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Old 09-17-2009, 06:52 PM   #1
the flyin' scotsman
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Default a/c low pressure switch

What are the symptoms of a bad LPS on the a/c system?

I was told by my local friendly GM delaer that the reason why my a/c is not as cold after running for an hour or so was because of the switch. He did 'top' up the system and it is cold on the 35km round trip.
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Old 09-17-2009, 07:03 PM   #2
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The low-pressure switch is a safety device to cut off the compressor when there is not enough charge in the system to properly circulate refrigerant and oil. It is there to save the compressor and should only kick in after the A/C hardly makes the air colder at all. It can in no way affect the air temperature by being defective, other than having a refrigerant leak itself. Maybe your tech was talking about the anti-freeze switch? If that is not working properly, your evaporator will eventually freeze up and airflow out of the vents will be reduced. See the other A/C thread about the freeze switch currently going on:

http://forums.rennlist.com/rennforum...r-the-car.html

Drama warning though

Or... maybe your low-pressure switch is failing in such a way as to think there isn't enough charge when there really is? That is also a possibility. An easy way to check this is to get out of the car when the air gets warmer and observe the compressor clutch for a minute or so to see what it is doing.

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Old 09-17-2009, 07:09 PM   #3
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The switch is what it says, at low pressure it opens the power circuit to the compressor and shuts it down. It is in series with the high pressure and freeze switch which all do the same thing, cut the power off to the compressor. If he topped it off then your R-134 or R-12 was low therefor triggering the switch. Also a low charge would not let your system cool correctly. If your charge got too low it would burn up your compressor if it remained on due to lack of lubrication.
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Old 09-17-2009, 07:20 PM   #4
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I do recall briefly reading the other ongoing thread............thx for the reminder.

The reason he 'topped' it up is when he filled the system some weeks ago he noted using 1.9lbs of R134a and I had pointed out to him that the system should take 950g or 33.44oz. Recalling that the other thread indicated a 'low' system would cause a loss of cooling after some running time I had the tech add more refrigerant.

As to the switch question the tech was the one who asked.............hence my question; thx for the inputs.
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Old 09-22-2009, 08:45 PM   #5
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Here's the latest symptoms:

when first engaging the a/c by pressing the dash switch everything works well to the point the air from the centre vent is very cold so I close the vent. After parking for a few mins and restart the compressor clutch does not engage.

After parking and engine off for +15mins all is back to normal.

Is closing the centre vent causing the frost switch to turn off the compressor or can the LPS cause this?

Appreciate your comments (Roger dont send the package yet )
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Old 09-22-2009, 09:19 PM   #6
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It's not the low pressure switch. Closing the vent? Why not lower the fan speed or (gasp!!!) move th etemp slider over to the temp you want and let the automatic system adjust things for you.

It's possible the freeze switch is the culprit. You can test for this with a little telltale light bulb wired to the two terminals on the freeze switch, in paralle with the two wires already attached there. If the freeze switch is opening and stopping the compressor, the bulb will light up. My home-made tesetr has a 194 bulb in a cheap store replacement socket, a couple extended leads with alligator clips and spade lugs that fit in fuseholders.
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Old 09-22-2009, 09:29 PM   #7
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Thx Bob ...................should further explain the 'auto system' is bypassed per Tonys write up; the hw valve closed and various solenoids held open by vacuum.

I just tried again in the garage; starting in the garage without the a/c switched then pressing the button all is well and stays that way...........+10mins.

Shutdown the engine and restart with a/c button still depressed fans engage but not the compressor.

Dis-engage the a/c button, shutdown engine, restart engine and re-engage a/c all is well.

It appears the system doesn't like the a/c button engaged before engine start but I dont see the relationship.
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Old 09-24-2009, 10:31 PM   #8
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The freeze switch tests closed before and after the a/c system self shuts down.

Is there an easy test for the head controller relay?
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Old 09-25-2009, 02:48 AM   #9
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Sure. At the freeze switch, see if you have voltage to ground while you have the AC button pressed and the Clutch doesn't engage. The low impedance of the coil and high resistance at the relay contact cause a lot of voltage drop. So voltage at freeze switch terminals will be pretty low if the relay resistance is high.
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Old 09-25-2009, 10:00 AM   #10
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Food for thought: The center vent contains a micro-switch. When the center vent is manually closed, the micro-switch is engaged and adjusts the flap step-motor. The step-motor adjusts flaps within the HVAC box and directs the air accordingly. Insufficient airflow over the evaporator coil may cause freezing; especially if mixed with cooler outdoor Alberta air. These flaps may sometimes require adjustment.

Wally P's got a great write-up on HVAC troubleshooting: http://www.928gt.com/T-WallyHVAC.aspx
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Old 09-25-2009, 10:16 AM   #11
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Thanks again Dr. Bob...............I'll test votage to grd @ the FS.

Andy............our normal cool air has been over 30C the last few days. When the compressor engages the system works very well but the more I use it the more frequent it dis-engages.

Wallys write up is well known; thx.
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Old 09-25-2009, 12:20 PM   #12
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This could be the little relay in the HVAC head... it works for a while until it gets hot enough.

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Old 09-25-2009, 07:42 PM   #13
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Thats what I'm now thinking................I may only need a/c once or twice for the remainder of the year but its sure nice when it all works.

I'll be searching for relay r&r now.
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Old 09-25-2009, 08:50 PM   #14
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Following up on Dr. Bobs suggestion I measured voltage from the freeze switch to ground with the a/c button engaged but the compressor was not.

Voltage readings fluctuated but on average I'd estimate 110mv.

Is that an indication of the bad HVAC relay? If so, please point to a thread to R&R with 'Radio Shack' part #...........TIA.

Gonna get this perfected yet.................so close
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Old 09-25-2009, 09:19 PM   #15
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I added an external relay to the head no more draw.
Remember you have high pressure, low pressure and a freeze switch all in series anyone will shut the compressor down. Make a jumper and bypass one switch at a time and see what happens. The other possibility is your clutch is going bad or you may have a corroded connection at any point. Have you placed a wire from your front power post to the compressor wire does the compressor work.
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Old 09-25-2009, 09:19 PM
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