This issue is perplexing me. The AC starts off working fine and cold, then at some random time after driving, it gets warm. The AC clutch has disengaged. I checked the low pressure charge when the compressor is running and it seems fine. Both cooling fans are running. I changed the relay for both the compressor clutch and condenser fan. Did not fix.
I had a neighborhood mechanic take a look and he saw the following codes when the A/C would not engage:
B2979 An open in the A/C pressure sensor circuit
B2980 A short in the A/C pressure sensor circuit
He then says he put gauges on the lines and that high pressure was getting into the low pressure line and this indicated a problem with the condenser or compressor.
I've never heard of that issue before and he seems like he doesn't want to mess with it any further. Any ideas on next steps? The randomness of it all is what is throwing me. Sometimes is stays on fine for 20 minutes in 90 deg heat. Sometimes it kicks off after 10 min in 80 degree heat. Does his diagnoses make sense?
The codes indicate a problem with the AC pressure sensor or its circuitry. What are the actual high and low system pressures?
@chucktobias Only info I have at the moment (as I don't have a good set of gauges and the mechanic did not give me the specifics) is it is about 45 PSI on the low side when the compressor is engaged and pegs really high (off the gauge) when the compressor is off. I am not sure of the high side as I don't have a gauge for that.
Are you using one of those so-called "recharge cans"? You can't do any meaningful diagnosis with those. The thing of it is you can get a real gauge set for not much more than one of those things costs.
Beyond that it should be possible to monitor what the pressure sensor is doing with an appropriate scan tool.
@chucktobias Yes. that was the only gauge I had at the time. I have a lower model autel reader that will tell me codes but it does not say anything about the sensor. Today is a scorcher in the midwest, like 95 deg and the A/C did its typical, ran fine for about 20 minutes then stopped. While it was in a stopped state I went to a local autoparts place that had loaner legit AC gauges. Both the low and high measured 160 psi with the engine off. The with the engine started the A/C would not engage and those pressures stayed the same.
With static pressure 95 degrees would equate to R134a pressure of about 114 psi. So if the engine compartment is at ambient temperature the system may be overcharged. However if the engine is hot the pressure would be higher. Really though pressures need to be observed under operating conditions for diagnostic purposes.
If you're going to work on AC you really need a set of gauges. Amazon offers several in the price range of $35-$50. This one for $43 even comes with can taps:
https://www.amazon.com/Orion-Motor-Tech-Puncturing-Refrigerant/dp/B07JLQTK9C