A/C help needed

[HISSIN50]

Hello,
i have searched and learned a bit, but did not find anything to really help me out.
need some A/C help. 88 gt Vert. i seem to have intermittant a/c of late.
while driving, if i set the HVAC to a/c, it often just blows hot air. i cant hear if the clutch is locking up. now if i come to a stop (idling), and turn the a/c on, sometimes i have to do this a time or two to get the a/c to kick on (i listen for the a/c clutch to lock up). sometimes i have to turn it to max a/c to get the clutch to engage.

then after about 15 mins of driving, the a/c gets warm. then it gets cool again a few minutes later.

i am gonna try to find an R12 gauge tomorrow to check pressure (old one was lost in a move). but since it is kinda intermittant, i wondered if anyone had any ideas, as i may have trouble troubleshooting since the problem is kinda intermittant.

thoughts i had:

im gonna get it to not work (with a/c on), then check for continuity at the low pressure switch and clutch switch. any help with what to look for and in what order (i would assume LP switch, then clutch switch)?

low R12 pressure, but then why does it work at all? and while revving, shouldn't pressure be greater, and thus kick it on, even if it has low pressure?

a low pressure switch on the fritz? likely?

i know the cycling time of the compressor is adjustable on some cars. i have not touched mine (and dont want to- creates one more variable). could that explain why it gets warm while still being on, then cools back off (sometimes)?

i hope to have more info for y'all, but i would really appreciate any thoughts or ideas or tips.
many thanks.

 

[HISSIN50]

update: (edit: when i refer to LPS, i am talkin about the low pressure switch, in case that is not a proper acronym).

i went out and played. here are some results:
a/c on, but not locking up clutch (no cool air inside car)
low pressure switch: all three wires showing 12 volts.
clutch switch: .5 volts

a/c on, clutch locked up (a/c workin nice)-
low pressure switch - all three wires showing 12 volts
clutch switch- 12 volts

so when the a/c is on, the LPS is giving 12 volts no matter what.
but when it does not work, im not getting 12 volts to the clutch switch.

so where can the problem be (and does this seem likely)? in lookin at Tom's awesome schematic:

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

from the LPS switch, i think the clutch switch feed goes through the WOT relay.

where is this relay in the engine compartment?

any ideas about why this is happening? i would not think the WOT relay would be failing and only allowing the a/c to come on intermittantly.... i wonder if i have a chewed wire that is intermittant.

but since the LPS has juice any time the a/c is 'switched' on (whether or not the clutch locks up), that means i have sufficient pressure/freon (assuming the switch works)? i would think so.

i would love help. i have no idea what is likely to have failed or what is common. many thanks for any ideas (no such thing as stupid ideas when dealing with me. LOL).

 

[shawn13]

sounds like your a/c s low but yes it will kick on at higher rpms I dont have my gauges on it to show you what I mean if it is low I would switch it to 134 any way yes it will work with you stock a/c stuff

 

[PRO50SC]

Your most likely just low on freon. You may have just enough freon in the system to turn on the compressor but not enough to get any cooling. That is the purpose of the low pressure switch. The low and high side pressures work opposite of each other. The more rpm's the lower the lowside reads and the higher the high side reads. As the rpm's increase, it pulls the low side down below the lps's threshold resulting in cycling the compressor. With the propper amount of freon in the system, the lps can not cycle because there is enough pressure in the system to keep it from doing so. The fact that everything works when you turn it on (although not to its optimum) basically tells you all the parts are doing there job. I wouldn't look any further until your sure about the propper freon charge. Hope this helps you out.

PS. Take a jumper wire and jump the lps. If everything is working the compessor will come on and run until you disconnect the jumper. Don't let it run too long like this because the freon also circulates the oil in the system and if there's not enough freon to circulate than the same goes for the oil.

 

[HISSIN50]

thanks for the replies guys. i know i typed a lot, and am not the most 'easy to read guy."

My LPS: whenever the a/c knob on the HVAC is on, reads 12 volts at all three wires.
this continuity is not broken. so it does not seem to me that i have low freon (or else i would lose continuity from the LPS, right?).


i still think i have an intermittantly open wire/circuit. i can start it up, turn the a/c on, and it can take two or three times of clicking back and forth (from vent to a/c on the HVAC control) to get the clutch to engage. as long as the clutch engages, i have decent cooling.
i think that im not getting a good consistant signal to the clutch switch.

does any of this sound right?

Big B, thanks for the help and explanation - knowing that low pressure side's pressure is inversely proportional to RPM helps me understand this stuff.

i will chase wires (if the LPS is working as it appears to, then the clutch should lock up all the time the LPS is working - and im not at WOT). this is not so.

back out i go. gonna go try to find the WOT switch to help chase wires.

i really appreciate your responses.....keep more ideas coming and let me know if my thinking is just wrong....

 

[HISSIN50]

ok, more done:
to me, it appears that the WOT switch loses ground (from the puter?) when the WOT switch is activated (when i rev it and the clutch disengages).....sound right?

the wires could look better, but they seem to function fine. it just seems that my WOT sensing is really sensitive (i hit the throttle linkage while monitoring the relay energizing/ uncoiling..... not a much of a throttle jab makes the relay kick out. relay should be fine - it is just a messenger for the coils wires to it.

lemme hear your thoughts!