was called to a shop for a complaint on a no AC operation on a 03 Chrysler Town & Country with a 3.8L (Figure 1)
.
This vehicle had a full AC charge and there were no codes stored in the ECM who had full control of the AC clutch
relay operation. The garage technician claimed he was able to command AC clutch operation by forcing the ECM to apply
the AC relay which in turn activated the AC clutch. This he did by using the actuator output test mode. This is a
powerful function of a factory enhanced scan tool that a generic OBD II scan tool would not be able to perform. By
performing this output test you have already proved out the ECM, AC relay, AC clutch and related wiring. The only
thing left was to find out why the ECM was unwilling to activate the AC relay.
When I arrived at the shop I placed my generic scan tool on the vehicle to see what was available using generic scan
information for training purposes only. There were no codes in memory (Figure 2)
.
Then I wanted to look at some basic PIDs that would prevent AC clutch operation such as coolant temp and TPS (Figure 3)
and everything seemed okay. There are a lot of other PIDs that are taken into consideration in the ECM's strategic decision to apply the AC relay and
these PIDs will not always be available on the generic side of the ECM. Your only resolve would be to use a factory
tool or an aftermarket scan tool claiming to provide all factory enhanced data. When using enhanced data you get more
available data PIDs not available in generic mode that will affect AC relay operation. These may include such sensors
as ambient air temp, AC pressure, P/S apply, AC command input or even AC clutch slippage. AC clutch slippage is
monitored by a revolution sensor mounted near the clutch coil plate and measures the speed of the compressor plate to
determine if an AC belt is loose or if the compressor is seized. If the compressor is commanded on and the ECM does
not see an output signal from the revolution sensor in a predetermined set time then the AC clutch will be commanded
off until the next key cycle.
I went to the AC control panel and selected AC operation. You will notice that the AC LED is lit (Figure 4)
.
This is important because it tells us that the panel is responding and has placed an AC command into the system. This vehicle
does not use a dedicated AC command line from the AC control head to the ECM. The AC control head on this vehicle uses a PCI communication network (Figure 5)
that allows it to send messages to other controllers on board. By placing my
Chrysler DRB on the vehicle you could see that the “AC Request Input” was
reaching the ECM proving the AC control head was doing its job but the "Desired
AC Clutch Relay" read off (Figure 6)
.
I next went to the actuator output test and selected AC relay activation and as I commanded the AC relay on (Figure 7)
.
You can see that the "Desired AC Clutch Relay" status now read on and the
"Actual AC Clutch Relay Control" read low indicating that the relay driver was
grounding the relay coil circuit. Okay so I proved out the technician's findings
so now it was time to dig deeper.I scrolled through the DRB to check the ambient temp sensor and it was within range at about 114 degrees Fahrenheit
(Figure 8)
.
These sensors are very common to fail in the field do to the locations
manufactures put them in the front of the vehicle. They are usually found behind front grilles or tucked away near the front bumper. If these sensors fall
out of range falsifying cold ambient temperature it will force the ECM to suspend AC clutch operation. When I continued
to scroll through sensor values looking for clues I saw the AC pressure sensor showing a value of 0 PSI with a voltage
value of 180 mV (Figure 9)
.
This was either a lack of an AC charge, a bad sensor or a wiring problem. The
garage assured me that there was enough Freon by placing gauges on the on the
system. I pulled a diagram from Mitchell-On-Demand to test the AC electrical
system (Figure 10)
and it showed me that this sensor was no different than a MAP sensor being a
3-wire pressure transducer sensor. I went to the sensor and hooked up my Vantage
leads across the ref power and ref ground sources to prove them out first (Figure
11)
.
I had a reading of 4.95 volts which was within spec. I then kept my ground lead on the ref ground and moved my positive lead to the signal line and it read about 120 mV (Figure 12)
.
This reading was too low for a full system charge so this sensor had to be bad.
I still needed to take one more precaution. I needed to make sure this wire was not partially shorted to ground between
the sensor and the ECM. I pulled the connector off the sensor and was surprised to see 4.95 volts on my Vantage screen
(Figure 13)
.
This was bias voltage feedback from the ECM which saved me some time on my
diagnostics. The ECM puts a bias voltage on certain circuits to test the
integrity of the circuit. I now knew that the wire was not open or shorted going
back to the ECM so now it was safe to say that the AC pressure transducer was the culprit. I now went back into the DRB to check for codes after I pulled the connector and there was now a stored code of P1598 (Figure 14-“AC Pressure Sensor Volts too High”)
.
This code indicated a problem of the circuit being high but not low. This is
because the ECM sensed an open circuit and immediately sent out the bias voltage
to check the integrity of the circuit. The internal processor was now reading
the bias voltage on the circuit instead of the sensor value and set the code
because it saw it as a value higher then its threshold. The real clincher about this bias ordeal is that the ECM never set a code on the generic OBD II
side. I even tried clearing the codes with the generic OBD II tool and cycled the key. The code P1598 returned on the enhanced side but still did not show up on the generic OBD II side (Figure 15)
.
I just want you to beware when using a generic OBD II scan tool to perform
enhanced diagnostics and not to fall into a trap of denial. Hope this story will
help you to better understand what is going on out there.