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Old 06-23-2009, 03:58 AM   #1
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Default A/C Low Pressure Switch

So I believe my low pressure switch is going bad on my 94 civic lx. My ac compressor's clutch kicks on and off too much. I heard this could be my low pressure switch which tells the comp. when to turn on and off. Could this be true?? If so where is the low pressure witch for the A/C system?? Also when idling and I turn the A/C on the motor drops in rpm, clunks then jumps up in rpm (when comp. is turning on and off) and repeats this until turned off. Oh and also if its really hot outside the a/c will NOT cool, but at night or in the morning it will keep Popsicles cold!

Please help!!!!!!!!!!!!
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Old 06-23-2009, 04:27 AM   #2
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Default Re: A/C Low Pressure Switch

I am currently reinstalling all of my a/c components on my car and I am quite familiar with where all of the parts are located. I cannot help diagnose the problem because I am still trying to learn a/c from a coworker of mine, who is helping me with it. The switch is located on the side of the condenser, there will be a wire plugged into it, with a connector round in shape similar to like an IACV harness. Hope you get it fixed man!
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Old 06-23-2009, 05:56 AM   #3
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Default Re: A/C Low Pressure Switch

have you check the system pressure or checked for leaks? sounds like you are just low on r134a.
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Old 06-24-2009, 08:39 AM   #4
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Default Re: A/C Low Pressure Switch

Put gauges on her and showed 40psi !! When its cooler outside she cools great. BUt when hot outside, not so well. And like said above when a/c is on and car at idle the rpm's studder 200-500 ??? Ill try the low pressure switch. Doesnt the low pressure switch turn the comp. clutch ??
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Old 06-24-2009, 08:57 AM   #5
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Default Re: A/C Low Pressure Switch

the low pressure switch should turn the comp off when it detects low pressure to prevent damage. if the comp was running with no freon it would probably burn up quickly so the low comp switch is a preventative measure.

are you getting 40psi when its hot outside and the a/c is on high? i think the DIY kit i have says that psi should be 45-50 when ambient temp is 80-85*. IMO it still sounds like you are a little low on r134a. worst case is you add some and it doesnt help. easier and cheaper to rule that out than change out the switch for no reason.
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Old 06-24-2009, 09:07 AM   #6
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Default Re: A/C Low Pressure Switch

whats high side pressure.. you need both numbers to diag. your car.
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Old 06-25-2009, 08:00 AM   #7
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Default Re: A/C Low Pressure Switch

I think the high side stays around 90 psi. Where exactly is the low press. switch located in case I decide to change it out?


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Old 06-25-2009, 08:01 AM   #8
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Default Re: A/C Low Pressure Switch

I think the high side stays around 90 psi. Where exactly is the low press. switch located in case I decide to change it out?


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Old 06-25-2009, 01:58 PM   #9
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Default Re: A/C Low Pressure Switch

40 low side and 90 hide side are both low numbers. low numbers means low freon the low side switch is shutting off your compressor proably because it wants to go lower then 40.

if you still think yoru low side switch is bad trying jumping the two terminals. all that switch is is just a switch located on yoru low side. should be 12v 12v on both wires. when the switch closes it basically doenst allow one wire to have 12v anymore. this breaks up the circuit so you wont get power to your compressor. this in turn shuts off your compressor.

so if you jump the harness low side connector the compressor will stay on regardless of low pressure. your basically bypassing the switch. but if your compressor is enagaging then we already know your low side switch is working.

have to find out why you have low pressure low side...low pressure high side...generally low freon is the result
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Old 06-25-2009, 02:33 PM   #10
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Default Re: A/C Low Pressure Switch

also for the low psi switch..its normally on the low line..low line is cold side....so generally next or close to yoru firewall....

high side switch is generally on the compressor itself....or on the hot side.. so somewhere between compressor and the line running towards yoru radiator
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Old 07-06-2009, 07:03 AM   #11
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Default Re: A/C Low Pressure Switch

