I was called to a shop early in the summer for a complaint of no air conditioning operation on a 2010 Nissan Sentra with a 2.0 Liter engine (Figure 1). The shop had done all the preliminary checks such as A/C pressure tests and scanning the A/C system for trouble codes. There were no blown fuses in the vehicle that would prohibit the operation of the compressor clutch coil and everything electrically visible seemed okay including the wiring at the A/C compressor. The shop needed a second opinion so they decided to give me a call.
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| Figure 1 |
It is not uncommon to get these calls in the summer months because here in New Jersey where it is very seasonal you would not expect these calls most of the year because people are usually not testing the full ability of their climatic control system to blast cold air. Most of these vehicles may not even alert a driver with an indicator light if there is a fault in the system so current problems may not be detected until the warmer weather moves in.
AA/C systems pre OBD II were less complex to diagnose and work on. Most of these systems were not electronically challenging unless you were working on the higher end vehicles with the fancy climatic control systems of the ’80s and ’90s. The basic systems all had a compressor clutch relay that was ground controlled through a series circuit. The command would start from the A/C switch and work its way through high and low pressure switches and also through a compressor cycling switch. If it was climatically controlled within the cabin of the vehicle then the bells and whistles would include an electronic controller with added ambient, interior and evaporator temperature sensors. It still was not hard to tackle for any reasonably competent technician.
The cars of today have gotten so complex that many control modules on board will interact to finalize a decision to apply the A/C clutch once a command has been directed from the vehicle driver. Most systems will use the Engine Control Module to monitor Wide Open Throttle, Coolant Temperature, Power Steering effort and outside bussed data before the ECM will activate the A/C clutch relay. Bussed messages may come from a Body or A/C control module that will monitor High and Low A/C Pressure, Sun Load, Ambient and Interior temperatures as well as Evaporator Core temperature.
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On to the Sentra
This particular vehicle used the Integrated Power Distribution Module to control the A/C Clutch relay directly and relied on bussed data from the Body and Engine Control Modules. There are many manufacturers now that are using a computerized fuse/relay panel to reduce the amount of wires needed to run from each controller to a common fuse/relay block. It would be much easier for each module to buss information into the computerized power panel to perform driver circuit tasks that are normally performed by each individual module. The only challenges are the expanded criteria list that each individual Control Module must meet before decisions are made by the IPDM to perform a requested task. It is very important to understand system strategy for all onboard control modules.
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| Figure 2 |
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| Figure 3 |
I scanned the network just as the shop had already done and did not see any trouble codes that would prevent the operation of the A/C clutch (Figure 2) so I decided to access controller PIDs to make sense of it all. My first place to start was at the A/C panel because this is where the command begins. I had to make sure the panel was initiating a command when the A/C button was selected. By viewing the Body Control Module data PIDs (Figure 3) you can see that the "AIR COND SW" signal was sent from the A/C switch and the switch was actively working. The A/C panel in this vehicle had a dedicated line that directly went to the Body Control Module to start the A/C command journey. The next guy on the journey was the Integrated Power Distribution Module. He would be the one to have the final say of applying the A/C clutch relay by grounding the relay coil.
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| Figure 4 |
I selected the IPDM data (Figure 4) and viewed the data with the vehicle running. It was interesting to see that the IPDM parameter for the "A/C COMP REQ" was off. The command was sent by the BCM but there was something in the network preventing the IPDM from making the final decision to apply the A/C compressor relay. It would have been nice if the manufacturer would have allowed you to activate the A/C Clutch relay through bidirectional control with the Nissan Consult 3+ scan tool but this was not an option on this model.
There was another player involved in this strategy game and that was the Engine Control Module. There have been many vehicles I have diagnosed in the past that had residual codes left in the network from prior problems that placed the vehicle in a failsafe mode. This in turn would keep certain component drivers from functioning until codes were cleared. So by simply clearing residual codes from a prior problem it would enable the A/C compressor to function again. This was not the case here so now I had to closely look at all ECM PIDs that could affect normal operation of the A/C system.
