Rick's Free Auto Repair Advice

Why car AC won’t cool

Most common reasons car AC won’t cool

Top reasons car AC won’t cool

When DIYers complain that their car AC won’t cool, it’s usually after they’ve tried to add refrigerant from a DIY kit. In their mind if they’ve added refrigerant and the gauge on the can is in the green, they should be getting cold air. In the real world, things don’t always work that way. So let’s take a look at the most common reasons your car’s AC system won’t cool.

Car AC won’t cool due to air in system

DIYers forget that these are sealed systems. If the AC system is low on refrigerant, it’s because a seal has been breached and refrigerant has escaped. Simple enough, right? Just add refrigerant. BUT, when you have a leak, you also get air in the system, and that can cause BIG problems. Look at these AC facts:

• A car AC with 2% or more of air will not cool properly, no matter how much refrigerant you add

• For every 1% of air in the system, you lose 1°F of cooling efficiency.

• When you reach 6% air, the system will stop cooling and possibly cause evaporator freeze up.

• If you don’t purge the air hose on your DIY recharge kit before you connect to the charging port, you will be adding air to the system (the air that was in the hose before you squeezed the trigger).

That’s why professional shops insist on finding and fixing the leak and then pulling a complete vacuum to remove all the air before recharging. If you leave air in the system, you’re going to have problems.

Car AC won’t cool due to overcharging

DIYers often add too much refrigerant when recharging their car AC. But R-134a refrigerant is VERY sensitive to overcharging. Just a 2-oz. overcharge is enough to dramatically drop the cooling efficiency of your system. Gauges can only give you a rough idea of charge level. But the ONLY way to get an exact charge is to evacuate the system and add refrigerant by weight, not by gauge readings.

Car AC won’t cool due to sludge

Air carries moisture into your car’s AC system and that moisture combines with the refrigerant and oil to form sludge. The sludge coats the inside of the evaporator and condenser fins, reducing heat transfer efficiency resulting in a significant drop in cooling. The only way to remove the sludge is to flush the system and replace the accumulator or receiver/drier.

Car AC won’t cool due to plugged orifice tube

Car AC systems are use either an orifice tube or expansion valve to meter refrigerant into or out of the evaporator. An orifice tube, as the name suggests, meters liquid refrigerant into the evaporator through a small orifice. Over time, debris from hose deterioration, desiccant from a ruptured desiccant bag, sludge from moisture accumulator, leak sealer or metal particles from compressor wear can clog the orifice and prevent refrigerant from flowing into the evaporator.

AC orifice tube

New and clogged AC orifice tube. Clogged tube shows evidence of “Black Death.”

Most orifice style car AC systems use a fixed orifice tube with a factory specified orifice size. However, some car makers and shops install a variable orifice tube that varies the size of the orifice based on temperatures.

Since the orifice is always allowing refrigerant to flow into the evaporator, an orifice tube system regulates cooling and prevents evaporator freeze up by cycling the AC compressor clutch. These systems are also referred to as “cycling clutch systems.” The car maker installs a low pressure switch near the outlet of the evaporator. When pressure falls below 20-something psi, the switch opens and the AC compressor clutch disengages. The compressor stops compressing refrigerant and the liquid pressure drops. Since cabin air is still blowing across the evaporator, the pressure near the low pressure switch increases and the switch closes, starting up the AC compressor again.

The only way to fix the problem is to flush the system, replace deteriorating hoses, replace the accumulator or receiver/drier, replace the orifice tube, add oil and recharge with refrigerant.

Symptoms of a clogged orifice tube

Car cools when AC is first turned on, but stops cooling. Why—the debris caught in the orifice tube screen falls off the screen, allowing refrigerant to flow. As refrigerant pressure builds, the debris gets caught in the screen and reduces refrigerant flow. OR, the debris never falls off the screen. In that case, you get a bit of cooling when you first start the AC, but the flow is so severely restricted, the AC can’t keep up with cooling the cabin.

The low side pressure is low. Why? Because not enough refrigerant is entering the evaporator. If you’re a DIYer and you’re using a kit with just a low pressure gauge, you’ll think it’s low on refrigerant and try to add more. It won’t help. If you have a manifold gauge and could see the high side pressure at the same time, you’d see that the low side pressure is too low and the high side pressure is too high.

The compressor is compressing the refrigerant. That increases pressure in the condenser. But the blocked/plugged orifice tube won’t allow enough refrigerant to flow. So you get high high side pressure and low low side pressure. Add more refrigerant to this system and you’ll just make the problem worse. In fact, in some systems that don’t have a high pressure relief valve, the extra refrigerant can cause the compressor to seize. The symptom? A screeching AC compressor belt.

Car AC won’t cool due to faulty expansion valve

A block style expansion valve meters the flow of refrigerant through the evaporator

clogged expansion valve

Clogged expansion valve

by varying the size of the orifice on the OUTLET side of the evaporator. It works by sensing the temperature and pressure of the exiting gas and moves a tapered needle/rod in or out of a small opening.

A capillary style expansion valve meters the flow of liquid refrigerant into the evaporator the same way but the valve is located on the evaporator inlet and uses a remotely mounted sensing bulb connected to the evaporator outlet tube.

An expansion valve can fail in three ways:

The Sensing bulb loses its charge. Once that happens, the tapered metering rod won’t move and won’t allow refrigerant into the evaporator.

The metering orifice gets restricted/plugged with debris, leak sealer, deteriorating hose debris, sludge or metal particles .

Sticking due to wear. Unlike an orifice tube system, an expansion valve has moving parts that can wear and stick.

Symptoms of a failed expansion valve

No cooling or too little cooling – clogged orifice or stuck tapered rod

Compressor clutch engages and immediately disengages. Pressure builds rapidly and high pressure switch disengages AC compressor clutch.

Sensing bulb has lost its charge

Low side pressures are too low and high side pressures are too high.

©, 2017 Rick Muscoplat


Posted on by Rick Muscoplat

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