Little AC Help Please

Discussion on Air Conditioning/Heat & electronic controls for the same, radiators/fans/engine cooling and related items.
13CapriceNC
Posts: 53
Joined: Sun Mar 31, 2019 8:33 am

Little AC Help Please

Post by 13CapriceNC »

So my Caprice is a weekend / Track car. I want to preface that before we get too deep into asking why i did what i did, lol.

I have removed the complete AC system. I dont need it / want it / care for it. The car is my little piece of hot rod "racecar" life. Now I know removing the AC and unplugging the sensors will cause the ECM to get a little funky. With the AC pressure Sensor unplugged the radiator fans will turn on high immediately and stay on because there is no refrigerant pressure to trip the switch and tell the ECM all is ok. My problem is to figure out how to trick the system into thinking the compressor is working correctly. The picture attached is the switch that is no longer there. I am not the best at wiring, so if anyone could point me in the right direction, it would be greatly appreciated.
Attachments
wire diagram
wire diagram
ac-high-pressure-sensor.png (9.38 KiB) Viewed 8507 times
AC Pressure Switch
AC Pressure Switch
4.jpeg (13.4 KiB) Viewed 8512 times
smwalker
Administration Staff
Posts: 2201
Joined: Mon Jul 29, 2013 1:44 am
Location: Azusa,CA

Re: Little AC Help Please

Post by smwalker »

Description of the sensor and function:

The A/C refrigerant pressure sensor is a 3 wire piezoelectric pressure transducer. A 5 volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0–5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts. The engine control module (ECM) converts the voltage signal to a pressure value.

It provides a 0–5 volt output and requires a 5 volt regulated power supply. In operation, the transducer senses applied pressure via the deflection of a 2 piece ceramic diaphragm with one half being a parallel plate capacitor. Changes in capacitance influenced by the refrigerant pressure under the ceramic diaphragm are converted to an analogue output by the transducers integral signal electronics. The pressure transducer's electronics are on a flexible circuit board contained in the upper section of the transducer. They provide linear calibration of the capacitance signal from the ceramic sensing diaphragm. Benefits of using the pressure transducer over a normal type pressure switch is that the transducer is constantly monitoring pressures and sending signals to the ECM. The normal type pressure switch only has an upper and lower cut out point. The ECM will disengage the A/C compressor at low or high refrigerant pressures and electronic diagnostic equipment can be used to extract system pressure information making it easier when diagnosing concerns. As well as acting as an input to the ECM for A/C operation, the ECM also uses the information provided by the pressure transducer to determine when to turn ON and OFF the 2nd Stage cooling fan operation.

THis chart shows the voltage needed to fool:
Screenshot 2020-02-01 at 2.01.36 PM.png
Screenshot 2020-02-01 at 2.01.36 PM.png (60.99 KiB) Viewed 8500 times
Then the how?? gets a little fuzzy for me here.
One approach using resistors: http://www.learningaboutelectronics.com ... istors.php

Calculator here. Use 200 ohm for R2 that seems to be a common approach, gives you a 467 Ohm R1.
http://www.ohmslawcalculator.com/voltag ... calculator

So looking around 200 and 470 seem to be common and available. That would give you 1.493 output.
Steve Walker
Azusa, CA
'11 PPV 9C3 Huron White
13CapriceNC
Posts: 53
Joined: Sun Mar 31, 2019 8:33 am

Re: Little AC Help Please

Post by 13CapriceNC »

OH BOY Steve, Thank you for being so in depth... Now that you have melted my brain, I will have to go back and read that at least 3 more times to absorb. What i get from that is that i need to utilize resistors to obtain 1.43 Volts to turn my AC off. I hope I am going in the right direction.
smwalker
Administration Staff
Posts: 2201
Joined: Mon Jul 29, 2013 1:44 am
Location: Azusa,CA

Re: Little AC Help Please

Post by smwalker »

Actually maybe I am not reading that right. Maybe what you need is at least 2.39 volts to tell the ECM the pressures are in the OK range. THe "AC off" is actually a command to turn the AC off becase the pressures are out of whack and that is what you already have with it being unplugged.
Steve Walker
Azusa, CA
'11 PPV 9C3 Huron White
Sanford
Posts: 356
Joined: Mon Dec 12, 2016 9:38 pm
Location: Texas Gulf Coast

Re: Little AC Help Please

Post by Sanford »

A quick fix may be to just plug the pressure switch back in. Might be able to cut off some of the switch to make it a little shorter. Without looking through the manual, I would think the switch would not come into play as long as the AC button is no pushed to turn the compressor on.
13CapriceNC
Posts: 53
Joined: Sun Mar 31, 2019 8:33 am

Re: Little AC Help Please

Post by 13CapriceNC »

I dont have any of the AC lines in the car anymore to put the switch back in, If i did plug it in, i dont think it would benefit at all seeing no pressure.
13CapriceNC
Posts: 53
Joined: Sun Mar 31, 2019 8:33 am

Re: Little AC Help Please

Post by 13CapriceNC »

Steve,

What lines would i be messing with to fool the ECM? And where would i hook them up? this is where i am getting confused.
smwalker
Administration Staff
Posts: 2201
Joined: Mon Jul 29, 2013 1:44 am
Location: Azusa,CA

Re: Little AC Help Please

Post by smwalker »

You got everything you need at the connector, 5 volts, ground and the signal going to the ECM. I looked for a picture of someone that did it but could not find anything so a crude drawing of it below. Basically the two resistors can be added to the end of the connector. Or chop it off and just add right to the wires.
Screenshot 2020-02-01 at 3.46.57 PM.png
Screenshot 2020-02-01 at 3.46.57 PM.png (18.47 KiB) Viewed 8483 times
Steve Walker
Azusa, CA
'11 PPV 9C3 Huron White
Sanford
Posts: 356
Joined: Mon Dec 12, 2016 9:38 pm
Location: Texas Gulf Coast

Re: Little AC Help Please

Post by Sanford »

You don’t need a line to screw the pressure switch on. I am guessing the switch will also act as a low pressure switch and prevent the compressor from engaging when low on R-134. Should be able to plug it into the harness and tie it up out of the way. The switch would just make the ECM think that the system has lost the refrigerant and would see no need to turn the gas on.

Steve’s method would allow you to shortened the wiring and sanitize the wiring under the hood.

Off topic a little, but was the compressor any good that you pulled off. I have have a 2011 with a locked up compressor. The 11 is an odd ball and it would be interesting to see if a 13 compressor would function correctly. I know the mounting bracket, compressor connection and discharge hoses are different. There are a few other differences, but they should not impact the fitment.
13CapriceNC
Posts: 53
Joined: Sun Mar 31, 2019 8:33 am

Re: Little AC Help Please

Post by 13CapriceNC »

Sanford,

I was confused by your previous post. Now I understand where you are coming from. The Compressor I pulled off was locked up as well and the ports were cracked. I do have the Condenser and it is still GTG, if you are interested in that.
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