AC compressor won't engage unless I do it manually
09-05-2016, 12:24 PM
#1
AC compressor won't engage unless I do it manually hi everyone. Lurk a lot, time to come forward. Mind if I pick your brains for a moment? 1993 RX7 with LS1 swap, 411 PCM 3 wire pressure sensor. I'm not able to get compressor to kick on. Manifold set shows 75lo/75hi on cold engine and its 75 degrees ambient air today. Push AC button, first relay activates and sends ac request signal to PCM...but that's where it ends. When I bypass PCM by sending ground signal to second relay (clutch control) the second relay kicks on and then 12v is sent to compressor and it all runs great, cold air, and manifold gauges show 45lo/220hi when car has been running awhile.
Something is telling PCM not to send signal trough the clutch control pin, ad in turn not sending 12v to compressor.
The pressure sensor is getting 5v and ground signal, and THE PRESSURE SENSOR VOLTAGE READOUT IS ONLY -.04v when engine running with AC button engaged. That is me with a multimeter readout at pin 14. That's with manifold gauges hooked up reading at approx 85psi hi/lo
FYI 3 wire pressure sensor wired thusly:
Purple (pressure sensor GND) to PCM 57 pin on red harness
Red (pressure sig) to PCM 14 on red harness
Gray (Pressure 5v) to PCM 45 on blue harness
What you thinking?
I'm thinking my AC pressure sensor is not reading correctly, no?
For helping other out, here's the formula to convert it AC pressure sensor psi readout to voltage and vice versa:
Equations for GM LS 0-5V 3 Wire AC Pressure Sensor
Pressure = (Volts*93.48625) -15.5388
Volts = (Pressure +15.5388)/93.48625
So in my case, I plug in -.04v to first equation and I get -19.27psi
Obviously that's incorrect as I'm not pulling a vacuum on my system.
Using the second equation, in my case knowing I have around 85psi, my voltage readout SHOULD BE somewhere around 1.075v
I'm no mathematician or ASE mechanic but am I correct in assuming my pressure sensor is trash since the readout should be 1.075v but it's showing -.04v? Assuming that's the issue, that explains why the clutch control wire is not getting ground signal sent from the PCM, because the PCM thinks my HVAC system is not only low pressure, but actually pulling a vacuum, so it won't give permission to AC compressor to kick on in hopes of protecting it from damage.
Is this sound thinking? Or maybe there junk at the pressure sensor valve preventing proper readout?
Something is telling PCM not to send signal trough the clutch control pin, ad in turn not sending 12v to compressor.
The pressure sensor is getting 5v and ground signal, and THE PRESSURE SENSOR VOLTAGE READOUT IS ONLY -.04v when engine running with AC button engaged. That is me with a multimeter readout at pin 14. That's with manifold gauges hooked up reading at approx 85psi hi/lo
FYI 3 wire pressure sensor wired thusly:
Purple (pressure sensor GND) to PCM 57 pin on red harness
Red (pressure sig) to PCM 14 on red harness
Gray (Pressure 5v) to PCM 45 on blue harness
What you thinking?
I'm thinking my AC pressure sensor is not reading correctly, no?
For helping other out, here's the formula to convert it AC pressure sensor psi readout to voltage and vice versa:
Equations for GM LS 0-5V 3 Wire AC Pressure Sensor
Pressure = (Volts*93.48625) -15.5388
Volts = (Pressure +15.5388)/93.48625
So in my case, I plug in -.04v to first equation and I get -19.27psi
Obviously that's incorrect as I'm not pulling a vacuum on my system.
Using the second equation, in my case knowing I have around 85psi, my voltage readout SHOULD BE somewhere around 1.075v
I'm no mathematician or ASE mechanic but am I correct in assuming my pressure sensor is trash since the readout should be 1.075v but it's showing -.04v? Assuming that's the issue, that explains why the clutch control wire is not getting ground signal sent from the PCM, because the PCM thinks my HVAC system is not only low pressure, but actually pulling a vacuum, so it won't give permission to AC compressor to kick on in hopes of protecting it from damage.
Is this sound thinking? Or maybe there junk at the pressure sensor valve preventing proper readout?
Last edited by Chucklee; 09-05-2016 at 02:53 PM.
