What are diodes and why are they important?
When a customer visits experiencing power related issues, the first thought is always battery. But it is just as important to ensure that the entire electrical system is functioning properly. Otherwise, you could install a new battery, only to have the customer return still experiencing issues.
Next to the alternator and battery you’ll find an essential, though often overlooked component; the diode. Diodes allow current to flow in only one direction. Often shaped like a simple cylindrical object with a stripe on it, the diode seems a bit low-tech, if almost unnecessary. But they most definitely are not! Functioning as a gatekeeper for back feed into the circuit, the diode protects essential elements of the car from current spikes.
If you get a car in your garage that has some suspicious issues with the keyless entry, remote start or start-stop system, it might just be that a diode is the real cause for concern.
How to identify a failing diode
A diode is an electronic component with two electrodes (connectors) that allows electricity to go through it in one direction and not the other direction. Diodes can be used to turn alternating current into direct current (Diode bridge). You can find at least six diodes between the alternator and the battery and electrical system.
Signs of a failing diode can be a poorly charging battery, or, if you detect “noise” in the electrical system. This can be caused due to a failing diode leaking AC current into the system, which can confuse electronic modules that are only expecting direct current. A shorted diode could allow excessive drain, or “phantom draw” symptoms. Also, it is possible for the battery to discharge faster, due to the energy leaking back through the failing diode.
Finding a failing diode can be complex, however, there are battery and system testers available that incorporate a diode ripple test into the overall system test process, to help you find abnormalities. Otherwise, to diagnose a diode you must disassemble the alternator and test each diode separately.
Whether you have a customer complaint, or are performing preventative maintenance, it is important to incorporate a diode test in your battery and system test procedure. Midtronics DSS-5000 incorporates a diode ripple test in the system test procedure.