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Chevrolet & GMC Charging System Duty Cycle Testing using Pico Scope

See how to test the PCM controlled alternator signal using a Pico Scope

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.

Next, select measurements tab, then select 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.

The duty cycle is high here about 55%, indicated by the wave pattern

The red current trace indicates the amperage output of the alternator, about 70 amps average