How a Module Controls a Circuit

Here is something that trips up a lot of technicians on modern vehicles. Most modules — PCM, BCM, TCM, and others — control components by switching the ground side of the circuit, not the power side. The component has battery voltage supplied to it at all times through a fuse. The module completes the circuit by providing the ground path. When the module commands the component on, it connects that ground. When it commands off, it disconnects it.

If you put a test light between the component terminal and ground and it lights up, that only confirms the power side is present. It tells you nothing about whether the module is commanding the component on. You could have perfect voltage at the component and still have a no-operation concern because the module is not providing ground. You have to test both sides.

Connect a test light to battery positive. Probe the ground-controlled terminal of the component while commanding it on with the scan tool. If the test light illuminates, the module is providing the ground signal — the module side is working. If it does not illuminate, the module is either not receiving the input signal to command it on, or the module output circuit has failed. Now you know which side to investigate next.