2014 Chevrolet Suburban 5.3L - A/C not working
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General Information: The customer brought this Chevy Suburban in with a complaint that the A/C would stop working after about two hours of driving. The A/C light on the instrument panel would flash and there would be no cold air coming from the vents. Any other time driving less than two hours, the A/C would work fine.
First Thoughts: I wasn’t quite sure where to begin with this one. I had a feeling that something was maybe overheating or building too much pressure that the HVAC(Heating, Ventilation, Air Conditioning) control unit was shutting it off. The only way to find out is to start looking.
Diagnostics: There were two main codes that were standing out to me in this vehicle.
P0645 - A/C compressor clutch relay control circuit.
B3933 - A/C Evaporator Temperature Sensor Circuit.
These two were the codes that could directly cause the problem the vehicle was having.
On this system, the engine control module(ECM) controls the operation of the compressor. The HVAC unit has a request to turn on the compressor via the A/C button, the ECM commands the compressor clutch relay to activate, then the compressor clutch engages.
I wanted to see if the HVAC unit was even giving a request for the compressor to turn on.
HVAC unit live data
On the right side column above, there is a request signal for the A/C which is great. This means that the HVAC unit can request the compressor to be on.
What is strange is that right above the A/C Request Signal data PID it says, “A/C Evaporator Temperature Sensor Out of Range” for the “A/C Compressor Clutch Inhibit Reason” data PID. The “A/C Compressor Clutch Inhibit Reason” data PID is telling me why the compressor is turned off.
The data PID on the left side says it is -22 degrees at the A/C evaporator core. At this time, the vehicle had only been running around three minutes and had been sitting/ turned off longer than that. -22 degrees at the A/C evaporator was not a correct reading.
Note: The A/C evaporator core is the part of the A/C system that gets really cold and the blower motor blows air through it, which is then blown throughout the vehicle. If the A/C evaporator core is too cold or on the verge of freezing over, the HVAC unit will make sure to request the compressor be turned off so that there is no damage to the system.
The one thing about the picture above though is that, on the left column, the A/C indicator is off. At the time of the picture, the button for the A/C was turned off, yet:
1. The HVAC unit still has a request for the compressor to be on.
2. The HVAC also said the compressor should not be on (from the “A/C Compressor Clutch Inhibit Reason” data PID)
3. The compressor was running constantly the whole time the vehicle was running.
4. It has an incorrect reading of -22 degrees at the A/C evaporator core even when the vehicle is off.
Something is not adding up.
A/C Compressor Clutch Relay with a relay terminal tool
My next step was to find the A/C compressor clutch relay and see if it is operational. There is a code for the control circuit for this relay. I used a relay tool to have better access to the terminals of the relay. What I checked for was proper voltage signals, ground signal, and command(or ground) signal. For this relay, the engine control module (ECM) sends a command (by sending a ground) to turn the relay on. The relay then sends voltage to the A/C compressor clutch.
I noticed, though, that the A/C compressor clutch was engaged all the time. It never went off, even with the button inside was pressed off. That is not good. If a compressor does not turn off, it can cause damage to the A/C system. The only time it would turn off was when I removed the relay. As soon as I would replace the relay back into its spot, it would immediately turn back on.
The things rolling through my head now are:
The A/C compressor staying on all the time must be the main problem of why the system shuts off after hours of running.
Something is telling the A/C compressor clutch relay to stay activated the whole time.
Some things that could possibly cause this problem are:
A bad HVAC control unit.
The command from the ECM being grounded from a bare wire.
A bad ECM that is possibly shorted internally.
Something else.
I used my relay tool and checked the powers and grounds of the relay. They were all good. The only thing that was odd was the command from the ECM was always there. Even with the A/C button off, the ECM command for the A/C compressor clutch relay was still there. It was very strange.
I decided to pick one of the possible problems that I listed above to run with. I wanted to trace the wiring harness to see if maybe a wire had rubbed bare and was grounding the command wire for the A/C compressor clutch relay.
The reason why I went with this route is that, even though it is possible, I did not want to believe the control units were the problem just yet without more evidence. Plus, I didn’t want to take apart the dash for testing the HVAC unit or tracing all the wires for the ECM to test it.
But it was a good thing I started with what I did because after about 1 minute of looking I found something quite unusual.
A little bit of a back story with this vehicle.
It was purchased from a used car lot.
The picture above is a small piece of wire spliced into another wire then alligator clipped to the body of the ECM, which is grounded. I thought to myself, “This can’t be that easy".
While the vehicle was running, I unclipped the alligator clip and the A/C compressor stopped. I clipped it back and it started.
SOMEBODY INTENTIONALLY GROUNDED THE COMMAND WIRE FOR THE A/C COMPRESSOR CLUTCH RELAY SO THE A/C COMPRESSOR NEVER SHUTS OFF!
All I could say was ‘Wow”. As a mechanic, you run into some crazy stuff.
But the vehicle is still not fixed. The A/C compressor now does not work at all.
From the HVAC live data set, we saw that the A/C evaporator core temperature sensor was causing the compressor to not turn on. We still have to fix that problem.
A/C evaporator core temperature sensor
Looking at the live data of the A/C evaporator core temperature sensor, I could see that it was not working. It was a steady temperature of -22. I decided to find the connector and plug-in a new A/C evaporator core temperature sensor. Plugging in the connector at this stage was only temporary to check if it worked.
When I plugged in the new A/C evaporator core temperature sensor, the temperature showed correctly, the A/C compressor inhibit reason stated “allowed”, and the A/C compressor started working.
Everything was working normally. But I was not done.
I can only assume whoever added the ground wire for the A/C compressor clutch relay either:
Did not know how to fix this vehicle or
Knew how to fix it but did not want to do it, so they did what they did.
The main problem with this vehicle was the A/C evaporator core temperature sensor. This sensor is not an easy replacement. This sensor is located in the HVAC housing which is behind the dash. The dash has to be partially removed, then the HVAC housing has to be removed. All for this little sensor.
It did not take me long to correctly replace this sensor; less than a day to diagnose and replace it.
After everything was back together, a good thorough test drive was done and everything was in working order. The A/C compressor operates as it should, the air is cold coming out the vents the whole time, and the customer is happy. Fixed!
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