AUTOMATIC AIR CONDITIONING SYSTEM : PTC Element Heater Control: Description

WARNING: This page is about a different car, the 2011 Nissan Leaf. However, it is still accessible from the selected car via links, so may be relevant.

Based on the air mix door position and signals input from each sensor, the A/C auto amp. Calculates the target coolant temperature [ambient temperature - approximately 70°C (158°F)].
A/C auto amp. transmits the operating rate to the PTC element heater via LIN communications so that the calculated target coolant temperature is achieved.
Based on the A/C auto amp. command, the control circuit inside the PTC element heater controls the heater output by the PWM method.

Fig 1: PTC Element Heater Control Circuit Diagram (Automatic Air Conditioning System)
Courtesy of NISSAN MOTOR CO., U.S.A.