How to Test an Outboard Regulator-Rectifier

Before testing the regulator-rectifier on your outboard motor as a potential culprit of a charging or ignition problem, you should eliminate the easier possibilities first. 

Check the battery to make sure the wires aren't corroded, and that the terminals are clean and tight. Make sure the battery has been tested and is healthy. Check the internal harnesses point-to-point to make sure there's no corrosion, breaks or kinks in the line that would be damaging the harness itself. Once you've eliminated all those possibilities, then the regulator-rectifier is the next thing to check.

Pro Mariner digital multimeter

Buy an outboard regulator rectifier

Regulator-Rectifier Testing

The process to test a regulator-rectifier is simple, but can also vary from unit to unit. Watch the video above and read on below to learn how to do simple diagnostic tests on an outboard’s regulator-rectifier.

NOTE: We tested the regulator-rectifier on a Yamaha T60 outboard, but these tests may be applied to regulator-rectifiers of other outboard makes and models. 

Find the voltage regulator-rectifier on your outboard, and grab a good multimeter to test the unit’s inner diodes. 

How to test an outboard regulator-rectifier

A three-phase regulator-rectifier’s diodes take the AC signal that comes out of the engine’s stator-rotor assembly, rectifies it and then smooths it out, usually through a capacitor. Its three phases then become a single regulated DC output voltage. 

When testing the regulator-rectifier with a multimeter, you’re looking to see if one or more of those diodes (in this case six) has failed. To perform a diode test, set the voltmeter to the symbol with an arrow and a block in front of it. You should see an OL (open loop) on the meter’s display when you switch to the diode setting.

Outboard regulator-rectifier diode testing

The diodes come in sets of three on the positive side and on the negative side that you're going to try to forward bias first, and they should each read around 0.5 on the meter. Next, you’ll reverse the lines, try to reverse bias them to make sure they're stopping the current flow like they should be. For reverse biasing, you should get an open circuit reading on the meter for both the positive and negative sides of the rectifier’s terminals. 

Regulator-rectifier bias testing

  • Forward Bias: Connecting the positive voltage potential to the P-type material and a negative to the N-type material.
  • Reverse Bias: Connecting the negative voltage potential to the P-type material and a positive to the N-type material.

Find the three wires coming down from the stator and rotor assembly connection, and the output that heads over to the battery. 

Outboard regulator-rectifier testing stator connector

Look for the positive and negative lines, which you’ll use to forward or reverse bias, depending on which set of diodes you’re testing.

Outboard regulator-rectifier tests

Testing outboard regulator rectifier

Diode Test 1: Forward Bias

Start on the negative side of the regulator-rectifier’s plug and forward bias the diodes across by going in with the positive side lead of the volt-ohm meter. Attach it to the negative side terminal, then use the meter’s negative side lead and touch each diode on the positive side terminal, which should provide individual readings of around 0.5 volts.

Outboard regulator-rectifier forward bias test

Outboard regulator rectifier diagnostics

NOTE: Our readings when forward biasing each diode were 0.524, 0.52 and 0.525, which means the current is passing in the correct direction. 

Diode Test 2: Reverse Bias

Reverse bias that same set, switching sides with the positive and negative leads on the meter. For the reverse bias, you should be getting no reading or zeros (OL) across the board for each diode, which we got when we tested all three.

Outboard voltage regulator diode testing

Switch to the positive side of the regulator-rectifier’s plug, then do the forward and reverse bias diode tests again on each diode. You’re looking for the same results, which is approximately 0.5 volts for each diode on the forward bias test and zero (OL) across the board for each diode on reverse bias. 

Outboard voltage regulator-rectifier testing

Yamaha outboard regulator-rectifier testing

How to Know if the Regulator-Rectifier is Faulty

If there was an issue with one or more of the diodes inside, an open circuit (OL) would be your reading when forward biasing any one of them. If you see something close to zero, that would also indicate an issue. On the forward bias condition, look for something in between 0.45 up to 0.5152 at the most, depending on what type of diodes are inside the unit. 

These tests don't work for every single manufacturer, as some of them have gotten a little more advanced with their ability to self-regulate. Also, you may run into other circuitry that your multimeter can't test past. The tests we ran work for our Yamaha 60 outboard’s particular regulator-rectifier and others like it, but they’re not all the same. Keep that in mind when you’re running these tests on your outboard’s regulator-rectifier. 



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