Expert: Roland Finston - 1/10/2012

Question
QUESTION: Roland ,
Your help last week on my problem was very helpful ultimately there was a high resistance short in the OTIS which had worked intermittenly over the past 2 years, so I disconnected it! I guess the summer heat in Louisiana finally destroyed it like it does most of the plastic on cars today.
Once I got all instruments working including the ATC panel the blower and duct buttons all worked but the compressor does not energize therefore no cooling. My wife now tells me that the A/C quit working a week before the instruments quit therefore I actually had two problems independently of each other.
The compressor clutch does not engage and I have applied 12 volts to the coil with a jumper and it is operation when energized. I have also put A/C gauges on the system and it has 70# of pressure so system should not be shut down by a low refrigerant pressure switch. I suspect the A/C pressure tranducer could be the problem but the factory service manual only shows 3 wires going to it without a diagram of the internal parts, Is there a way to check it with a meter to see if it is defective. I have also looked at th A/C compressor clutch relay which when swapped with another relay in the panel I get the same result. The relay seems to get it coil pwr from the PCM marked (wide open throttle) Does this mean that when the key is in the run position it is energized then when starting the engine or WOT the clutch is deenergized and also acts a a normal relay when thermostat calls for cooling. I also ran self diogonostics on the ATC and got no faults .
Any help would be appreciated.
Morris

ANSWER: Hi Morris,
Glad to learn of your progress. As I observe the pressure and temperature graph it appears to me that 70 psi is possibly too low.
The wiring check would be: The black/light blue wire goes to pin 4 of the pcm and is a common sensor ground. The violet/white is a 5v supply wire from pin 6, and finally the dark blue wire goes to pin 48 and is the signal wire.
The clutch relay will only close when there is sufficient pressure in the system and A.C. has been requested, and of course will not it you are starting the engine or have it at WOT. I would be inclined to add some refrigerant as the easiest 'test'.
Roland

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QUESTION: Roland,
According to my pressure temperature chart R134A has a pressure of 71.1 Pounds at a temperature of 70 Deg F which was the ambient temp yesterday as I had not run the engine to raise the temperature of the engine compartment when I checked it. In addition I leaked checked with an electronic leak detector the entire system and there is no evidence of leaks. Should the system have a low charge it would start and then shut down as the suction side of the compressor pressure drops causing a short cycle, by the way does the system even have a low pressure switch on the low pressure side of the system as I see no reference to one in the manual.

On the transducer the wire colors and there potential will be read from the PCM what I need is when the connector is disconnected from the pressure tranducer how is the internal wiring of the pressure transducer connected so that I can check for continunity or resistance.
Any help would be appreciated.
Morris

Answer
I don't have any schematic showing the sensor internal wiring but I believe that its function is to sense for both over pressurization and under pressurization. But no instruction is offered as to testing the sensor. The output voltage of the transducer when the compressor should normally be activated is 0.451 to 4.519 volts. Below or above that reading will be intrepreted by the pcm as to not have the correct pressure (too low or too high). So you could see what is shown on the signal wire with the engine running as a measure of what it is telling the pcm. Whether it is wrong or right can only be checked by an independent assay of the actual pressure.
Roland