How to Test an Alternator Diode

Updated April 17, 2017

An alternator can be considered the heart of an engine's electrical charging system. By generating electricity through a rectifier, windings and diodes, the alternator supplies extra voltage to run all of the vehicle's accessories and recharge the battery when it becomes low. A dash indicator warning light can be the first indication of a discharge problem in the alternator circuit. Certain parts of the alternator can be prone to failure first, such as the diodes. Diodes can be checked by following a few easy steps and using a few diagnostic tools.

Charge the battery to its full potential by using a personal portable charger or having it charged at a repair facility. Have a battery cell load test and voltage check performed after charging. This will rule out a shorted battery or a battery that will no longer hold a charge. A low battery charge condition or weak cells often can be mistaken for bad internal parts of the alternator. Make sure the battery terminal posts and cable terminals have been cleaned before installing the fully charged battery.

Remove your alternator with the appropriate sockets and wrench. Take it to a parts repair store and have it bench-tested for charging output. A short circuit in the rectifier, armature windings or burnt diodes will show a no-charge condition. In the case of a complete no-charge condition, the alternator should be replaced or rebuilt with the correct kit parts.

Examine the brightness of the dashboard warning indicator light that indicates charging failure. Do this with the engine running at an idle and with all the accessories turned on. A slightly dim indicator light can mean one or two diodes have failed. Since an alternator has six diodes, three positive diodes and three negative ones, the indicator can get progressively brighter as more diodes fail. No other alternator part will fail with this type of progressive symptom (from dim to bright) other than diodes.

Run the engine at idle with accessories on. Hook up one lead of the voltmeter to the positive battery terminal and the other lead to the BAT terminal on the back of the alternator housing. The reading should be less than 0.2 volts. Any voltage reading higher than 0.2 volts means the system has high resistance. In this case, check and clean all wiring connectors on the back of the alternator. Clean them with carburettor cleaner and a small wire brush.

Touch the voltmeter test lead to the alternator housing case, and the other lead to the negative battery terminal (not the cable end). If higher than 0.2 volts, check all wire grounds, including the engine-to-chassis ground strap on the front, side or rear of the engine. No voltage reading at all can point to burnt out diodes.

Things You'll Need

  • Voltmeter
  • Carburettor cleaner
  • Wire brush (small)
  • Battery terminal cleaner tool
  • Sandpaper (if applicable)
  • End wrenches (small set)
  • Battery charger
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About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.