Hi.
Inserted a thermometer in the dash AC vents. In a few minutes 40F cold air comes out, comfortable interior.
Ten minutes later, temperature gradually increases to 70F. Condenser and evaporator fans work fine. This is highway 60mph or city, 90F ambient.
Where do I start diagnostics ?​​​​​​​

 

You'll need to hook up an automotice AC machine to monitor running pressures of the compressor and watch them cycle with the expansion valve opening/closing, and verify proper charge in vehicle

 

+1 for incorrect charge level as the starting point. There are two different fill levels (refrigerant measured in g) based on whether you have rear air or not - make sure that they're filling to whichever is appropriate. Should be on the sticker under the hood - maybe the shop got confused and didn't give you enough, or you have as low leak that has it below spec.

Curious - if you leave it running long enough, it ever get cold again, or is it one time then never again until you restart the truck? I'm guessing that the compressor kicks on initially which gives you a burst of cold air, then takes a long time to kick back on due to a low charge. You should also be able to look at the front of the compressor and see when the clutch kicks in and off - by design it'll cycle on and off. There's a pressure sensor behind the grill that controls the cycling.

 

In both my Rover and in my Accord, the vent temperature cycles up and down as the climate control is constantly adjusting the airflow to achieve the temperature selected. It's not unusual for vent temperature to move up, the question is whether it moves back down again on its own.

 

Thanks.
Nope, will never be cold again unless I stop vehicle for about 15+ minutes.
While driving, have not checked opening the hood to confirm compressor cycling. Will do that pulling to a parking spot, leaving engine on when ceases to produce cold and will post findings.

From what I understand; the compressor disengages as in cycling when pressure rises in normal operation and also disengages if there is lack of refrigerant gas pressure. If there is enough refrigerant gas level, or the evaporator has 'consumed' the flow from last 'on' cycle, the compressor engages again if the temperature control calls for cold.

 

Yes - the switch will cut off the compressor if pressures get too high or too low. It's measuring on the high pressure side of the system. Here are the specifications:


It's also possible that the system is overcharged and, as a result, once the gas heats up due to proximity to the engine, it cuts out.

I'd say confirm that your refrigerant charge is the correct volume before you start troubleshooting electrical issues.

 

Drove in city only, somewhere around 35mph / 1400 rpm. Got 42F out of the vent on 90F ambient. Nice cold.
Reached destination in about 7 minutes and temperature out of vent had risen to 60F. Parked, opened hood and observed compressor clutch at idle : always engaged, never cycled off. Rose idle to 2000 rpm a couple of minutes and vent temperature decreased to ~50F. No cycling compressor, always engaged. That seems logic.
Next try may be freeway, will come back.

 

If your A/C charge seems correct and pressures are in satisfactory range...l'd say it might be your control head, mounted in dash. It may be kicking into what is defrost mode, where the inner vents are opened to run a little warm air over the evaporator to keep it from icing up. If humidity levels are extremely high, icing will happen more often and there is a temp sensor on the evaporator and when it senses cooling is diminished (from icing) it either lets a little warm air travel through/over the evaporator coils or cycles the compressor off. Are you seeing the standard/normal amount of drainage from the ports underneath? If not, then l'd say the evaporator is icing up and not thawing out enough to allow air flow.

Now, it could be overfilled, as mentioned...and it could be overfilled just enough to diminish cooling, without building enough pressure to trigger high pressure switch. Filling R134A is much more difficult than let's say R12. To me, if you were getting 40 degrees out of the vents...at 90 F ambient temp...then you may be slightly overfilled. Usually/normally, depending on humidity levels, you'll see about a 35/40 degree difference between outside temps and cabin temperatures.