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how to chek a ac refrigerant pressure sensor

goth on Tue May 12, 2009 10:46 PM User is offline

Year: 1994
Make: pontiac
Model: firebird
Engine Size: 5.7 Lt
Refrigerant Type: r134
Ambient Temp: 95
Pressure Low: 20
Pressure High: 175/200
Country of Origin: United States

I have a 94 firebird 5.7 Lt, ac was working fine until I park the car for several weeks, I tried to turn AC and compressor didnt came on, I suspected loss of refrigerant so I connected the gages, it was 95 without the ac running, So I force the ac compressor relay and the pressures looks fine, 20 on the low 175/200 on the high, I think for some reason the PCM is not sending a signal to the relay, I look to the haynes digram and I see the PCM has three sensors engine coolant , evaporator temperature and refrigerant pressure sensor, I didn check the engine coolant temp sensor and its fine however I dont know how to check the refrigerant pressure sensor wich I suspect is bad, and the evap pressure sensor as well, I have search the internet and some ac book for some info on this but no sucess !

GM Tech on Tue May 12, 2009 10:52 PM User is offline

Did you reset your code 46 that may have been set- by unhooking the battery for 10 seconds? Code 46 is a low charge code- that the HVAC algorithm sets when there is not enough cooling going on

The test for the transducer is to use a Tech I scan tool- to read the voltage and psi output....

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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......

goth on Tue May 12, 2009 11:00 PM User is offline

Thanks for your quick response, The car had no battery for over two months, I have not reset any codes since I dont see the check engine light on, shall I see the check engine light on when the code 46 is active?

iceman2555 on Wed May 13, 2009 9:03 PM User is offlineView users profile

Once the electrical problem is rectified....recover and recharge the system....the pressures are not 'fine'. The system is undercharged.

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

GM Tech on Thu May 14, 2009 12:33 PM User is offline

On your "F"-body- the code 46 does not set the SES light-- it is internal to the system- you need a scan tool to see it- although it may work by jumping a and b on the ALDL connector-- never tried that since I have a Tech I.....

But it does shut down the system- PCM does not allow the relay to energize--- do you have a negative signal on the coil side of the a/c relay when a/c is requested?

-------------------------
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......

goth on Thu May 14, 2009 10:06 PM User is offline

I dont have the negative signal on the relay, it looks like the pcm its not grounding it, I will have to scan it, unfourtunately I only have an obd 2 scanner, do you know of any cheap obd 1 scanner?

GM Tech on Fri May 15, 2009 7:47 AM User is offline

Well if you can ever get it hooked up to a good scan tool- it will tell you both refrigerant pressure from the transducer and evap temp sensor temperature- in volts as well.

Why do you suspect a bad transducer? - they very rarely fail unless.....someone tries to "jumper" them to try to get the compressor to come on.....

-------------------------
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......

goth on Fri May 15, 2009 6:09 PM User is offline

I didnt know that if you disconnect the battery for a while a low refrigerant pressure code is set, And since the PCM wasn´t enabling the compressor relay, First I suspected the refrigerant was gone, but after I hook the gauges I realize that was not the case, So I suspected something was not leting the PCM turn the compressor on, that is why I wanted to check this pressur transducer, I Did not bridge it because I new it was not a simple on/off pressure switch like other cars.

Regarding the scanner I saw this freescan software on the web, I will purchase an aldl cable and try it out (or probably make one if i am able to find all of the pieces) . It is suppose to scan OBD1 for error codes.

http://www.andywhittaker.com/ECU/FreeScan/tabid/70/Default.aspx

Matt L on Fri May 15, 2009 7:18 PM User is offline

I know that on OBD1 Fords, for "key on" tests you can ground the test lead on the connector, sit in the driver's seat and count the flashes of the MIL. Is this the case for GMs also? All that the Ford OBD1 box did was flash a lamp or run a buzzer when the MIL was (or would have been, if the key were on) illuminated; unless you had something fancy that counted the flashes for you.

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