hi all. I have a 1990 f150 4x4 5.8 auto trans w/overdrive.My a/c doesnt get cold . maybe a little cool but not much. The compressor and /or clutch cycles off and on. I went and bought a retro kit and put the fittings on (so i could test the pressure) its about 60 degrees outside,and the gauges read about 50 until it cycles then it drops to about 25-30. If i open up the valve to let the 134a into the system(iknow its supposed to be r12) its jumps up to 70-80(in the danger zone) so i quickly close it off and remove it. Any ideas?I bought the truck about 4mnths ago so i dont know how well the a/c ever worked. thanks for any help.

 

oh and i am hooking it up to the low pressure side up by the fire wall .

 

Is it R12 or R134a in the system?

Also if your using the gauge that comes with that little canister retrofit kit, junk it. Get a A/C manifold gauge set, can pick one up from summit for under $50.

and with the AC @ 60 degree ambient temp the pressures are as follows

High Side pressure :
120 to 170 psi.
Low Side Pressure :
(STV,POA, or VIR systems) 28 to 31 psi
(Expansion valve systems) 7 to 15 psi

@ 80 degrees ambient temp

High Side Pressure :
180 - 275 psi
Low Side Pressure :
(STV, POA, or VIR systems) 28 to 31 psi
(Expansion valve systems) 7 to 15 psi.

 

Ok heres a dumb question. I know the high side and low side but where is the expansion valve system?

 

An Expansion valve is a temperature senstitive valve that controls refrigerant flow and the evaporator temperature.

The location is just before the the Evaporator it looks like a pancake on top and it has a upside down L shape valve when viewed from the side. Your high pressure port charging system could be located just before it. But its mounted after the Receiver-drier and the hose from there goes to the Expansion valve and then the other end goes to the evaporator.

 

Is this the same as a high pressure switch/valve that is electrically controlled? Ive got another thread going about my moms monte carlo and this thing is kickin my az.

 

They wouldnt be electronic. They use a temperature sensing bulb and a fluid that expands and contracts with temperature.

Your probably thinking of the Expansion Tube system, It controls refrigerant flow and evaporator temperature by cycling the compressor on and off. Basicly its an Orifice tube.

 

Ohhh that does cycle the compressor on and off hmmm interesting. Well I will keep that thread updated and I will let everyone know what happens.

 

An expansion tube isnt electronic either. Im just saying all systems now adays are expansion tube. They have either a pressure cylcing switch that turns the compressior on and off with refregerant pressure, or a teperature sensing bulb

 

The pressure cycling switch is located in the top of the Accumulator (on most new systems the Accumulator is where the low pressure port plumbs into the side) Now with this switch, if you have NO refrigerant or very LOW refrigerant in the system, the clutch will not engage on the compressor, you can even hae an orifice tube that is clogged up and the compressior pulls a vacuum on the suction side which is where the pressure switch is located at and it can also fool the switch into cycling the compressior off. It also works in the sense if the pressure gets too high it cycles the compressor off. Some new cars have another feature that if your engine starts over heating, the clutch will cycle off to stop the engine from over heating. And even more so there is yet another system that when you hit the passing gear the cluch kicks out.

The big thing to remember is that on your car which is most likely computer controlled, the computer might be the problem. When you click the AC button, your making a request to the computer that you want A/C. The computer looks at the ambient temperature sensor (measures outside temp) and looks at the engine temperature (if over heating it wont engage the clutch) with the Ambient temperature sensor if its below 70 degrees, the computer wont turn the clutch on on the compressior. Your problem can be as small as a computer sensor being faulty or even worse, a computer that isnt carrying out your request when the engine isnt over heating and the ambient temp is above 70 degrees.

 

Ive had the someone mention the sensor on top of the accumulator. Unfortunately in 96 gm didnt think the monte carlo needed one. The switch Im dealin with right now is between the accumulator and the firewall and I think it does double duty. My 99 F-150 has the switch on the accumulator and the one the line so I dont have much for reference around here. The ol' computer issues are on the back of my mind but Im hopin I dont have to go there.

 

well that is probably the pressure switch. They are usually located atop the Accumulator on GM but sometimes their not. I have a 79 elky that has nothing on the Accumulator.

The pressure switch will be located somewheres between the Evaporator and the compressior in the suction hose. Thats the only place that it can control evaporator temperature.

With A/C systems when pressure goes down temperature goes down. When pressure goes up, temperature goes up. Thats how its albe to cycle the clutch to keep the evaporator at the right temp cause it measures the amount of pressure in the suction line.

But it still remains if the A/C system had a leak in it, and it hasnt been repaired or have the right amount of refrigerant, you wont be able to get the Compressor to engage unless you trick the clutch into thinking the pressure switch is saying that pressure is higher than it should be and to turn the compressior back on to get the temperature back down as close to 32* as possible

 

Hmmm so if theres a bad leak the compressor wont engage? The only way I have gotten the clutch to engage so far is to jump the relay or run 12v directly to the compressor. This is some good info!

 

If theres a leak, there wont be enough static pressure (pressure without the compressor engage) to engage the compressor. Its a safety feature, cause lubricating oil is only flowed through the A/C system with the refridgerant. If there is no refrigerant there is no lubricating oil and you can destory the compressior so the factorys put a fail safe in that if there isnt pressure in the system it wont engage.

Static pressure should be in the area of 50 psi on the high side. If you have 1/4 charge you would have 12.5 psi and 1/4 charge isnt really enough to circulate the lubricating oil efficently. You will be pumping the refrigerant and create a vacuum effect in the suction side cause you dont have enough refridgerant and you start pumping nothing then to pressurise the highside.

 

That makes sense. Ive tried to add some freon and it takes only what the bottle pressure will give out even with the compressor running its like it just wont pull anything from the bottle. I doubt I can get to 50 psi but it does seem to hold what little pressure I can get in there.