We have a 2012 Sprinter and this past weekend was the first time I tried to use the AC. It never seemed to kick on. The temp was the same on fan or AC, and there was no change in the sound coming from...
With a bad motor start cap you should still hear a change in the sound of the unit when the motor tries to start. If nothing is happening, it may be the compressor relay. You should have schematics that will show which wires to jump out to actuate this relay.