Central air unit compressor won't run. low side is 120+ psi and high side is 178 psi. what reasons would cause this.

CHECK REFRIGERANT CHARGE. USING A MANIFOLD GAUGE SET. HOOK MANIFOLD GAUGE SET TO LOW PRESSURE PORT AND HIGH PRESSURE SIDE PORT. LOW SIDE PRESSURE SIDE SHOULD HAVE 25 - 36 PSI AND HIGH PRESSURE SIDE SHOULD BE 178 - 237 PSI. IF LOW SIDE PRESSURE SIDE READ BELOW 20 PSI THE LOW PRESSURE SWITCH WILL NOT TURN ON THE AC COMPRESSOR CLUTCH NEED TO ADD LITTLE MORE REFRIGERANT. IF REFRIGERANT LEVEL CORRECT CHECK AC COMPRESSOR FUSE AND RELAY.IF POWER GOING TO AC COMPRESSOR CLUTCH YOUR COMPRESSOR CLUTCH HAS OPEN WINDING. AC COMPRESSOR CLUTCH NEED REPLACING.

It could be several things.I would start by making sure that the compressor is working.Check high and low pressure while unit is running. If they both read the same then your compressor is not working.The first thing I would do is to add refrigerant to see if it is just shutting down due to low pressure.Depending on how old your unit is and what type of gas you are using you should see pressures on the high side from 250-350 and low side between 55-85.Hope this helps you. Thanks.

check the refrigerant level. The compressor is running until the low pressure or high pressure switch switch cuts it off.

Running the unit without enough refrigerant will damage the compressor because the lubricating oil is dissolved in the refrigerant. This is the reason for the low pressure switch cut off--to protect the compressor from damage.

If pressure gets too high, the compressor seals will blow. Thus the reason for the high pressure switch. Too much refrigerant or too much air in the system (due to a leak) will cause high pressures.

If refrigerant level appears to be correct, then suspect trapped air from a leak causing a high pressure cut-off. If refrigerant is low, suspect a leak causing a low pressure cut-off.

Hello RR Hard start can't hurt BUT dosen't sound like the problem.Do you have a scroll compressor?They don't need a kit.Does it start every time?It could be short cycling on LOW PRESSURE SWITCH.If the unit is (pumping down) during the run time you have a restriction in the system.Make sure the service valves are open ,do you have a filter dryer colgging up or a bad TXV valve also could cause short run times.20-30 PSI the switch cuts out.

low side psi will run about 30 psi, hi side pressure will go as high as 350 psi then the pressure switch will kick the engine fan on then drop psi down to 120-150 psi

if the compressor is running - the suction pressure should be 65-75 psi - if it is very low - not enough freon - if it is very high - the compressor valves could be damaged - both will cause this

By George, you've got it.Front seat (screw in) the high side and run the unit. Do not let it pull into a vacuum as this will allow air to be sucked into the system when you open it. Also, watch you high side gage. Don't go over 400 psi. This is usually the High pressure cut out setting on most safeties. Pull low side to about 2-3 psi and kill unit. Let sit for about 5 minutes and then run it again back down to 2-3 psi. Front seat the low side any you should be good to go.
I strongly suggest that you add a filter/dryer to the system. It's good practice to do so once the system has been open. If you are going back with a 410-a system, don't leave it open to the atmosphere for more than 15 minutes. That is, the condensing unit. The only way to remove moisture from the oil on this system is to use filter/dryers. You can not vacuum it out like in the old days with R-12.

Good luck and hope this is what you are looking for.

If your pump is shutting off before it reaches it's max psi. then you need to adjust you switch. Under the black case there are two adjustment screws. One is the high\low adjustment, the other is high pressure cut off.
The first will change when the pump turns on and off for your low psi setting and the high setting simultaneously.
The second will allow you to raise or lower just the high setting. There should be a diagram under the black cap to show which is which. Adjust the high side only and just a 1\4 turn then run the compressor and see if that takes care of the high setting.

The cap tube is clogged and the unit is overcharged. Get used to this on beverage air units.