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 in order to observe their 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.
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 important to ensure safety for test the operator.
An additional factor to consider for functional testing is the stimulus voltage and current required to activate the devices in the product assembly. 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 both cases the power supplies must be capable of supplying the required current. In some instances, it’s necessary to apply more than one working voltage at the same time. In such cases the test system must support multiple stimulus busses.
It is important to understand that not all cable and harness testers are designed to support functional testing. Test systems that support functional testing must be designed such that the test points are able to withstand the stimulus voltage to which they could be inadvertently subjected. Test systems that support functional testing vary in the levels of voltage and current they can provide. 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.