Functional Testing Defined

Harness assemblies can include passive components such as resistors, diodes, and capacitors, and active components such as relays, lights, actuators, and sensors. Functional testing energizes active components by applying their working stimulus voltage and current to activate the devices to observe their correct response. In the case of relays, the expected response is switching of the contact(s) that results in changes to the pattern of connections which can be verified with an electrical test. In the case of lights and actuators, the test system may prompt the test operator to verify the expected response or the test system can evaluate the change of state during test using the output of sensors.

Good Practices in Functional Testing

When testing components, the best practice is first verify the presence of active component electrical characteristics, as well as verify the correct connection pattern of the tested device in an unenergized state. After this first step, the next step is to energize the active component and observe the correct response. This two-step approach can prevent damage to sensitive components in the device under test, and in some cases, is helpful to ensure the safety of the test operator.

Functional Versus Non-Functional Test Systems

It is important to understand that not all cable and harness testers are designed to support functional testing. Test systems that support functional testing are designed such that the test points can withstand the stimulus voltage to which they could be inadvertently subjected. Automated test systems that support functional testing apply the stimulus voltage to active components through the test system in an automated way, without operator intervention.

Choosing the Right Functional Test System

Test Systems that support functional testing vary in the in the levels of voltage and current they can provide to energize active components. When choosing a functional test system, you also need to evaluate if a single stimulus voltage, or multiple stimulus voltages are required for your testing needs. Assemblies that include active devices that all have the same working voltage can be activated by routing the stimulus from a single, inexpensive power supply. However, assemblies with active devices that have multiple working voltages require either a programmable power supply or multiple power supplies as the source. In some instances, it’s necessary to apply more than one working voltage at the same time. In such cases the test system should support multiple stimulus busses. Some test systems support multi-bus test points, which can both test and energize. Moreover, testers with functional test capability vary in whether they allow the stimulus voltage, or even multiple stimulus voltages, to be run through a distributed test system from a central supply or supplies.