Rick's Free Auto Repair Advice

Jump a low pressure switch

When it’s ok to jump a low pressure switch

There’s only one reason to jump a low pressure switch

Would you knowingly drive your car with no oil in the the engine? Well, that’s what you’re doing if you ump a low pressure switch. That’s because the refrigerant carries the lubrication oil throughout the system. The low pressure switch cuts off compressor specifically to prevent compressor damage due to oil starvation. Low refrigerant charge = low or no oil circulation.

In other words, you can jump a low pressure switch in a car momentarily to make the AC compressor clutch  engage for testing purposes ONLY. But if you leave it connected too long to try and recharge the system, you risk serious and even catastrophic compressor damage. Seriosuly. It AC low pressure cutoff switchcan damage your compressor in a way that throws shrapnel throughout your AC system. That can easily cost you well over $1,500 to repair. If want to jump a low pressure switch to add refrigerant to your car AC, think again. That’s not the way to do it!!! Here’s why

AC compressors can’t compress liquids

As refrigerant flows through the evaporator in your dash, the heat causes the refrigerant to boil and change from a liquid to a gas. Depending on the type of system in your car, the gas exits the evaporator and goes into either an accumulator in an orifice tube system or directly into the compressor in an expansion valve system.

However, even with an accumulator in the system, a small amount of liquid enters the compressor. It’s designed this way specifically so the liquid refrigerant can carry lubricating oil into the compressor. The issue that comes up when you jump a low pressure switch for longer than a few seconds is that you’re running the compressor without oil. That will destroy it.

Jump the low pressure switch only to test the circuit

Low AC pressure will prevent the relay from sending power to the AC compressor clutch. If you jump the low pressure switch with the engine running, do it only for a few seconds. Do NOT jump the low pressure switch to operate the compressor non-stop so you can add refrigerant. I’m not kidding about this. You will damage the compressor.

How to add refrigerant when the AC compressor clutch won’t engage?

When you turn off a car AC system, the pressure difference between the high and low sides equalizes after a while. How can the pressure equalize if the compressor isn’t running? Simple. The orifice tube or expansion valve continues to release liquid into the evaporator as the car heats up. That liquid changes to a gas and flows into the compressor and out whichever compressor reed valves are open at the point. There’s always an opening between the high and low sides when the compressor is off.

So you can add refrigerant to a system even when the

jump low pressure switch

Fill with refrigerant until gauge reads above 25-psi

compressor clutch won’t engage. It’ll just take a lot longer. To speed up the process, warm the refrigerant can in a bowl of warm water. That will boil the liquid and increase the pressure. Swap out the water for more warm water once it cools. Repeat this process until the gauge on your recharging kit shows a pressure above 25-psi. At that point the low pressure switch should allow the AC compressor to engage.

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat

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