Meeting the Test Needs of Automotive Wiring Harnesses

by Dan Romanchik

Automotive cables are different. Cables you find in a PC or other electronic gear consist mainly of a set of wires with a connector on each end. To test those cables, you plug the cables' connectors to the test connectors on a cable tester.

The tester checks each circuit for continuity and then for shorts between circuits and then tells the operator if the cable passed or failed. The operator unplugs the cable, puts it into the appropriate bin and moves on to the next cable. Some testers get fancy and allow you to run hipot tests on the cable's circuits.

Automotive wiring harness tests are another matter, though. Automotive wiring harnesses are more complicated than the cabling found in most electronic gear because they often include electrical components, such as relays and switches, in addition to conductors and connectors. They may also include mechanical components, such as grommets and retainer clips. The presence of these components make testing automotive wiring harnesses more complicated.

Take, for example, the case of a wiring harness with a relay panel. Most cable testers can easily test the cable wiring, but when the assembler installs the relay panel, the relay coils and contacts will short some of the circuits together. Some of the relays may also have internal diodes, which complicate testing. To properly test a harness with a relay panel, a tester should be able to test resistances of both the coil and the contacts, and to test that the relays actuate properly, it needs a power supply to switch the relays.

Another interesting test challenge is the resettable fuse. This device consists of a layer of specially formulated conductive polymer sandwiched between two metal electrodes. When a short circuit or overcurrent condition occurs, the resistance of the polymer increases and limits the current. The resistance of the device remains high until the current is switched off. The device then cools and the polymer's resistance falls back to near zero.

To test this device, the tester must first be able to apply a current to the device for an appropriate amount of time and then check the resistance of the device. To test that the device resets, you must be able to program the tester to wait for the component to cool and then check the circuit resistance again.

Another example of a component that a cable tester may be required to test is an airbag module. The unique characteristic of testing this device is that these modules have a shorting bar, which grounds the device and prevents the module from inflating the airbag during assembly. To properly test a harness with this module, the tester must have a mechanical interface that removes the bar, verifies the circuits, replaces the shorting bar, and then verifies that the shorting bar is back in place.

According to Ed Luby, sales engineer for Dynalab (Rey-noldsburg, OH), testers for automotive wiring harnesses are also frequently used to verify that mechanical components have been installed before shipping. Some harnesses, for example, have grommets or clips installed that assemblers use during final assembly to mount the harness to the vehicle. If the grommet or clip isn't there, the assembly can't be used.

To detect these mechanical components, manufacturers often outfit the assembly fixture with a switch or optical sensor that detects the presence of the component. The cable tester must have an input for this switch and be able to check the state of this input while running a test.

Luby also notes that many harnesses have connectors with “wedges” or “retainer clips” that lock the connector's terminals into position once they've been installed and the circuits verified. Verifying that the wedge is in place is important so that terminals aren't pushed out when the harness is connected at final assembly. To verify that the wedge is in place, you can use a switch or sensor just as you would for a mechanical component.

What's Ahead?

Like all automotive technology, cables are changing, although perhaps more slowly than other areas. One development that is affecting wiring harness design and test is the increasing amount of data being sent via harnesses from one electronic control module to another. This development holds the promise of reducing the number of circuits in a typical wiring harness, while at the same time increasing the test requirements on those circuits.

One way test requirements could become more stringent is that in addition to shorts and opens testing, tests may be required to ensure data integrity. At present, testing data does not seem to be a big concern, but as data rates increase, it may be more critical. Also, according to Tim Horacek, an applications engineer for Weetech (Hollis, NH), we will soon see fiber optics being included in automotive wiring harnesses.

Another development that will affect harness testing is the move to a 42-V power supply. Although not a big change, some test fixtures may have to be upgraded to accommodate this change. (See “42-V Power Scheme Will Affect Test,” for more information.)

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