Low Pressure Switch Lockout

[threewood]

Help me understand the process. Home A/C system is not blowing cold. Breaker is good. Inside fan runs, filter is new. No iced up lines. Condenser fan not running, capacitor is good, fan spins freely.

I pull the cover on the condenser unit and the display light on the circuit board shows red light (low pressure switch lockout). OK, might be low on refrigerant.

So I started thinking....how can a tech hook up a manifold gauge set to check pressures if the compressor isn't running? And how does he add refrigerant if the compressor isn't running?

I don't want to dig into the issue of finding and fixing a possible leak. I'm just curious as to the process. Google is worthless.

 

[American Locomotive]

The pressure ports always work. If the system has a low pressure switch, there needs to be a certain minimum pressure in the system for the compressor to turn on. If they pressure in the system is below that threshold - the system is probably almost out of gas. At pressures that low, the natural pressure in the bottle is enough to push gas into the system. Once the system has enough gas to satisfy the low pressure cutout, the compressor can start, and you can then continue to feed more gas in.

 

[PoorUB]

So I started thinking....how can a tech hook up a manifold gauge set to check pressures if the compressor isn't running? And how does he add refrigerant if the compressor isn't running?

Without the unit running a tech cannot get a good pressure reading, it has to be running. If I showed up to check your AC I would reset the low pressure switch, get it running and see what it is doing.

The tech can also just force the contactor on, but the low pressure side, with the unit off, will be higher than operating pressure. The low pressure tripped while running, unless the switch is faulty. A R410 unit will run around 100 PSI low side when running. When it is off, it will be well above that, maybe 200 PSI, but t will vary.

As far as adding refrigerant, you can to a point, but again, you need it running to get a proper reading.

 

[threewood]

Without the unit running a tech cannot get a good pressure reading, it has to be running. If I showed up to check your AC I would reset the low pressure switch, get it running and see what it is doing.

The tech can also just force the contactor on, but the low pressure side, with the unit off, will be higher than operating pressure. The low pressure tripped while running, unless the switch is faulty. A R410 unit will run around 100 PSI low side when running. When it is off, it will be well above that, maybe 200 PSI, but t will vary.

As far as adding refrigerant, you can to a point, but again, you need it running to get a proper reading.

Thanks. It's a 410a system. There is no reset on the switch. Do they jump the wires to get it running?

I'm not certain if it tripped while running. We had a good snap of cooler weather so we shut it off, had the windows open. We turned it back on and I didn't notice whether the compressor was running, the blower turned on though.

 

[PoorUB]

If there is no reset, killing the power to the unit and turning it back on may clear the control board.

Anyone's guess why it tripped at this point.

 

[threewood]

I've pulled the outside breaker to the unit and turned the thermostat off but that didn't work to reset the switch, if that is the issue. I kind of want to test the switch which will either tell me it is closed (working properly) or open (faulted because it is operating as intended). I'm going to call a tech tomorrow. I just really like delving into problems to see if I can figure it out. I don't have a gauge set or R410 so I really can't do much but test.

I'm curious how often circuit boards go bad? This unit was working great all summer, turned it off, turned it back on, then it wasn't working. Unless there is a slow leak somewhere. I can't imagine it dumping all the refrigerant overnight.

I am stocked up on capacitors waiting to save the day but mine have been rock solid, of course.

 

[cvairwerks]

On mine, when the LP sensor trips out, you have to pull power for at least 1 minute, then wait for 5 minutes for the board to reset after power is restored.

 

[chris142]

Thanks. It's a 410a system. There is no reset on the switch. Do they jump the wires to get it running?

I'm not familiar with house AC but on cars we do jump wires temporarily to get a compressor to come on . I assume a house AC is similar

 

[threewood]

I was at work when the tech showed up. System took 1lb 10oz. I guess he went by his gauges for top off, not total weight. Complete system takes 11.4lbs. So any leak hopefully is small since this system is 5 years old but who knows. It's working as it should now. Thanks.