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Category: Saturn
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I have 2000 Saturn SL1. The AC Compressor will not turn on

Customer Question

I have 2000 Saturn SL1. The AC Compressor will not turn on when I hit the on switch from the dash board. If I go to the relay box and take out the relay and jump it with a wire. The compressor runs. I opened the relay box and found that there is only one wire that is connected to the AC Relay. Am I missing a wire or what could the problem be? The wires to the compressor seem ok and the wires to the dashboard seem ok. What do I need to do? Does any one have pictures of the wires that should connect to the AC relay?
Submitted: 8 years ago.
Category: Saturn
Expert:  GM-FRANK replied 8 years ago.




On your vehicle, the AC compressor relay is energized by the PCM. The PCM looks at several inputs and will dis-allow compressor engagement if it sees something it does not like. Such as low pressure in the AC system.


Here's a look at the compressor control circuitry.




You see the Gray wird circuit 729 that leads to the PCM is what turns on the AC compressor relay.


The body control module looks at the pressure when a request for AC is commanded by you.




A good basic starting point is to connect a set of AC gages to the system to be sure there is sufficient charge to allow the entire system to function.


I hope this helps



Customer: replied 8 years ago.

My friend was looking at the information you sent me and had questions.


1. How does the PCM communicate with the BCM so that the compressor activates?

2. Can an AC gauge from Autozone do the job for checking the pressure?

3.He wonders if the problem could be over at high/low pressure switchover on 66 light green connection?

4 How hard is it to check and or change out the parts? And is it worth the cost?

5. Are the PCM and the BCM micro-chips or what are they?

6.Any other information would be appreciated?


Thank you.

Expert:  GM-FRANK replied 8 years ago.


Hello againCustomer


Let me address your questions.


The PCM is the Powertrain Control Module. The BCM is the Body Control Module.

For the most part, these modules do not fail when it comes to AC problems. Although they can fail. The Modules communicate between each other via serial data.


The BCM handles the Blower Fan Circuits and the Pressure switch and then tells the Powertrain Control Module via serial data to adjust the engine for the load the compressor is going to put on the engine.


They simply look at Your AC High/Low Pressure switch and if they see the AC pressures within a safe range, will allow the compressor engagement.


So the Body Control Module tells the PCM....OK we have a request for AC, and the pressures are OK acording to the Pressure Switch input to the BCM. THe PCM then Grounds the primary side of the AC Relay, and this energizes the Compressor.


If the pressure is low in the AC system, The PCM will not ground the relay.


A shortcut would be to bypass the pressure switch and trick the modules into thinking the pressures are good. Then attempt to turn on the AC at the Interior switch.

If the Relay energizes and the Compressor Engages, then you either have a bad pressure switch, or the system is low.


Since you stated that you already bypassed the relay and the compressor engaged, then I'll assume that the compressor clutch is functioning.


A set of AC gages from Autozone should tell you if you have pressure in the system.


Keep in mind, that if the system is empty, there is a chance you have a leak somewhere, and you will need to repair that leak and then evacuate/vacuum the system to remove any moisture before recharging the system.


So by all means, temporarily jump the light green to the dark green wire at the pressure switch just to see if you can trick the PCM AND BCM into energizing the compressor clutch relay. If everything seems to work with the switch bypassed, remember that this pressure switch is there for a reason. If the compressor runs under conditions of very low or very high pressure, the compressor can fail.


I hope this information helps.



Customer: replied 8 years ago.

frank, One last question. What is the freon type on the SL1 2000? I know there are two types out there. The kind that a professional has to deal with and the kind that a person like me can use.

What would happen if I put in the R-134 instead pf tje r-134a? Is there an issue in doing this?

Customer: replied 8 years ago.



I bought a gauge to check the pressure and found it to be low. We added more freon and now the low is reading about 49 and the high reads about 50. From what I have found for directions is that the compressor is faulty. Is This right? Is there anything I can do other than buying a new compressor. Again the Low had high pressure and the High had low pressure. The compressor did start once we added the freon. It may not have been electrical afterall.

I did some more reading and I added a total of about 23 ounces of freon. I do not know if this is to much or not? I checked the pressure and the low is reading normal and the high is still running low like at about 100. From what I reasearched, it is a leak some where. I also saw that I have to evacuate the freon that is in the system if I am going to open the lines in any form. My next question is if the air vacuum pumps from Harbor Freights would do the job of evacuating the system or am I looking at the wrong type of pump? I was looking at the ones that were ITEM 96677-7VGA or ITEM 98076-2VGA. They do not say much on what they actually do. What is mentioned is that the first one removes moisture from the air but nothing about removing the freon so as to open the system to to repair and then extract all the air that got into the system.

Second I will Accept what you had previously told me for my first question.

Thank you,