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| Figure 5 |
I selected some critical operating PIDs in the Engine Control Module such as the Coolant Temperature, Throttle Position, Power Steering Pressure Switch and A/C Pressure (Figure 5). I was viewing each one to see if they all met the criteria for A/C clutch operation. If the engine temperature were too hot, the throttle was at Wide Open position, power steering was being applied or the A/C pressure was too high or too low the ECM could send a message to the IPDM to not allow the A/C clutch to apply. The data all seemed to be normal except for the A/C pressure sensor reading that showed about 4.6 Volts. This seemed a little high to me but I was not certain where it was supposed to be or what pressure value it represented.
Information please
By looking up information in the resources I had at my disposal I found that 0.1 Volts represented 10 psi. So doing the math, the reading on the A/C pressure switch with the compressor off was about 460 psi. There was no way there was that much pressure in the system and it made sense why the ECM would not okay IPDM A/C clutch operation. There were no codes in the ECM because the sensor value was not within the threshold of a short or open circuit. The engineer who designed the system did not write rationalization into sensor valve input. If the reading was abnormally high or low some manufacturers could set codes for this type of problem so you could at least have a sense of direction what to address with an inoperable A/C Clutch.
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| Figure 6 |
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| Figure 7 |
The most probable cause would be a bad A/C pressure transducer but I had to rule out the wiring going to the sensor because a weak ground could elevate readings on most sensors that require a reference ground. I had the shop remove the front grille to gain access and I noticed that someone had repaired wiring at the A/C pressure sensor (Figure 6). I turned the ignition switch on and unplugged the connector and as soon as I did this my A/C pressure input on the scan tool went to 0 Volts (Figure 7). This told me that the sensor input line back to the ECM was not an open circuit or partially shorted to voltage. It was a quick test to rule out one of the wires so now I had two more circuits to check.
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| Figure 8 |
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| Figure 9 |
Looking at a wiring diagram to match wire color, pin #'s and seeing where the wires originate from doesn't always guarantee pin configuration. The harness connector was previously repaired and that changed the dynamics of my diagnostics so I had to validate the wiring repair. The diagram (Figure 8) shows the pin #2 to be Blue and at the middle of the connector and that was correct. The diagram also shows Pin #1 to be Red/Green for the reference Ground and this wire checked okay with my logic probe at 0 Volts (Figure 9). Pin #3 was shown to be Green/White for the 5 Volt reference and this also checked okay with my logic probe (Figure 10). The problem here is that the diagram is not a direct visual of pin orientation but only for a pin assignment.
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| Figure 10 |
I went back to my information system to acquire a connector view for proper pin orientation (Figure 11). Looking at the connector from a back view with the lock on top the Pin #1 was to the right and Pin #3 was to the left. The 5V line and the Ref GRD were mistakenly crossed by whoever repaired the connector. I simply removed both outer wires from the connector and placed them in their proper cavities and plugged the connector back in. I now had a reading of 1.5 Volts that equated to about 150 PSI. I cleared any codes in the vehicle that might have been triggered during my diagnostics and started the vehicle. I pressed the A/C switch on the panel and I heard the A/C clutch kick in and the A/C was now blowing cold once again.
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| Figure 11 |
This diagnostic adventure was a true "A/C Curve Ball” that really tested a technician's ability to fully understand system operation and the strategies involved. This was not your normal run of the mill component or wiring integrity failure. There was no direction to follow because there were no trouble codes but only a customer's complaint. When someone changes the dynamics of a vehicle with wrong parts or poor workmanship it becomes a different ballgame on a different playing field. I could easily see a shop throwing many parts at this job such as an A/C pressure switch, A/C Control Head, BCM, ECM and even an IPDM but the clues were hidden and masked by only the unknown values within the data stream. Engineers design these vehicles to work within certain parameters and it is up to a good technician to figure out these diagnostic puzzles based on his acquired knowledge he or she accumulated and his or her will to keep learning.