ACtion Magazine - March 2015 - (Page 15)

Compressor clutch magnetism Dear Editor, I would like to offer comments about electromagnetism, just to add a bit to your well chosen words. Most of you know an electromagnet may be made by wrapping coils of thinly insulated wire around a soft iron core. The magnetic strength can be measured in ampere turns, meaning the product of multiplying the number of coils by the amperes flowing through the wire at a given voltage. This voltage would normally be read with the key on engine on, or usually between 13 and 14 volts. Remember, this is important: the higher the voltage the higher the current flow. For the purposes of this example, let's declare 6 amps and 100 turns (coils) of wire, or 6 X 100 = 600 ampere turns. If we had a failure of the wire insulation that resulted in a short to voltage half way through the coil, we would cut the number of turns in half. Reasonable logic would conclude you have reduced the holding power of the clutch by 50%; but wait just a minute. If you have a short to voltage half way through the coil, the short puts wires that were in series (or a single path) in parallel which would reduce the resistance of the wires by 50%. This would cause the current flow to double. The numbers would calculate out to 50 windings at the same voltage with half of the total resistance, or 12 amps and 50 turns (coils) equaling 600 ampere turns. The magnetic force is the same and the clutch would hold properly. Bear in mind that the relay or other control circuit was designed to switch about 6 amps. If you double the amps, the relay could fail or the control circuit may automaticity shut down and not reset until the current demand is reduced. A failed relay may not be repaired by replacing it. The same thing applies to diagnosing the module, BCM, ECM, Quad Driver, or other control unit that has failed. Obviously you may see the reduced resistance if you check the disconnected coil with a properly zeroed ohmmeter. Another tool I like to use is a jumper lead with an ATC fuse receptacle and a couple of alligator clips connected to B+ and the grounded clutch coil. Crank up the engine and let it run for about 5 minutes. If a 5 amp fuse fails, try a 7.5 amp fuse. If that fails, there is trouble in River City. Regards, John Brunner Reader Reply March 2015 * ACTION 15

Table of Contents for the Digital Edition of ACtion Magazine - March 2015

Engine cooling systems: Electric cooling fan operating strategies
System Charge Determination
Service Port
Leonard's Law
Virtual View
Heavy duty and off road
Last Watch
Coolin Corner
Letters to the Editor
By the numbers
Industry News
Association News
In Memoriam
New Products

ACtion Magazine - March 2015