First off you need to know how these thing's work ,computer controlled .
Trunk Release Function
When the PZM receives a serial data message for the activation of the trunk release solenoid from the instrument cluster or from the RFA, the PZM energizes the trunk release relay under the following conditions:
• The transmission is in PARK
• The valet lockout switch is not activated.
This vehicle has electronic components which the service technician controls in order to provide valuable self-diagnostic information. These components are part of an electrical network that controls various engine subsystems and various body subsystems. A description of each of these subsystems is in the appropriate subsection of this service manual. These major components that compromise the computer system monitor the system sensors and the system switches. These components are the following:
• The powertrain control module (PCM)
• The instrument cluster
• The air controller module (ACM)
• The sensing and diagnostic module (SDM)
• The electronic brake and traction control module (EBTCM)
• The continuously variable road sensing suspension module (CVRSS)
• The body control module (PZM)
• The integrated radio controller (IRC)
• The remote function actuator (RFA)
• The climate control panel (CCP)
• The cellular telephone (PHN)
The body control module (PZM) performs the following functions:
• HVAC afterblow control
• Battery guard
• Door lock and unlock
• Fuel level
• Retained accessory power (RAP)
• Theft deterrent
• Exterior lighting control
• Interior lighting control
• Trunk release
• Time of day for the radio display
• Rear defogger
• Fuel door release
• Vehicle personalization
For more information about these functions refer to the appropriate sections.
Your probably have a lose connection somewhere , ground for the BCM or instrument cluster . Behind the dash or instrument cluster ! or other places . G-400 ,G205
The instrument cluster supplies the turn on signal for the alternator
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The generator provides the voltage to operate the vehicle's electrical system and to charge the battery through CKT 1 (RED). When the ignition switch is placed in RUN, narrow width pulses on CKT 23 (GRY) are supplied by the powertrain control module (PCM) to the rotor, creating a weak magnetic field. When the engine is started, the regulator senses the generator rotation by detecting AC voltage at the stator through an internal wire. Once the engine is running, the regulator varies the field current by controlling the pulse width. This regulates the generator output voltage for proper battery charging and electrical system operation.
The PCM turns on the VOLT indicator via Class 2 serial data line CKTs 1807 (PPL) whenever undervoltage, overvoltage, or a stopped generator is detected on CKT 225 (RED).
When the ignition switch is placed in the RUN position, voltage is supplied to generator terminal L by the instrument cluster, turning on the regulator. The generator's voltage regulator controls current to the rotor, thereby controlling the output voltage. The rotor current is proportional to the electrical pulse width supplied by the regulator. When the engine is started, the regulator senses generator rotation by detecting AC voltage at the stator through an internal wire. Once the engine is running, the regulator varies the field current by controlling the pulse width. This operation regulates the generator output voltage for proper battery charging and electrical system operation. The generator F terminal is connected internally to the voltage regulator and externally to the PCM. The PCM monitors the field voltage on the generator F terminal.
It's more then likely just a lose connection somewhere . Could take half your vehicle apart to find it .
My first place to look would be here , G400
In the left rear of the passenger compartment, under the rear seat
BCM ground .
on Mar 30, 2017