Problem with Passat Climatronic 1996

I have installed a Climatronic system from a Passat 96' 2.0 16V (VAG number 3AO907044A) into my Passat 94' TDI, which has the part number 028906021B. The problem is that when the engine first starts, the compressor doesn't engage until the car reaches 25 km/h. After that, the compressor works fine even when the car isn't moving. Is it possible to fix this so that the compressor engages when the engine starts? There are no fault codes in either the Climatronic system or the engine.