A/C recharge?

Thank you, I love this site BTW.

Does anyone know where this is and how one tests it?

It is hot this week and the air conditioner doesn't work at idle. When driving the air is cold until I slow down in traffic and then HOT air comes from the vent. Revving the engine restores the cold air.

At idle the clutch is not engaged. If I rev up the engine a little the clutch engages and the the air coming from the vents is really cold.

I was going to add refrigerant, but when the compressor is running (above idle) the can gauge indicates that low side pressure is okay.

When the compressor isn't engaged, the low side pressure goes up to over 120 psi.
I bought this truck two years ago, and this is the first time I have noticed a problem.
This truck doesn't have a separate fan for the AC, and I verified that the radiator fan is turning.

At idle the compressor clutch is disengaged and the low side pressure rises. That is enough that when I increase engine rpm the compressor clutch pulls in.
The high side pressure should increase with increasing rpm (?) So that shouldn't be keeping it from running at idle. The high side pressure should be at a minimum when the compressor clutch is disengaged, so that shouldn't be keeping it from running at idle.

I checked the sight glass on the receiver and it looked normal to me. Turbulence with no bubbles when the compressor is running, placid when it is not.
So I think that leaves the thermocouple (thermistor?) And the interaction of the engine control unit and the compressor control.
I tried an experiment; I raised the idle ever so slightly (I've been wanting to do that to get it off the rough idle it had when I bought it) and the compressor immediately came on. I think that my idle is still in spec, but I'll have to get my hands on a tach to check it.

That's a good enough fix for the remainder of the heat wave, but I want to understand more about the compressor controls.

Thanks for the insights. They lead me to a workable solution.

My Ac works good between 25 and 45 miles per hour and after that it turns off or stops working and I get nothing but warm air. I twice drain the AC system and added the proper amount of R-134a using a gauge and check it well. My car takes 1.5 # of R-134a. I do not understand why it works at slow speeds only. If I drive fast and I no longer feel the cool air coming out, I slow it down and the Ac gets very cool again.I am very puzzle over this. I could use some advice. Thanks

(not hot but warm enough). At idle engine overheats and heater blows cold. 4 cyl 22R 2.4L 5sp with 246k miles. Recent tune-up, O2 sensor, and complete exhaust with new cat. Replaced radiator, cap, hoses, thermostat and water pump. Thermostat opens, coolant circulates, and coolant level stays full. Fan appears to be working properly, and the fan clutch looks new from previous owner(shiny & I can still read the codes printed on it). I'm stumped- any ideas?

I flushed the entire system yesterday, took the (new) radiator out and flushed it as well. All the hoses are new and none are collapsing. I put it all back together and tested the system from cold start. Same results- coolant starts circulating at 195 degrees, fan clutch engaged, engine overheated as heat blew cold. The only other thing I can imagine to be the problem is the head gasket, but I don't want to go tearing into the engine if that isn't the issue.

A bad head gasket (or cracked head) will show up in a compression check? Sorry, I don't have experience in this.

I have a junkyard compressor. Tag says NipponDenso 10P15C, R12. I need to find out what type of oil/viscosity and how many ounces to put in the compressor and other components for a upgrade to R134a. Is it true I should use 75% R134a Freon versus R12? What should my ac gauges read? Thanks

No it won't show up in comp test. If your losing heat out of vents means air is building in heater core. Assuming you burped the system correctly you have a blown head gasket.

Thanks so much for the good info.

Hi, can you tell me if I should buy an $82 vacuum pump or use the venturi vacuum device I already have? Thanks

Hi, how can I find out which PAG oil to use? 46 or 100 viscosity?

Hi, I found a chart and with my compressor 10P15C NipponDenso it says use 46 PAG oil. Do you agree? Thanks