Automotive Air Conditioning Information Forum (Archives)

Mojongo on Mon August 02, 2010 7:11 PM

Year: 2000
Make: Cadillac
Model: Deville
Engine Size: 4.6 V8
Refrigerant Type: 134a
Ambient Temp: 98
Pressure Low: 55
Pressure High: 225
Country of Origin: United States

AC clutch not engaging on a 2000 Deville. Showing trouble code P1540- AC pressure too high. Gauges showed about 115 on each side not running so I bypassed the AC relay and clutch engaged and pressures seemed normal. I'm guessing it is the three wire pressure sensor on the high side line but I was wondering if anyone could tell me how to test it. It's not a very expensive part but I just don't understand how it can be defective and yet it is still operating the electric cooling fans. With the relay bypassed the vent temp is in the 40s and pressures seem normal.

GM Tech on Mon August 02, 2010 10:11 PM

With youe DVOM- read the gray wire to pressure transducer- (does not have to be plugged in)-it should have 5volts to ground with ignition on.

then read the black wire (does not have to be plugged in) should be continuity to ground- if okay then

read the red/blk with transducer plugged in-- it should relate to system pressure--between 0 and 5 volts to ground- so at 100 psi it should read near 1 volt- at 200 psi it should read near 2 volts etc..if not okay then yor pressure transducer is scewed and needs replaced..

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Mojongo on Mon August 02, 2010 10:59 PM

Thanks! I've been searching for a wiring schematic with no luck. I'll run these tests tomorrow. I'm guessing any continuity between the gray/blk-rd wires turns the fans on but excess continuity shuts off the compressor? I hate to throw parts at a problem and not understand how it got fixed. Thanks again.

Mojongo on Tue August 03, 2010 12:07 PM

Mojongo on Tue August 03, 2010 12:18 PM

The red/blk wire was reading 4.8 volts even when the transducer was not connected to the high side line. Thanks for the correct terminology also. The parts store was unable to find the part looking it up by application. Their computer menu only offered the choice high pressure switch but when I told them it was a transducer they were able to enter that as a search and locate it. I thought I was transcending from mechanic to computer nerd but then I see I hit the reply button before I typed anything. Thanks for the vocabulary lesson!