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1991 Siverado no power to A/C low pressure switch
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Floydroberts54
Novice
Aug 29, 2009, 10:21 AM
Post #1 of 16
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1991 Siverado no power to A/C low pressure switch
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1991 Chevrolet Silverado 1500 5.7 liter engine with 108,00 miles. Air conditioning has been low on freon for a very long time. When I went to recharge the a/c found that the low pressure switch has not power to it. I wired the compressor direct and charged system, but still not power to switch. When you start the truck or push the a/c button, you get the a/c light flashing for a couple of minutes. I have checked the fuses all are good. I have found three relays behind the glove box. When you push the a/c or recirculate button, two of the relays click, one does not. All the relays are just alike so I have swapped them around to see if the ones that are working would stop but the same two work every time. What do I need to check now and how.
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Hammer Time
Ultimate Carjunky
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Aug 29, 2009, 10:30 AM
Post #2 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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That switch is not supposed to have power to it. It provides continuity through the switch for a ground supplied by the PCM to trigger the relay. The flashing light at the control head is an indication of a problem with the request signal, probably a bad control head.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Sidom
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Aug 29, 2009, 1:17 PM
Post #3 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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Disconnect the battery for about 5 minutes and then hook it back up.
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Floydroberts54
Novice
Aug 29, 2009, 1:37 PM
Post #4 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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Is there any way to test the control? Are the other two relays that are working not connected through the same circuit and control?
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Sidom
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Aug 29, 2009, 1:53 PM
Post #5 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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Have you tried resetting the system as I suggested?
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Floydroberts54
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Aug 29, 2009, 1:59 PM
Post #6 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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No, I was going to try that first thing. Just wanted to check in on that switch, had one of the parts dealers say the same thing was wrong. No one could tell me how to check it. Thank you for your help, I'll post back after Church in the morning.
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Floydroberts54
Novice
Aug 30, 2009, 8:41 AM
Post #7 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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Sidom It worked. I now have 12.1 volts at the low pressure switch and the A/C works like a champ. Never would have thought of the battery. I and my 16 year old son who has inherited my old work truck, THANK YOU VERY MUCH.
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Hammer Time
Ultimate Carjunky
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Aug 30, 2009, 8:44 AM
Post #8 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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What you have done is erased the code and reset the computer. That didn't resolve the problem that caused it in the first place so it will likely be back.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
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Aug 30, 2009, 9:19 AM
Post #9 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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Floyd - it ain't fixed yet. You really need to see the pressures with gauges as systems are made to shut down when things aren't right. Not paying attention as to why it shut down could cost you or owner a fortune!
If you can't find 40sF center vent temps you are in trouble unless you are dealing with incredible temps outside,
T
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Sidom
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Aug 30, 2009, 12:10 PM
Post #10 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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Glad to hear its working for you....
I believe the system will be ok.. Now this is just what I've noticed and never really had the curiosity or need to research it any further so this is just my take..
On that year range of Chevys when ever I had a complete system I finished, after I evacuated it and started to charge it...Without fail after the 1st lb, the system would cutout, the panel would blink & need to be reset. After I got the 2nd lb in, all was good & I could finish charging the system. It only happen to me twice, after that I wouldn't even waste my time, I would just bypass the low pressure cutoff sw until I got 2 lbs in.
My theory is that it was a safety for the clutch. With just 1 lb in the system the system is cycling the clutch so rapidly that it could actually damage the clutch due to overheating. Like I said I've never researched it any further because the only time I've seen this was on a low system that had been operated.. On the ones that came in that way, I repaired the leak, reset the system & didn't see them back for the same problem...... Just my 2 cents...
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Hammer Time
Ultimate Carjunky
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Aug 30, 2009, 12:17 PM
Post #11 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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SYSTEM DIAGNOSTICS
This system does NOT generate Codes.
The microprocessor monitors the actuators position and flashes an indicator should a malfunction occur.
Should a malfunction occur, the respective indicator will flash for a period of two minutes at every ignition-- "ON" cycle and every time a control head key-press command selects the malfunctioning actuator, until the malfunction is repaired.
The first time an actuator malfunction is diagnosed the corresponding flashing indicators will begin after a five second delay.
The assignments of the flashing indicators to the malfunctioning actuators are listed in the table.
If the Mode or Temperature indicators flash, use the information under Control Assembly and/or Air Door Actuators, to diagnose the problem. This is all that is supplied by the manufacturer.
LOW REFRIGERANT DETECTION
The controller monitors the state of the low-pressure cycling switch, if the compressor has been activated, and measures the duration of the compressor "On" time.
The minimum compressor "On" time limit will be 1.5 seconds. Once this limit (less than 1.5 seconds) has been exceeded for 10 consecutive cycles, the compressor will be turned off and remain off until the servicing has been performed and the battery disconnected to reset the memory.
The A/C indicator will flash for a period of two minutes and then turn "Off" when the low charge condition has been detected. It will also flash at every ignition cycle for two minutes until the servicing has been performed and the battery disconnected to reset the memory.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Sidom
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Aug 30, 2009, 12:25 PM
Post #12 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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LOL.........Very nice HT & very fast... impressive 
I was pretty sure it had a low freon mode in........now I know.
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Hammer Time
Ultimate Carjunky
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Aug 30, 2009, 12:29 PM
Post #13 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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The Broncos vs Bears game is boring................LOL
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Sidom
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Aug 30, 2009, 12:32 PM
Post #14 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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Damn it.........I forgot all about that  ... Man I wanted to see the 1 quarter........
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Hammer Time
Ultimate Carjunky
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Aug 30, 2009, 12:46 PM
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Re: 1991 Siverado no power to A/C low pressure switch
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Most of the Broncos are busted up now incl the Qback
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Floydroberts54
Novice
Aug 31, 2009, 9:09 AM
Post #16 of 16
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Re: 1991 Siverado no power to A/C low pressure switch
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I know what the initial problem was, the low pressure switch had a leak and would also not stay connected to the wiring very well. This was a long time ago ( 7 years ). The system was low on freon when I fixed the switch and I never got back around to recharging the system. This has been my work truck for the last ten years. Just used the old 2/40 - A/C and kept right on going. The Kid needed a vehicle and I gave it to him. He wanted the a/c working and I knew the only thing wrong was the low freon. Just guess I had a brain laps on needing to reset everything or it never tripped out before when it lost a charge.
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