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Low pressure switch cutoff point?
Posted: Sat Aug 10, 2024 7:20 am
by mschlott
I have a 2003 Chevy Silverado 1500HD. At highway speeds it does not cool as well as it does around town. When checking the pressures, they seemed fine, but I could get the compressor to cycle by bringing the RPMs up to 2000 RPM and spraying the condenser with water. Thinking the low pressure switch was cutting the compressor on the highway, I added some refrigerant.
I think I'm a little overcharged at this point but the high side is only 10-20 psi high. It still cut out at highway speeds. I jumpered the low pressure switch and now it cools properly at highway speeds.
Watching the pressure with the low-pressure switch connected, it looked like it was cutting off at about 30 psi. I have cheap gages, so they could be off slightly, but I suspect that the low-pressure switch is cutting off at too high of a pressure. I don't think this is a common failure mode, but I could be wrong. I'm not an AC tech.
Is this a normal failure for a low-pressure switch? If not, is there a way to look up specs on low-pressure switches to see what pressure they cut off at and possibly replace it with one with a lower cut-off point?
Re: Low pressure switch cutoff point?
Posted: Sat Aug 10, 2024 4:49 pm
by Cusser
mschlott wrote: ↑Sat Aug 10, 2024 7:20 am
I think I'm a little overcharged at this point
but the high side is only 10-20 psi high. It still cut out at highway speeds. I jumpered the low pressure switch and now it cools properly at highway speeds.
You mean 10-20psi higher than where you think it should be? Jumping a switch except for testing is not a good solution. Are you sure that switch is not high pressure or combination low-high switch? Can you post pictures of it, and where it is located?
Was that 10-20 psi high measured at 1800 rpm or so?
mschlott wrote: ↑Sat Aug 10, 2024 7:20 am
I have a 2003 Chevy Silverado 1500HD. At highway speeds it does not cool as well as it does around town.
Are both electric fans behind the radiator both spinning?
Or is your 2003 equipped with a fan clutch? Malfunctioning fan clutch can definitely cause poorer AC performance at city traffic or idle.
Re: Low pressure switch cutoff point?
Posted: Sat Aug 10, 2024 5:03 pm
by tbirdtbird
Wow.
First, no jumpering is allowed. You can do damage.
Check to see if there is a cabin air filter, if so change it.
Adding refrigerant Willy nilly is a no-no.
And I fear you used one of those so called death kits from the parts stores which contain sealer, which is a mistake, it can clog your system. If it had only a blue low side gauge, it was a death kit, and those gauges are notoriously inaccurate.
If you are going to do AC work, whether a DIY or a pro, you need a proper manifold gauge set.
Hi and low pressures need to be recorded at 1800-2000 rpm and reported here.
Is the condenser clogged with bugs and lint and grass, etc?
What most likely happened is you had what is called ‘seasonal’ losses from the comp shaft.
The correct approach would have been to have a pro shop use their RRR machine and pull out what refrigerant was left. Then have them install the correct amount, which is usually on the under hood sticker.. they would use pure refrigerant to do this.
This is your starting point to doing a diagnosis, and in most cases is all that is needed.
You should also check the clutch gap, searching here will reveal how to do that. It should be set to .020.
Re: Low pressure switch cutoff point?
Posted: Sat Aug 10, 2024 7:27 pm
by Cusser
tbirdtbird wrote: ↑Sat Aug 10, 2024 5:03 pm
What most likely happened is you had what is called ‘seasonal’ losses from the comp shaft.
The correct approach would have been to have a pro shop use their RRR machine and pull out what refrigerant was left. Then have them install the correct amount, which is usually on the under hood sticker.. they would use pure refrigerant to do this.
This is your starting point to doing a diagnosis, and in most cases is all that is needed.
You should also check the clutch gap, searching here will reveal how to do that. It should be set to .020.
THIS
Re: Low pressure switch cutoff point?
