Auto Service World
Feature   January 1, 2001   by CARS Magazine

Don’t ignore “back EMF”

Remember learning about basic ignition theory way back in high school? The coil set up a magnetic field when energized, and when points or transistors interrupted the current, the magnetic field colla...


Remember learning about basic ignition theory way back in high school? The coil set up a magnetic field when energized, and when points or transistors interrupted the current, the magnetic field collapsed, inducing a powerful secondary voltage. There’s nothing new there, but consider other coils in the vehicle’s electrical system. They’re everywhere that a solenoid is used, from door lock actuators to the A/C compressor clutch. Relays also contain coils, and the common feature of them all is that they dump a reverse current, called “back EMF” (electromotive force) into the coil’s supply circuit. If the coil is substantial, the current can be considerable, and if it’s in series with the alternator’s output diodes, the unit’s rectifiers can be adversely affected. To get around the problem, diodes are often installed to give the back EMF an easy path to ground, while allowing current to energize the coils readily. CHECK THOSE DIODES. And don’t wait for the unit to fail completely, because most will function with a damaged or open diode for some time, spiking everything in the supply circuit with current every time the coil is switched off. The A/C clutch circuit is especially important, because it can damage the alternator directly. Unwrap the tape around the harness near the compressor’s connector if you can’t find it in the manual, and remember to isolate one end of the diode electrically before you test.

The comeback you save may be your own!


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