Wiring Harness News WHN Sept-Oct 2020 WHN Sep-Oct 2020 - Page 52

52 SEPTEMBER/OCTOBER 2020 Wiring Harness News INDUSTRIAL INFO-TAINMENT Wire Harness Testing Evolution and the Advent of Test Fixtures Continued from page 50 ____________ Figure 3. Switch Pin switch pins that engage the components to verify their presence (Fig. 3). Switch pins look similar to spring probes. The difference is that current passes immediately through a spring probe upon contact, whereas the current only passes through the switch pin when it compresses and reaches its switch point. A wire is attached to the barrel of the switch pin and another is attached to the stem coming out of the back end. The stem is insulated from the barrel. The two come into contact when the switch pin compresses to the switch point. The circuit closes between these two points if the lock or clip is present. It remains open if they are missing. The switch is wired and programmed into the test sequence like any other circuit. Don’t Sweat the Small Stuff Ring terminals, spade terminals, insulated and uninsulated male and female fast-ons and flag terminals, blunt cut wires, stripped, partial stripped, and terminated wires are all common on harnesses. Test fixtures of differing varieties have been developed for all of them. A ring terminal fixture can consist of a simple plastic base with a metal stud machined to the inside diameter of the ring terminal. If both the ID and OD need to be verified, a plastic stud can verify the ID and a plastic shroud can surround the OD while spring probes contact the underside of the ring terminal (Fig. 4). Figure 4. Ring Terminals Insulation piercing clips can be used to test wires, or fixtures with properly positioned contacts can test blunt cuts or stripped wires of different sorts without impacting the insulation. Spring probe constructed fixtures will effectively contact straight faston terminals and flag terminals without violation and simple clips can be used to secure and test spade terminals (Fig. 5). Figure 5. Spade Terminal Test Clip (2) The Unseated Terminal Issues with unseated terminals have plagued the industry for years. Most connectors are push to seat and operators are trained to push-clickpull. But the repetitive nature of the process and the quest for speed can lead to errors. While some connector designs lend to easy seating, others can be more challenging and error prone. During the testing process, terminals that are barely not engaged can make contact and pass the continuity test. The challenge for manufactures is to catch these instances. The most simplistic and least costly approach as it relates to test fixtures is to set the depth of the probes in the fixture to allow minimal engagement with terminals that are fully seated. Typically probe manufacturers will recommend travel/engagement of 2/3rds of the probes full travel. By limiting the engagement, most cases of unseated terminals may be detected as open circuits. In the testing of connectors with female terminals, a type of probe called a “Hat Pin”, “Shoulder Pin” or “Step Pin” (Fig. 6) can be used in many instances to limit engagement with a fully seated terminal. These spring probes have a shoulder that stops on the face of the connector housing (with Deutsch DT04 and DT06 series connectors and their Amphenol and Molex crosses, the shoulder stops on the wedge). The dimension of the probe from its shoulder to the tip of the probe controls how much engagement is made with the terminal. These types of probes are slightly more expensive than standard spring probes and are a cost-effective approach to controlling the amount of engagement. Figure 7. Push Test Fixture A third and more costly approach would be to test using heavy pressure switch pins rather than a spring probe (Fig. 7). The heavy pressure switch pin can push the unseated terminal back, and though making contact will not pass the current because it has not compressed to its switch point. Conversely, if the terminal is seated the switch pin engages, compresses to its switch point and passes the current. It allows the for a concurrent continuity test and terminal seating test. This can also be done as a stand-alone terminal seating test which would require a continuity test later in the process. The number Figure 6. Hat Pin _____________ Continued on page 54