Not to hijack the thread but i am having the same problems as OP. Except when i bought one of those kits with a pressure gauge to recharge the system it said my low side pressure was at 60psi. the can said that was in the red and it should be at like 30psi and owners manual says 22psi. I don't know which numbers are right. ambient temp is in the mid 90's btw. I dont wanna replace the pressure switch for no reason, but i heard that could be the problem if compressor is short cycling and the charge is good.
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Old 05-30-2010, 06:48 PM   #12
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Default Re: A/C Low Pressure Switch

IMO the short cycling can be caused from the evaporator freezing up do to low freon or the evaporator temp sensor or AC thermostat malfunctioning. First make sure you are good high and low side for r134 next replace the ac thermostat...
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Old 07-30-2010, 04:50 PM   #13
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Icon2 Re: A/C Low Pressure Switch

My moms car is pretty much doing some of the same things. It isnt blowing cold air at all at idle or at cruise. The condenser fan was toast, so I replaced it with a new one. I knew that wasnt gonna fix the problem but it needed changed anyway. When I turn the a/c on the engine surges but the compressor does not engage so I thought it might be low on 134. I bought one of those big cans of 134 with the gauge, and when I attached it to the low side port, the needle pegged. So I bled off the pressure to the green band, around 55 psi and it didnt fix anything. I am thinking the maybe the high or low pressure switch is bad. Also I can turn the compressor by hand, so its not froze up. Its just free spinning on the pulley. Any ideas????
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Old 07-30-2010, 05:37 PM   #14
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Default Re: A/C Low Pressure Switch

when i took mine to the shop and they were able to free spin the compressor by hand the guy told my the compressor needed to be replaced. Its not supposed to free spin like that.
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Old 07-30-2010, 08:58 PM   #15
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Default Re: A/C Low Pressure Switch

Quote:
Originally Posted by dcraig1000 View Post
My moms car is pretty much doing some of the same things. It isnt blowing cold air at all at idle or at cruise. The condenser fan was toast, so I replaced it with a new one. I knew that wasnt gonna fix the problem but it needed changed anyway. When I turn the a/c on the engine surges but the compressor does not engage so I thought it might be low on 134. I bought one of those big cans of 134 with the gauge, and when I attached it to the low side port, the needle pegged. So I bled off the pressure to the green band, around 55 psi and it didnt fix anything. I am thinking the maybe the high or low pressure switch is bad. Also I can turn the compressor by hand, so its not froze up. Its just free spinning on the pulley. Any ideas????

With the A/C on, check whether the Red wire at the compressor has battery voltage. If so, the gap between the armature plate and clutch pulley may be too wide:

Click the image to open in full size.
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Old 08-01-2010, 04:19 PM   #16
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Default Re: A/C Low Pressure Switch

Okay... I dont have any power on the red wire going to the compressor. when I unplugged the high pressure switch and checked for continuity on the switch itself, it read closed. shouldnt it read open? i also grounded out the condenser fan connector to try to get it to come on and nothing. i checked the 134 pressure again and its fine, right around 55 psi. i pulled the low pressure swtich connector off but couldnt get to it to read with the meter. I jumpered the low pressure switch connector and nothing. Im lost and dont know which way to go with this now.
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Old 08-01-2010, 04:37 PM   #17
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Default Re: A/C Low Pressure Switch

Without the compressor running, the pressures should be equal on the low and high sides and read around 100 PSI. The low charge is probably preventing the pressure switch from closing/switching on, which is continuity across the two terminals.
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Old 08-01-2010, 05:55 PM   #18
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Default Re: A/C Low Pressure Switch

So do I need to bring the pressure up until it reads 100 psi on the low side? And not past 100?coz the other day when I checked it, it was pegged and I bled it off, probably too much?
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Old 08-01-2010, 09:29 PM   #19
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Default Re: A/C Low Pressure Switch

not sure if this is going to help you but i had a similar problem.
my a/c would work just fine on cool days, at night or first thing in the morning only. also, the led lights on the control panel for the a/c on/off switch, fresh or recirc modes would go on or off intermittantly if you pushed them.

