Ok now let me start this out by saying that I am a mechanical engineer with a little background in applied thermodynamics. I am trying to get my trucks ac system working and have been having a problem. I can not get it to blow cold through the vents. I vacuumed the system for 30 min before adding the proper cans. After adding the proper amount of 134, 1.7-1.8lbs, I cannot a) get the system to blow cold and b)get the compressor to work under its own power. When adding the cans of 134 I jumped the compressor wire to the battery to get it to pull in cans faster. After adding the proper amount of refrigerant the pressure was only 20psi on the low side and 100ish on the high side with the compressor engaged. Any help in this matter would be greatly appreciated. Also the system does hold coolant and all fuses behind the glove box are good. I changed the relays with one out of a truck with working air with no change in symptoms. Thank you for your time. Derek C

 

it sounds like it is undercharged. are you sure the cans of refrigerant are drained? maybe a bad compressor? what truck do you have? put that in your signature so that when you ask questions people can easily see what vehicle you have

 

I have a 95 4runner. I know the compressor is not bad because I can watch it engage when I jump it to the battery. I know it is not undercharged due to the fact that like a AFC(average frustrated chump) I put in 4 cans to see if it would start to work. Now I pretty much know it is overcharged and will need evacuated. Also the motor was just put in the truck by myself. When I disconnected the old compressor the system had a full charge. The motor swap was done in a shop and in one weekend and made me assume that no debris would of gotten in the hoses/lines. I'm leading towards some debris may have gotten in the line causing a restriction and now I will need to flush it and re vacuum it out. Was just looking for a second opinion before having a go at it.

 

with what you did in your first post, you should have had a perfectly working system. most people do this without a vacuum and thats a big no-no. your pressures are a little odd, but it depends if the comp. was running or not. heres about where your pressures should be. probably about 100 lbs on the low and high side, static. (comp not running) when the compressor is running, about 20-40 on the low side, but you should be closer to 200 on the high side. so....if the pressures you stated where while its running, your high side is too low, probably the reason it doesnt work.

the reason its not coming on by itself - first thing i would look at is the low pressure switch. thats what basically turns the compressor on and protects it when it gets low on refridgerent by shutting it off. this item should be located on the filter/drier. if your controls are set to a/c with the engine running, there should be 12 volts running through both sides of the switch. if you only have 12 volts on one side, its most likely a bad switch. no 12 volts, well now you should make sure all fuses good, and you are most likely going to have to go behind the dash and start at the a/c switch and follow the circuit until you find the faulty component. the circuit should kind of run something like this - fuse, a/c switch, fan switch, and possibly a resistor type deal in the fan box, low pressure switch, a/c compressor.

if you only have 100 lbs on the high side when its running, i would say you have a faulty expansion valve or orfice tube. actually your compressor could be bad as well. just because it engages doesnt really mean its functioning properly. i would guess this isnt a restriction. if it were restricted i would expect higher pressures.

my knowledge of a/c is good, but my knowledge of toyota's a/c is minimal, cause mine works just fine. lol. if you have a mack or peterbilt, dont hesitate to bring it to me.

 

Unfortunately, the pressure switch is buried in the underdash module. However, it is possible to jump it at the connector behind the glove box.

Just because the compressor engages when jumped, does not mean it's not bad. Did the a/c blow cold before the engine swap?