My girlfriend was saying her a/c was blowing hot air the other day.So today I put a guage on the low pressure and idling with the a/c on max it was slowly cycling from 45# up to 65#.At 65# the a/c fan would come on and the pressure would drop back down to 45# after five or six seconds.Car was warm and outside temp was about 85*. Is this typical or does it indicate some issue? The car was blowing cold air. It's a 2000 with r134a.

 

You have an issue if the low side pressure goes up when compressor is on and then goes down when condensor fan comes on, low side, [vapor/suction] pressure should drop when compressor is on and stabilize at around 10-20 psi.
It sounds like you have the gauge on the high side, [liquid/discharge] that pressure will go up until condensor fan comes on, not sure what 45# and 65# mean but high side pressure will climb to about 100 - 150 psi and the condensor fan will turn on and high side pressure will stabilize at whatever the the psi is when fan turns on.

Without a proper set of A/C gauges it can be very hard to tell what may be wrong, if the compressor turns on and continues to run but air out of vents is not cold it can be a compressor problem, a low refrigerant charge or a defective expansion valve/orifice tube.
A very low charge would result in "fast cycling", compressor would turn on for a few second then turn off then back on for a few seconds and off again and so on.

The other possibility is the blendoor is not moving over to direct air through the evaporator or the valve on heater feed line is not closing, [depending on system used].

If the compressor turns on and stays on, run the engine with the A/C on fro a few min, the check the temp. of the suction and discharge lines coming off the compressor, CARFULL, one line should be very cold and the other can get very hot.
Let me know what you find, also MM&Y of car will help. 94

 

Car is a 2000 Mercury Mystique (Ford Contour).Guage is on low pressure side.
# is my lazy way of saying pounds or psi.The compressor seems to stay on the whole time I was working with it.It was blowing cold air the whole time I was working with it.I guess I just expected the pressure to remain more stable at idle.

 

The low side pressure should stay stable as long as compressor is on, it will drop when compressor turns on and go back up if compressor cycles off and down again when compressor cycles back on again.
Again, your gauge is acting like it is on the high side. 94

 

bump cuz my accord is being retarded too, i got one of these gauges http://copronymous.com/wp-content/up.../07/ac-007.jpg

and stuck it on the low side with the car off and the end of the gauge shot out air and the needle is stuck almost at the red zone its in the caution area

then i tried it with the car on, no difference with the gauges, did i break the stupid little gauge?

mine tends to work well and cold but only in the morning the first time i drive the car, later in the day after ive stopped and parked a few times it doesnt work for ****

halp! this is annoying cuz i have leather seats so without ac my back gets all sweaty FTL

 

how many psi does that guage go to? With the vehicle off, the pressures equalize in the two sides (high and low) and both wind up reading about 80 psi (70-90 psi, depending on descent charge level and variations in systems). Might be under-charged just enough to where the condenser can only exchange enough heat when the air is cool. Hard to say for sure though.

And to the OP, when you say the A/C fan came on, were you basing this on sound or visual? Not to say your incompetent, but sounds like you thought the compressor cycling was the fan (usually the A/C fan is on the whole time the system is cooling). Without knowing the high side pressure, I can't be too confident, but if the compressor was cycling fast, or not cycling, the system is probably low on refrigerant.

 

Low charge wont give such a high low side reading. Sounds like an expansion valve that isnt restricting enough. Id like to know outlet vent temp, ambient temp and high side pressures.

 

Could also be a cracked reed valve in the compressor. Or at least one that isnt shut all the way. Does the gauge needle bounce a lot?

 

A single gauge on the low side, [or high side] will not tell you much, you need a proper set of A/C gauges if you are going to try and diagnose an A/C problem with gauges, you need to see what both the high and low sides are doing when compressor is on and off. 94