1999 GMC air cond. AC clutch wont engage. system is charged. relay, fuze,and low pressure switch are replaced.

most of the time ,that is caused by low refrigerant charge

Sort of.
If the system is empty, you would need to jumper the low pressure switch to power the ac clutch to put the first can or two of freon in.
Keep in mind the system is supposed to be vacuumed out before adding freon, so if it is full of air you won't be able to put the full amount of gas in.

ADD Freon. It's a R134 system and the pressure is too low. Trust me add Freon and it will work fine. With a low amount of Freon if you jump the low pressure switch at the accumulator it will be very cold but will not have enough pressure to kick on the low pressure switch.

It is likely low on refrigerant. There is a low pressure switch that inhibits the operation of the compressor when there is not an adequate amount of refrigerant. If you have an R-12 system, you can retrofit it to accept the R-134A refrigerant charging equipment.

If jumpering the relay contacts does not engage the compressor, the clutch coil may be burned out or wire broke. If it is a failed clutch coil, you may need to replace the compressor because many of those parts are not being carried anymore.

The low side pressure switch in the AC low pressure line signals the PCM (computer control) to close the AC compressor relay and raise the idle speed. If the system is low on refrigerant charge the relay will not allow the system to engage the compressor, this is because the oil that lubricates all the works is suspended in the AC refrigerant, if that is low then the system gets no lubrication and will quickly seize up..

If you do not have enough refrigerant in the system. The low pressure switch will prevent the compressor from engaging to avoid damage.

Check your pressure.

I don't recommend this, but you may need to jump your low pressure switch to get the compressor going to add more refrigerant to the system. But if your low on charge, it may be "best to take it to a shop to have them fill the system".

hi.
it is possible for either to be faulty, but it far more likely to need regassing.
there is a pressure switch in the freon system that will prevent the A/C clutch engaging if the pressure is too low.

i would get the refrigerant checked and topped up before starting to worry about anything else.

Can't see filling with freon without compressor turning. Put jumper wire on high/low pressure switch harness to see if clutch engages. If so, bad high/low switch. If not,check power to high/low harness via fuse,relay, or dash control.Battery voltage applied to compressor if accessible will also engage compressor clutch and read your low side with your fill gauge. 30-35lbs is good

to prevent damage to the air conditioning system there is a "low pressure switch" located on the condenser near the low pressure hose (large one) .. if the refrigerrant (R134A) gets too low then the clutch will be blocked from activating by that switch .. you can have your system tested and recharged or you can do that yourself .. the R134A is available at auto parts stores .. some come with a gague that is required for proper charge .. if you dont already have a gage then yoiu will need that one ... recharge kits often come with instructions .. other less likely but possible problems could be a fuse or bad AC clutch relay. sometimes damaged wiring or bad clutch will be the problem but much more likely just low fluid ..

The AC compressor does not have to be running to charge the AC system, and the low pressure switch will even prevent the AC compressor from operating until there is sufficient pressure to operate the AC compressor without damaging it.

Start the engine and turn on the AC, then charge the system with the R-134a and as soon as there is enough pressure built up to trip the low pressure switch off, the AC compressor clutch should then engage and then finish charging the AC system.