A/C Pressure Sensor Voltages

Car: 2013 Chevy Sonic 1.8L, M/T, LUW engine code
Issue: A/C compressor is not being commanded on

All of my troubleshooting has pointed me to the refrigerant pressure sensor. For brevity's sake I'll leave out the details of everything I tested to get there, unless anyone is curious. At 65°F ambient, with the car having not been run recently, the static pressures measured low-side 72 psi and high-side 77 psi. This pressure should be within the range to allow the compressor to be engaged (32-450 psi on high-side), so I started checking the pressure sensor. Diagram is attached for reference.

With sensor connector mated to sensor and X1 demated from ECM, I measured 11.53 kΩ from 26 to 13, and 6.82 kΩ from 12 to 13. (I also verified wire integrity, no shorts to chassis, etc; again, details if you're interested). I applied 5.00 V across +26/–13 with my own power supply, and I measured 0.974 V across +12/–13. Given those resistances I measured I would have expected 2.958 V (=5*6.82/11.53). I also measured 6 mA total current, where I would have expected 0.434 mA (=5/11.53k). I can't explain the voltage or current measurements.

1. Does anyone have a volts-to-pressure chart for this sensor? I can't find one, but I suspect 0.974 V might be less than 32 psi.
2. Is there anything wrong with my methodology? Measuring resistances, applying external voltage, comparing voltages and currents to expected values - are those meaningful tests?
3. Is there anything different about the way the ECM applies 5V versus the way I did?

Thanks in advance! First time posting here, but I'm a long-time viewer of SD's videos.

aluiaam wrote:

Does anyone have a volts-to-pressure chart for this sensor? I can't find one, but I suspect 0.974 V might be less than 32 psi.
 

Hey aluiaam! I couldn't find a pressure/voltage chart in service info, but I did find this bit in the scan tool data parameters chart:

 

According to this, it would seem that your voltage reading of .974V should be sufficient to allow compressor engagement.

Is there anything wrong with my methodology? Measuring resistances, applying external voltage, comparing voltages and currents to expected values - are those meaningful tests?

I wouldn't say that there's anything wrong with your testing or methodology, necessarily. Thorough, certainly! But maybe a bit indirect, or possibly unnecessary.

I mean no offense here, but is there a reason you didn't want to do voltage readings at the A/C pressure sensor with the engine running and the A/C requested on? Or, was there a reason you didn't trust the 5V regulator in the ECM?

1. Is there anything different about the way the ECM applies 5V versus the way I did?

Not that I can see. The only difference I can think of between your setup and the vehicle would be that the ECM may employ a bias voltage on the pressure sensor signal line, which may result in a difference in circuit behavior.

As far as your inoperative A/C goes, I think your next best bet is a scan tool. GM typically provides excellent scan data regarding A/C operation. You'll be able to see the pressure sensor signal, plus the A/C compressor clutch relay command AND the A/C request from the HVAC controller. Between those three PIDs, you should get a pretty good idea of where the problem lies.

Thanks for the info, Tyler! I actually didn't have any back probe pins, so that's why I disconnected at the ECM and applied my own 5V. There are no DTCs related to 5V ref circuits or anything related to A/C. I went out and got some back probe pins and retested with the full circuit intact. With ignition on, at 75 °F ambient (over 50% humidity) and static gauge pressures around 90 psi on low and high side, voltage was 1.15 V (and 5V Ref was good too). I started the car and turned on the blower and A/C, and of course it started working, blowing cold and everything . I let it cycle for ~10 minutes at idle while I watched the gauges and multimeter. Low side cycled between 20-40 psi, high side cycled between 140-200 psi (depending on whether condenser fan was on). Sensor voltage tracked the high side pressure too, 1.6-2.1 V. All in all the system seems to be working fine when it does come on, so I'm chasing an intermittent problem.

Can you recommend a scan tool that will read those HVAC PIDs? Bidirectional commanding for compressor clutch would be nice to have too. I have an Innova 5210. It reads 42 PIDs in total, but no HVAC PIDs (AC pressure, evaporator temp, etc).

Gotta love intermittents!   Glad to hear it's blowing cold for now, anyway.

As for a scan tool, it's tough go to wrong with the TopDon TopScan:

www.topdon.us/collections/service-tools/products/topscan

The TopDon (Launch) coverage for GM vehicle is usually excellent. HVAC scan data and bidirectional controls will be there, as well. As long as you have a smartphone, it's an incredible value at $60. Paul even used it in a recent Premium video:

www.scannerdanner.com/scannerdanner-prem...sor-diagnostics.html

Happened to have a '14 Sonic in today, had a look at the A/C data while I was at it:
 

Also checked that it has the compressor clutch bidirectional function:

 

Thanks for the recommendation and the screenshots! The TopDon looks like it has a pretty good feature set. I'll check it out.