Ac Recharge Question

[aikitect]

My AC is not blowing cold air.

I have a R-134 AC recharge kit that has a gauge attached. I hooked up the gauge and discovered something interesting:

The pressure starts at about 10 psi (needs charge) and goes to about 40 psi (full charge). There is a fairly loud click sound and the pressure drops back down to 10 psi, goes to 40 psi, click, 10-40-click, etc., etc.

Each cycle lasts about 3-4 seconds.

I did not add any refridgerant - I wanted to check with yopu guys first to see if:

1. I need to add freon so that the lowest part of the cycle is in the charged range

or

2. Freon is not the problem and I need to have a professional take a look.

It worked fine last year, but I had my tranny replaced over the winter. This is the first it has been warn enought for the climate control to have to really cool the car, so there is the possibility it is related to the tranny change.

Thanks

[BillAileo]

You are probably hearing the compressor turned on and off by a low pressure switch. The following discussion is pasted from http://members.tripod.com/~jbabs714/autoac.htm. You might want to read more at that site.

The most common problem is that the refrigerant (Freon) leaks.

When the system does leak, the first thing that happens is the a/c does not work well when you need it the most (hot!! summer day). At night it will work better. What causes this is a pressure sensor that turns off the compressor if the pressure drops below the pre-set setting of this switch.

The low side of a working car a/c should be around 40psi. when the outside temperature is hot (summer).

As the system looses more and more freon the low side pressure will drop more and more. The switch will cut off the power to the compressor when the pressure drops to about 25-30psi.

When the compressor stops the high pressure side will bring the pressure up as the system equalizes. The switch will sense the pressure rise in which the compressor will come on again. Then it starts over again.

The less freon there is the more the switch will turn the compressor off/on/off/on ect.. After a certain point the a/c will not work at all.

This low pressure switch is there to 1: Regulate the compressor. As the temperature of the air in the car gets cooled the pressure on the low side drops, if you run the a/c with the fan on low the cold coil can get so cold the water condensat will freeze up the coil. 2: The switch will keep the compressor from running if the system gets low of refrigerant.

Some vehicles use a switch on the high pressure side, it does basically the same thing. Except it senses the high pressure and will shut down the compressor when the pressure goes to high or to low. For Saftey.

[aikitect]

Great stuff.

It looks like I need to go ahead and add some of the freon.

Having never done this before, I wanted to make sure I did not do anything stupid.

Thanks for your help.

[MadeInJapan]

It looks like I need to go ahead and add some of the freon.

+1 on that.

Don't be afraid to add gas, just be careful of your fingers near the Serp. belt as it spins very close to where you add it. From what I understand, the pressure from the cans is not great enough to create a dangerous amount of pressure in the system. It's not like you're using a big tank of the R134-A, like the pro shops do.

I had to add a can and a half the other night. Did the same thing last year about this time. No need to worry, but don't get any of those crazy cans that are for high mileage, sealing leaks, etc. Just get the regular gas. It's cheaper anyway. Add a can and see how it goes. measure the pressure (It should actually stablize between on and off of the compresor to the point that it's the same when car is on and off). You'll be amazed that the clicking of the compressor going on and off will slow down considerably. Mine pretty much quit doing that altogether after I got the temp. in the cabin's air vent down to 40degrees. Just turn everything on high/recirculating in the cabin, close all vents except the center one and blower on high. Let the car continue idling as you add, and through the center vent measure the temp. with a thermometer. You're good to go when you get the air temp. exiting the vent down to 40, and you can turn the car off, screw down the opener to the gas can and unhook the hose to the low port of your A/C system.

Hint on adding the gas: If you place the can upright in a warm bowl of water as you add the gas, it will go in faster.

Good luck and let us know how you do.