In this article and video, how to acquire and interpret the alternator duty cycle control circuit on GMC Vehicles. In this example, a 2018 Acadia is used. The information shown is for a normally working charging system.
Using a wiring diagram, locate the control circuit to the alternator from the powertrain control module (PCM)
Insert a specialized backprobe into the connector
Connect the backprobe to a test lead
Connect the backprobe to channel 1 (A) on your scope
Connect a high amp probe to channel 2 (B) on your scope
Connect the scope ground to battery negative
Turn On the high amp probe, set the scale and zero the probe
Confirm the direction of current flow through the probe
Then clamp the alternator output cable
Start the engine and allow it to idle
Set scope channel 1 to 20v DC and 20 ms per division
Set scope channel 2 to 100 A DC
Once pattern is displayed, set cursors to measure duty cycle.
When measuring duty cycle, you can select an area between cursors or the entire trace
Pattern shown in low load on charging system
Vehicle at idle, all loads turned OFF.
The duty cycle is low here about 30%, indicated by the wave pattern
The red current trace indicates the amperage output of the alternator, about 8 amps average
Pattern shown here is high load on charging system
Vehicle at idle, headlights and heater blower ON.