High A/C Vent Temperature for 1999 Dodge P/U

Maybe the former owner would know what the problem is. Have you asked?

Have you checked the specs? What's it supposed to be? If it's 110°F outside, maybe it is OK.

This is another case of TLI (too little information).

The former owner purchased the truck new (1999). He observed that, recently, the air wasn't blowing as cold as it had been; but was under the impression that the system was low on coolant. I haven't been able to find the specs yet. Still looking.

Is the AC compressor kicking in or is it cycling on/off quickly?,

It could be either low gas, or the hi/lo pressure switch is faulty.

Is the drive belt hooked onto the compressor?

Sorry I'm asking generic questions I'm not directly familiar with your vehicle and you haven't detailed all the symptoms.

The compressor doesn't appear to be cycling in/out unusually quickly. We checked the coolant and observed that ir is reading 40-45 pounds, which is supposed to be within specs. Drive belt in properly connected and is in good condition.

Another potential problem is the air mix door. Automotive air conditioners have a door or damper that allows part of the air, after passing through the evaporator coil, to go through the heater coil. This is controlled by the temperature control, often a slide control. In the 'Cold' position, the damper should be completely closed. If the control linkage binds up, a seal strip comes loose, or anything else keeps this door from completely closing, some of the cold air will go through the heater core and the air exiting your vents will be warmer than expected. As you can imagine, it doesn't take much of an air leak here to make a substantial difference.

40PSIG is 45 degree F saturated boiling temp for R134A. Have you fuddled with the charge? I'm not sure of the metering device used, but most likely a pressure restricter (for this age of truck). What is the high side pressure/temp? Assume it is 20degrees above the outside temp relative to saturated condensing temperature. If it is high, there could be non condensables in the system or overcharge of refrigerant.

On cooling side, 20 degree F rise of air temp to evaporator temp. is about normal, could be a little cooler.

But trucks typically do not have recirculate feature like cars do, so if the outside temp is high this is probably working as good as it can.

it does't have a txv it has an evaporator pressure regulator

Back a long time ago they used pilot operated absolute suction valve. That was in suction line to the compressor to prevent evaporator freeze up. Memory was these were used with a TXV that controlled constant superheat.

Without high side pressure, the OP is not giving the information needed to troubleshoot the system. Either non condensables (ok it's air) or overcharge can be cause high suction pressure. Or a failure of the metering device. Or as Lyn suggests, too damn hot.

I haven't owned a car with AC since my 1962 Pontiac grand prix, but now I have a 2013 Nissan Versa, and hope I never have to fix it. I did install an aftermarket in a 98 Honda Civic CX, but all it had was a TXV.

So where did the OP go with more information?

I'm flying absolutely blind when it comes to the technical aspects of the air conditioning system. I really appreciate the help you folks are providing. I'm checking back with the man who put the test gauges on the truck, but I believe the readings were 40-45 on the High side and 20+ on the Low side. I'm doing more online research to see what the pressure readings should be for this truck.

Now your giving totally confusing information.

I thought the 40PSIG was the low side pressure (evaporation pressure). The high side pressure will tell us the condensing temperature.

Be accurate.

Some owners inject a sealer to stop AC leaks and this gums up the works.. Had it happen couple of times. Have a AC tech run some pressure checks it will be very inexpensive in the long run.

What is the gas used.If the system is with R134a the suction pressure has to be 16 to 20 PSI since R134a boiling point is -26.3 deg C.

In your system the expansion valve is clogged with air, what happens when air in the system the discharge pressure will become high and air will start freezing in the expansion valve. To avoid this evacuate the system with a vacuum pump upto 30 Hgmm,replace the filter drier, charge new gas until the required pressure reaches and the liquid indicator gets cleared with slight bubbles randomly. Pls do not over charge the system system with gas. Before attending the repair please thoroughly clean the condensor and the evaporator with a aid of a pressure washer.

Another possibility is that air flow to the condenser unit is obstructed in some way. Er, check for dead squirrels in the duct?

We need to know the outside ambient temprature....humidity..and both the high and low side pressures......vent temperature as well....taken at the same point in time...

We can't make any guesses without all that information to know if its ndercharged or overcharged..or is in an acceptible range.

The outside temperature, when I checked the vent temp, was 105 degrees. Humidity was approx 20%. Vent temp was 60-65 degrees while sitting still. Have not checked the vent temp while driving. The Freon pressure was 45 on the High side. Not sure what it was on the Low side.

Your system would be using R134a. The 45 pressure (we are talking psi, right?) would be on the low side of the system. At 45 psig (60 psia), the evaporation temperature is 50 degF. At 105 degF outside air, the condenser (high side) would be at 135 psig minimum, probably closer to 170, or even higher. If the high side were really 45 psig, the system would be seriously low of refrigerant, but you would also be getting no cooling. By the way, how hot was the interior of the truck during this time? I would say that an idling truck, sitting still in 105 degF air, that is still able to maintain a 45 psig low side pressure is doing pretty good (although the temperature of the refrigerant leaving the evaporator/entering the compressor would be useful information to have). My previous post mentioned the air mix door. Although another poster dismissed an outside air door on pickup trucks, I think yours may have one. If there is a 'Recirc' setting, it does. If this door is open or not fully closed, it allows outside air into the vehicle. As you can imagine, it doesn't take much 105 deg air to overload the system, especially at reduced capacity due to idling and standing still.

Assuming the previous owner is being up-front, I think you can dismiss air in the system as a problem, mentioned by previous posters. Recent automotive A/C systems have a pressure switch on the low-side which turns the compressor (ok, the clutch) off before a vacuum is pulled. Unless the switch has malfunctioned or someone has allowed air in while servicing the system (adding refrigerant, replacing a hose, etc.), which he indicates hasn't been done, there is no reasonable way for air to have gotten in.

The pressures I gave are on startup, after running for a while they should both drop to around ~35 and ~195lbs respectively....

here's a helpful video...

http://www.youtube.com/watch?v=80jSH8VQZms