Posted: Sun Aug 11, 2024 7:33 am
by mschlott
Cusser wrote: ↑Sat Aug 10, 2024 4:49 pm
mschlott wrote: ↑Sat Aug 10, 2024 7:20 am
I think I'm a little overcharged at this point
but the high side is only 10-20 psi high. It still cut out at highway speeds. I jumpered the low pressure switch and now it cools properly at highway speeds.
You mean 10-20psi higher than where you think it should be? Jumping a switch except for testing is not a good solution. Are you sure that switch is not high pressure or combination low-high switch? Can you post pictures of it, and where it is located?
Was that 10-20 psi high measured at 1800 rpm or so?
mschlott wrote: ↑Sat Aug 10, 2024 7:20 am
I have a 2003 Chevy Silverado 1500HD. At highway speeds it does not cool as well as it does around town.
Are both electric fans behind the radiator both spinning?
Or is your 2003 equipped with a fan clutch? Malfunctioning fan clutch can definitely cause poorer AC performance at city traffic or idle.
Hi. Yes, I mean 10 -20 lbs higher than I think it should be for the current temperature and humidity at about 1500 RPM. When I rev it to over 2000 RPMs, the low side drops to about 30 and the high side goes up about 40-50 psi. Since I'm in the driveway I don't get the same amount of cool air as when I am driving 70. That is why I hit it with water to see what would happen. When I do that it drops below 30 and cuts out.
I don't have electric fans. It is mechanical and seems to be working properly.
Re: Low pressure switch cutoff point?
Posted: Sun Aug 11, 2024 7:36 am
by mschlott
tbirdtbird wrote: ↑Sat Aug 10, 2024 5:03 pm
Wow.
First, no jumpering is allowed. You can do damage.
Check to see if there is a cabin air filter, if so change it.
Adding refrigerant Willy nilly is a no-no.
And I fear you used one of those so called death kits from the parts stores which contain sealer, which is a mistake, it can clog your system. If it had only a blue low side gauge, it was a death kit, and those gauges are notoriously inaccurate.
If you are going to do AC work, whether a DIY or a pro, you need a proper manifold gauge set.
Hi and low pressures need to be recorded at 1800-2000 rpm and reported here.
Is the condenser clogged with bugs and lint and grass, etc?
What most likely happened is you had what is called ‘seasonal’ losses from the comp shaft.
The correct approach would have been to have a pro shop use their RRR machine and pull out what refrigerant was left. Then have them install the correct amount, which is usually on the under hood sticker.. they would use pure refrigerant to do this.
This is your starting point to doing a diagnosis, and in most cases is all that is needed.
You should also check the clutch gap, searching here will reveal how to do that. It should be set to .020.
I'm just jumpering to test. I know it has a charge and is not dropping dangerously low.
I don't have a cabin air filter. I have good air flow to the cabin.
I did not add it willy nilly. I believed the AC was cutting out due to low pressure, and I wanted to see if bringing up the low side would keep it from happening. It helped, but did not fix the problem. I will take it down to the correct pressure, when I am done testing. It does not cool as well at idle because it is overcharged.
I cleaned the condenser.
Re: Low pressure switch cutoff point?
Posted: Sun Aug 11, 2024 7:37 am
by mschlott
After doing some additional reseach, I found that the cut off point for the low side switch should be 25 lbs. Since mine is cutting off at 30 lbs, I'll replace it and report back.
Thanks for the advice and feedback.
Re: Low pressure switch cutoff point?
Posted: Sun Aug 11, 2024 7:54 am
by JohnHere
As mentioned earlier, it's not a good idea to jumper anything--not permanently anyway. If what you're dealing with is an undercharge--and we don't yet know for sure--you'll "starve" the compressor of oil, and it will quickly destroy itself internally.
I was also about to say that your LPCO switch appears to be defective, but you beat me to it. A reading of 30 PSI on the low side is considered normal. If the LPCO switch is shutting off the compressor at that pressure, then the switch is defective.
Re: Low pressure switch cutoff point?
Posted: Mon Aug 12, 2024 6:17 am
by Windery
You could get a replacement switch with a lower cut-off pressure if you confirm that’s the issue. But I say is better having a pro take a look.