My 2007 Impala's A/C is not working the compressor is kicking on but the on/off switch on the dash keeps flashing on and off.. it was working fine until my wife took it in to have it sereviced and they added more R134a. Now it just flashes on and off what could cause this?

Could be an electrical issue. You're going to have to take it back in.

What symptoms caused you to take the car in for servicing?

The A/C ON/OFF switch is likely flashing on and off to indicate something is wrong with the A/C, just like a Check Engine Light staying on indicates a problem affecting engine emissions.

Here's my take, if they added too much freon and did not observe high and low pressure manifold gauges during the process. It's best to add freon, by a CoolTech machine, to the low pressure port. Too much freon is dangerous, just like too little freon is dangerous. Just like the A/C Clutch Cyling Switch works for an established range of pressures (cut-off and cut-on pressures), so does the Pressure Cutoff Switch. The A/C Clutch Cycling Switch attempts to control the evaporator core pressure at a point where the plate/fin surface temperature will be maintained slightly above freezing. This prevents icing of the evaporator core and blockage of airflow. Similarly, the Pressure Cutoff Switch, which is mounted on a schrader valve-type fitting on the compressor manifold and tube assembly, is used to interrupt A/C compressor operation in the event of high system discharge pressures. Too much freon can result in unusually high pressures and drown the compressor in freon (slogging). The compressor can compress freon gas, not liquid. My primary hunch is that the Pressure Cutoff Switch is protecting the compressor from too much freon, causing it to come on and off. My secondary hunch is that too much freon is affecting the A/C Clutch Cyling Switch, which is sending a signal to the PCM to turn the clutch on and off frequently.

Have a CoolTech machine recover, evacuate, and recharge with the proper amount of R-134a and refrigerant oil, before any serious damage is done. Consult tables for how much freon (e.g. 32 oz.) and refrigerant oil (e.g. 6.5 oz.) your system should have. Recovery will likely remove nearly all of the refrigerant oil. Look into refilling with the superior BG Universal Frigi-Quiet Refrigerant Oil.

If you believe the refrigerant and oil levels are fine, the problem may be something as simple as a bad A/C Clutch Cycling Switch. Personally, I've had bad ones come on brand new OEM accumulators. These are very important switches. That's why I'd like to know what symptoms you were observing, prior to having the system serviced.