ACtion Magazine - March 2015 - (Page 15)
Compressor clutch magnetism
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
March 2015 * ACTION
Table of Contents for the Digital Edition of ACtion Magazine - March 2015
Engine cooling systems: Electric cooling fan operating strategies
System Charge Determination
Heavy duty and off road
Letters to the Editor
By the numbers
ACtion Magazine - March 2015