That is actually a diode, which is a one-way valve for electrical current flow. When you have current flowing through a coil of wire, it sets up a magnetic field around it. When you interrupt that current flow suddenly, that magnetic field has no choice but to collapse instantly, and doing so generates a huge voltage spike. That is the desirable and necessary characteristic of an ignition coil. It develops about a 300 volt spike that is amplified in the coil's secondary winding to fire a spark plug.
The same spike is developed in the coil in a relay, but to a smaller degree. There are usually diodes inside relays too to short out those spikes. A lot of GM's relays use a resistor instead to just dampen those spikes to a safe level. The advantage to doing that is their relays can be installed two ways without worrying about which way the diode is connected inside it.
The AC compressor clutch also uses an electromagnetic coil, and just like in the ignition coil, when the relay cycles off, the collapsing magnetic field in that coil generates a huge reverse voltage spike. During normal operation that diode is in the circuit backward, so it is turned off, meaning it is not conducting any current, just as if it was not there. When the clutch cycles off, the reverse voltage spike makes current want to flow the other way, and that puts the diode in the circuit temporarily "forward-biased". Current from that spike flows through the diode effectively shorting it out so it cannot damage other components.
In this application, problems with these diodes are not common. If one were to short, it would be like a piece of wire, and a fuse would blow. Given that there is not much else tied to that part of the circuit, if the diode were to be open, or missing, at most you might hear a little pop in the speakers when listening to a weak AM radio station.
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Tuesday, August 21st, 2018 AT 8:26 PM
