I'm an old school type of guy so the first time it got cold outside I thought I had a bad zener diode in my alternator and thus the voltage fluctuations at idle but now I find out it's completely normal and the following is why.
The PCM, or ECM receives a voltage input from the generator (5) and also a battery voltage input (7) from the TIPM (Totally Integrated Power Module), it then compares the voltages to the desired voltage programed in the EVR (Electronic Voltage Regulator) software, and, if there is a difference it sends a signal to the generator EVR circuit to increase or decrease output. It uses a Pulse Width Modulation (PWM) to send signals to the generator circuitry to control the amount of output from the generator. The amount of DC current produced by the generator is controlled by the EVR circuitry contained within the generator.
So unlike my 82 Corvette that has the regulator built into the alternator the Cummins 6.7L is software regulated, pretty trick.
Oh by the way if you're wondering what's causing the fluctuations in the first place it's the post-heat cycle of the intake manifold heater system. The
number of cycles and the length of the cycling
operation is controlled by the engine control module,
this time will not exceed 150 seconds. The
needle should then stabilize at the approximate
operation point.
• The cycling action will cause temporary dimming of
the headlamps, interior lamps, and also a noticeable
reduction in blower motor speed which is what I was seeing.