when it was cool outside the leds would work no problem and the compressor would engage.

my problem was the control unit (the part that sits just above the radio and has all the switches and levers. specifically, bad solder joints for the on/off, fresh, and recirc switches.

you have to remove the unit and take it apart down to the circuit board. there you will find the terminals for the switch. if you look closely and yours are bad it will appear as though the solder has a smalll circular crack around the terminal. i bought a solder sucker for $9 at radio shack and just re-soldered the terminals... 3 sets of 6 terminals.

a/c works great no matter the temp outside.

btw... had this problem on 2 other civics as well.
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Old 08-02-2010, 03:44 AM   #20
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Default Re: A/C Low Pressure Switch

the green a/c light comes on and stays on, no problem. the engine even surges at a constant while the a/c switch is on. theres just no movement of the compressor end disk. i think next i am going to put 12vdc to the red wire to the compressor and see if it kicks on. then put 12 vdc to the condenser fan to see if it kicks on too.
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Old 08-04-2010, 05:36 PM   #21
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Default Re: A/C Low Pressure Switch

Okay, this is what happened today. I put 12 vdc directly to the compressor, no joy. then i put 12vdc directly to the condenser fan and the fan and the compressor kicked on. it was still blowing warm air though. i found hte 20 amp fuse blown for the condenser fan, replaced it took the jumper wire off and everything worked, except it was still blowing warm air until i put a can of 134 in. now everything is great. ???? i swear i checked the 20 amp fuse days ago when i first started working on it. who knows. f...... magic.
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Old 08-04-2011, 06:51 PM   #22
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Default Re: A/C Low Pressure Switch

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Originally Posted by dcraig1000 View Post
My moms car is pretty much doing some of the same things. It isnt blowing cold air at all at idle or at cruise. The condenser fan was toast, so I replaced it with a new one. I knew that wasnt gonna fix the problem but it needed changed anyway. When I turn the a/c on the engine surges but the compressor does not engage so I thought it might be low on 134. I bought one of those big cans of 134 with the gauge, and when I attached it to the low side port, the needle pegged. So I bled off the pressure to the green band, around 55 psi and it didnt fix anything. I am thinking the maybe the high or low pressure switch is bad. Also I can turn the compressor by hand, so its not froze up. Its just free spinning on the pulley. Any ideas????
the low side will build pressure up to over 100 psi when the compressor is not on bleeding off the pressure just wasted freeon. if the engine ( surges) idles up then you know the control unit is good. definatly could be the ac clutch relay or the cycling relay , most hondas have two 1 for keeping the compressor from running too long goes to the timer and 1 that simply turnes the clutch on. use a jumper wire on the 3 and 4 post of teh relay if the clutch engages then its the relay. replace
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Old 08-04-2011, 07:02 PM   #23
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Default Re: A/C Low Pressure Switch

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the low side will build pressure up to over 100 psi when the compressor is not on bleeding off the pressure just wasted freeon. if the engine ( surges) idles up then you know the control unit is good. definatly could be the ac clutch relay or the cycling relay , most hondas have two 1 for keeping the compressor from running too long goes to the timer and 1 that simply turnes the clutch on. use a jumper wire on the 3 and 4 post of teh relay if the clutch engages then its the relay. replace
also to all honda owners that when it gets too hot out and blows warm air, compared to a gm ac system that the condenser is enclosed inside the fan shroud with no air gap inbetween, (creates a vaccume through the condenser) you could make fins or a shroud to go inbetween the radiator and condenser to help the vaccume of the fans, or like i did get some 8' or 10' shallow fans to put on the outside of the condenser blowing in and wire to a switch. i only did this because in town stop and go was unbearable and a wast of gas to even run the ac this was my solution. also in areas with temperatures our ac system will handle up to 50 psi on the low side when cycling. the more freeon the better cool. just be careful when the sun goes down the ac lines and or expansion valve can freeeze up with too mutch freeon and all 4 fans going at the same time so just be sure to turn your custom *** cooling fans off lol.
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