How to Check Alternator Diodes: 9 Important Steps to Know!

Reviewed by Dr. Deepakkumar Jani

To check alternator diodes, use a multimeter set to the diode test function. Disconnect the alternator’s electrical connector and place the multimeter’s positive lead on the alternator output terminal and the negative lead on the alternator case. A good diode will show low resistance in one direction and high resistance (or OL “Open Line”) in the reverse when swapping leads. Repeat for each diode. A faulty diode will show low resistance in both directions, indicating a short, or high resistance in both directions, indicating an open circuit.

Challenges, Troubleshooting, and Solutions Related to Alternator Diodes

Challenges/IssuesTroubleshooting StepsSolutions
1. Identifying Diode FailurePerform a diode test using a multimeter.Replace the rectifier assembly if any diode shows abnormal readings, indicating failure.
2. Inaccessible Diode ConnectionsDifficulty accessing diodes due to alternator design.Consult the vehicle’s service manual for disassembly instructions to access diodes for testing.
3. Differentiating Diode Problems from Other FailuresIsolate the diode issue from other potential alternator problems like worn bearings or regulator issues.After confirming diode failure, check for other potential issues with the alternator to ensure comprehensive repair.
4. Misinterpreting Test ResultsMisreading or misunderstanding multimeter readings.Familiarize with the multimeter’s functions and ensure it is correctly set to diode test mode. Review what constitutes normal and abnormal readings.
5. Safety Precautions During TestingRisk of electrical shock or short circuits while testing.Ensure the alternator and vehicle’s electrical system are powered off. Disconnect the battery to prevent accidents.
6. Deciding Between Repair and ReplacementDetermining whether to replace the diodes (rectifier assembly) or the entire alternator.Evaluate the cost and effort of replacing the rectifier assembly versus a new alternator, considering the age and condition of the alternator.

In this advanced guide, we’ll provide DIYers with an in-depth look at how to properly check your alternator diodes using a digital multimeter (DMM).

Understanding Alternator Diodes

Alternator diodes act as one-way valves for the electricity generated by the alternator rotor and stator. They convert the generated AC into DC current that can charge your car’s battery and power the electrical accessories. Most modern alternators have six diodes arranged in a diode trio configuration. Each trio has:

  • One positive diode
  • One negative diode
  • One regulating diode
How to Check Alternator Diodes

 Image Credits: Alternator by Andy Arthur is licensed under (CC BY 2.0)

The diodes must be in good condition to prevent the battery from draining and provide stable voltage for the vehicle’s systems. Symptoms of failing diodes include dim lights, battery drain, and other electrical gremlins.

Preparing to Test the Diodes

Before testing the diodes, you’ll need to gather the proper tools:

  • Digital multimeter (DMM) capable of testing diodes
  • Appropriately sized wrenches for removing the alternator
  • Vehicle repair manual for your specific make/model
  • Replacement diodes if needed

Additionally, make sure to disconnect the negative battery terminal before starting to prevent short circuits during testing. Take all necessary precautions if the engine is hot during testing as well.

With the tools gathered and the battery disconnected, locate and remove the alternator from the vehicle per the repair manual instructions for improved access to diodes during testing.

How to Test the Diodes with a DMM

To check the diode trio, set your DMM to diode test mode. This mode sends a small voltage through the diode and measures the drop across it. A good diode will allow current to flow only in one direction, while a faulty diode will have issues preventing the “backflow” of electricity.

Here is how to check each duo one by one:

Positive Diode

  • Red (positive) leads to the positive diode terminal
  • Black (negative) leads to the rotor terminal
  • Note forward bias voltage drop
  • Reverse leads and note backflow

Negative Diode

  • Red leads to the negative diode terminal
  • Black lead to the rotor terminal
  • Note voltage drop in the forward direction
  • Swap probe polarity and check for backflow

Regulating Diode

  • Red probe to regulator diode terminal
  • Black probe to rotor terminal
  • Check voltage drop when forward-biased
  • Check for continuity in the reverse direction

The actual voltage drop will vary based on the DMM and alternator model, but you should see a significant difference between the forward and reverse readings. Any diode that fails to block backflow is defective and will need to be replaced.

It is imperative to repeat the diode tests 2-3 times for consistency, as a transient issue can allow temporary conduction in the reverse direction during an initial test. This will help diagnose any intermittent diode failures or weak diodes.

Determining Root Cause of Failure

If one or more diodes are found to be faulty during testing, the underlying cause of failure must be determined before replacing the components. Possible root causes include:

  • Excessive heat: Install an alternator heat shield if the alternator is getting too hot for the diode capacity
  • Voltage spikes: Install voltage spike suppressors if electrical issues or load dump is damaging the diodes
  • Factory defect: Replace the alternator if the diodes fail prematurely with no underlying cause

Without addressing root cause failures, the replacement diodes can be easily damaged again leading to repeat faults.

Replacing Faulty Alternator Diodes

Once the root cause is corrected or a factory defect is suspected, the actual diode replacement process is straightforward for most DIYers:

Supplies Needed:

  • Soldering iron & solder
  • Heat shrink tubing
  • Replacement diode trio
  1. Unsolder the bad diode(s) from the alternator terminals. Heat up each terminal as you gently twist the diode to remove it.
  2. Test fit the new replacement diode trio and trim the wire leads if necessary for proper fitment.
  3. Apply heat shrink tubing over any exposed lead wires from the new diodes to prevent shorts.
  4. Carefully solder the new diodes back into their correct positions on the alternator by reheating each terminal.
  5. Allow the alternator to fully cool before reinstallation to prevent diode heat damage.
  6. Securely fasten the alternator back into the engine bay, reconnect all wiring harnesses that were removed during removal, as well as reconnecting the negative battery terminal.
  7. Start the engine and rev to higher RPMs to load the alternator while double-checking voltage output with a DMM to confirm 14V+ charging voltage.

Following proper diode testing and replacement procedures helps keep your charging system in peak condition for years of reliable service. Be diligent about assessing root causes of any failures that are discovered to prevent repeated diode issues down the road.