Diodes

A diode is an electrical one-way valve. Current flows through it in one direction and is blocked in the other direction. That is the entire concept. Think of a check valve in a plumbing line — water goes one way, the valve blocks it from flowing backward. A diode does the same thing with electrical current.

A diode has two terminals — the anode and the cathode. Current flows from anode to cathode. That is the forward direction. If you try to push current the other way — cathode to anode — the diode blocks it. On a schematic, a diode looks like a triangle pointing at a vertical line. Current flows in the direction the triangle points. The line is the cathode — the blocking end. On the physical component, the cathode end is marked with a band or stripe.

The alternator uses six or more diodes in a rectifier bridge to convert AC current from the stator into DC current the vehicle can use. Without these diodes, the alternator output would be useless. Diodes are placed across relay coils and solenoid coils as suppression diodes — when the coil de-energizes, the collapsing magnetic field creates a voltage spike that can damage the module controlling the coil. The suppression diode absorbs that spike. LED lights are diodes — Light Emitting Diodes. They produce light when current flows through them in the forward direction. Isolation diodes in power distribution prevent current from backfeeding between circuits that share components.

A diode fails in two ways. It fails open — no current flows in either direction. The circuit acts like it has a broken wire at that point. It fails shorted — current flows in both directions. The one-way valve is now just a pipe. In an alternator, a shorted diode allows AC current to leak into the DC electrical system, causing ripple voltage that makes gauges flicker, lights pulse, and modules behave erratically. A shorted suppression diode across a relay coil creates a constant current path through the coil, which can keep the relay energized or drain the battery.

Set your meter to the diode test setting — the triangle-with-line symbol on the dial. Touch the red lead to the anode and the black lead to the cathode. You should read approximately 0.5 to 0.7 volts — this is the forward voltage drop across the diode and confirms it conducts in the forward direction. Now reverse the leads — red to cathode, black to anode. You should read OL — open line, no current flow. That confirms the diode blocks in the reverse direction. If you get OL in both directions — the diode is open. If you get a low reading in both directions — the diode is shorted. Either result means the diode has failed and must be replaced.