How to Test an Alternator Diode


An alternator diode is a very important part and is considered as the heart of an engine's electrical charging system. It generates electricity using a diodes, windings and rectifier and supplies extra voltage to run all the vehicle's accessories and thus recharges the battery when it becomes low. A warning light in the dash indicator will be the first sign of a discharge problem in the alternator circuit. Some parts of the alternator such as the diodes, may cause the failure. It should be kept in mind that all the lead connectors from the back of an alternator should be clean and tight , to avoid failures. An alternator diode can be tested by following a set of steps and making use of some diagnostic tools.

Tools and Materials Needed:

  • Battery charger
  • Battery terminal cleaner tool
  • Voltmeter 
  • Sandpaper (if needed) 
  • Carburetor cleaner
  • End wrenches (small set) 
  • Wire brush (small)  

Step 1 - Charging the Battery

Use a portable charger to charge the battery to its maximum potential or by charging at a repair facility. After charging completely, perform a battery cell load test and voltage check which ensures that the battery can no longer hold further charge or  there is no shortage in the battery. Often, weak cells or low battery charge state can be misinterpreted as the damaged internal parts of the alternator. Remember to clean the battery terminal posts and cable terminals before setting up the fully charged battery.

Step 2 - Testing the Charge

Remove the alternator using suitable sockets and wrench. Take the alternator to a parts repair store and bench-test it for charging output. No-charge condition will be indicated by a short circuit in the rectifier, armature windings or burned diodes. If a complete no-charge condition occurs, then the alternator should be replaced or rebuilt using the appropriate kit parts.

Step 3 - Observation

Observe the brightness of the dashboard warning indicator light which indicates the charging failure. Observe this with the engine running at an idle state and with all the accessories turned on. Dim indicator light indicates that 1 or 2 diodes may have failed. An alternator has 6 diodes, 3 positive and 3 negative diodes. The indicator may turn brighter progressively as more diodes fail. Only alternator diodes show this kind of progressive failure symptoms.

Step 4 - Taking the Readings

Run the engine with all the accessories on. One lead of the voltmeter must be connected to the BAT terminal which is located behind the alternator housing and the other lead to the positive terminal of the battery. See to it that the reading is less than 0.2 volts, otherwise the system will have a high resistance. Using carburetor cleaner and a small wire brush, clean all the wiring connectors that are located behind the alternator. 

Step 5 - Inference

Stroke the voltmeter test lead to the alternator housing case, and the other lead to the negative battery terminal. If the voltage is above 0.2 volts, check all the wire grounds, including the engine-to-chassis ground strap on the front, side or back of the engine. No voltage reading indicates burned out diodes. You have completed testing the alternator diode.