I have just picked up a 1999 4.2 XLT from a family friend and have a couple of questions. I was told when I bought the truck that the A/C wasn't very cool. Well when i select A/C or MAX A/C on the dial the temperature of the air coming out of the vents doesn't change, it is not very cold either. I metered the electrical connections behind the temp control **** and the voltage di change from high to low when **** was turned. Whats the first thing to check to see if the a/c is working? Also when I select A/C or MAX A/C the idle rises for a second and drops back down and then rises then drops constantly, is this normal??
Any help would be greatly appreciated

 

its a 99 it takes the 134a coolent can be bought at autozone or advance auto parts our walmart a can might run you $7 fill line $10 to 12 it not that hard to do 2 or 3 cans should do it be ice cold sorry the second thing the ac unit runs off of pressure so if the 134a is low or bad it need recharged that will cause the pump kick on and off so i would try to recharged the system than see what happens for the idle to speed up than down yes that comon with low coolent the pump is kicking on and off add a few can of 134a and see what happensn i would clear up the up and down idle is from the cpu telling that the motor speed up couse of a load and idle up a bit to take place of power loss hope that helps

 

Wow. Do not add 134 *****-nilly unless you like high pressure dishcharges of refrigerant scattered over your engine, your hood & fender, and perhaps your eyeballs and face.
There, a sentence with a beginning and end.
Get gauges or a kit with at least one gauge for around $30. One kit will allow you to check hi & lo but not both at the same time.
There is a suction switch that will disengage the a/c clutch when the pressure falls too low. This is to prevent the evaporator from freezing up. If the compressor was allowed to run unchecked, it would lower the evaporator to below freezing and would eventually block the air flow.
The idle is changing because the ECM 'knows' that you turned on the air, and is bumping up for the expected load of the compressor. It varies because of the cycling and perhaps a slow or dirty IAC.
tom

 

Tom pretty much has it nailed.
A few years back I had an Expedition that did just what you describe.
Rather than buying all the stuff needed to dod a proper A/C check and charge I just to it to a local shop.
They put the gauges on and sure enough, the charge was low which caused the compressor to cycle on and off.
They charged it and all was fine.

 

So I took the truck to a garage today to have the system checked cost me $30 for them to tell me it was low on pressure. Anyway I drove home and picked up a couple of cans from autozone. When I get home to add the cans I turn on the ac and nothing, the compressor has stopped cycling so I tried to add some fridge and it wasn't having any of it. I think the shop has done something to the system to maybe try and get me to go back and pay them big bucks for repairs. What are the first steps in checking the system out??