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Tackle an A/C curveball with a solid diagnostic process

Friday, December 1, 2017 - 08:00
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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.

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.

Figure 2
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.

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.

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