How do i engage the ac compressor clutch to charge the ac system

That's a sign that your system is very low of refrigerant. So it's best to find and fix the leak before refilling. Adding refrigerant on a system that's low and leaking is a really bad idea. See these two posts about how to diagnose a compressor clutch that won't engage and how to recharge http://ricksfreeautorepairadvice.com/ac-compressor-clutch-not-engaging/
http://ricksfreeautorepairadvice.com/ac-recharge-on-your-car-or-truck

Did you charge it with the proper amount of freon? Is the ac clutch engaging? One of the ac lines should be cold, is it? Check the freon level to make sure that you don't have a leak. You should have pulled a vacuum on the system after replacing the compressor. This would have shown if there was a leak and removed the air in it so the coolant can be added.

You need 22oz for a "front only" a/c system and 32oz of R-134a refrigerant for "front and rear" a/c system.

The compressor is off likely due to insufficient refrigerant charge in the system. Once the minimum required charge has be provided, the compressor clutch should engage (check for blown fuse if it does not). Refrigerant is added only to the low or suction side of the compressor and only as a gas. Do not invert the refrigerant tank in an attempt to add refrigerant as a liquid! Placing the tank in warm water is a better way to fill the system (raises the pressure in the tank) without concern for slugging the compressor.

If all you get is hot air there are several possibilities. The compressor is driven by the serpentine belt and is engaged and disengaged by an electric clutch mounted on the compressor drive pulley. With the engine running, have an assistant turn on the AC while you observe the AC compressor clutch. If the clutch engages, you can be reasonably sure that the system has freon, and the electric circuit is OK. If the clutch doesn't engage, which is the most likely situation, either you are low on freon, the AC clutch relay is inop, or AC compressor fuse is open (blown). A search of Google images will give you the location of the AC relay. You can pull out the relay, carefully remove the cover, reinsert the relay, and engage the compressor clutch by manually closing the relay contacts. Warning: do this only for a second or two in order to see the clutch engage. If the clutch does engage, then you are probably low on freon. There is a safety switch that prevents the clutch from operating if there is not enough freon in the system. Hopefully, this is the case. Go to Walmart and get two cans of r-134 and the inexpensive kit, and following their instructions, charge your system. If this is not the problem, you'll need a set of AC gauges and the skill to use them. Hope this helps. Dano

It will not come on until you start adding freon. If you start the engine and turn on the a/c to max a/c, you can add refrigerant through the low side port. Once there is enough in the system, the compressor clutch will engage and allow the rest of the refrigerant to be drawn in. Hope that answers your question! :)

u should first evacuate ur system down to vacuum bfore adding freon. chances are system is full of air,therefore not allowing full charge to enter ac system. also make sure ur clutch is engaging at the compressor, u can see wen its on and wen its off.

If the noise started after you added the Freon, then it perhaps is your AC clutch. Normally if you have an automatic AC system, then when the Freon gets low, the auto system will not allow the AC to engage, only allowing the fan to circulate outside air. In order to add Freon to the system, the AC lock out has to be over ridden, in order for the compressor to **** in the Freon. This in turn engages the AC system so that the clutch engages, thus engages your AC system. The clutch bearings may be at fault.

could be a problem with the heater controls ,or you do not have the right machine to recharge the vehicle properly with R134 ,the vehicle needs to have the system reclaimed then oil recovery and then a vacumn before the oil is added and the gas by weight.For this you need a special machine

you can jumper the low presser switch to in gage the clutch until you have enough freon in the a/c unit to keep the Freon low presser switch in gaged.hope this helps.

The compressor will not run without a minimum amount of R-134A. In some cases the compressor will shut itself down and require a reset (disconnect neg battery cable for 20min) prior to adding refrigerant. Best of luck with your AC issue.
Greg