Make sure the outdoor unit is not mounted upside down, the fan should be facing forward (do not face the wall) as this can hinder the performance of the unit and cause the flow of lubricating oil the compressor will enter the cooling circuit and damage the AC. In the meantime, perhaps you also need to visit www.airconservicingsingapore.com/ to find the excellent AC repair experts in the business.
Choose an outdoor installation location that is open, dry and free of any dirt that can be caught in the exhaust fan blades. Remember also that the outdoor unit is quite noisy and blows hot air so do not bother other people around you.
Also, make sure the distance between the indoor and outdoor units is not too far away to make it easier for you or a technician if you need to take care of both units.
Avoid installing an outdoor unit too close to the window as it will produce a rather disturbing hum sound.