Mercury Outboard Charging System Testing

While working on a 2002 Mercury 40 outboard, we noticed after taking it apart that the stator connections to the regulator-rectifier had overheated and the wiring was burnt out. 

We went ahead and cut them loose to determine whether this was just a bad connection that caused them to overheat, or if there’s a problem inside the regulator-rectifier or the stator. Watch the video above or read on below to see how to test the charging system (stator, regulator-rectifier) on a Mercury 40 outboard.

First thing is to open the conduit for the wires so you can take measurements with a multimeter. These multimeter tests will help you determine whether you need to replace the stator and/or regulator-rectifier, or if you can save a lot of money by putting new ends on the wire connectors instead.

Start off by cutting off the charred connection with wire cutters, then using a wire stripper to strip the wires back a little bit so you can take some measurements.

Outboard Stator Static Testing

The stator is a single-phase unit, so there are only two wires instead of the three found on the regulator-rectifier. On the stator, it’s difficult to measure really low resistance, so you want to make sure that neither one of the yellow wires are shorted to ground or in the case or the mounting points of the stator itself.

When checking for resistance through the windings, because it’s a single phase system, you're going to be looking for a reading in the 0.1. to 0.2 ohm range. Check each end of the winding going back to ground on both wires to make sure it's not shorted, and then go across to see what kind of resistance you get. Our results show we’re in the mega ohm range for both wires, which tells us we’re good to go. 

Next, adjust for your leads by putting your multimeter’s leads together to find your zero point, which in our case is roughly 0.2 to 0.3 ohms. That means that whatever we measure, we have to take that number into account to actually get to our zero point. 

Our measurements showed 0.4 to 0.5 ohms, which is a good reading, as it's within range of our zero point.

NOTE: The preferred method of testing a stator to see if it works is to do a live AC measurement with the engine running, but that wasn’t an option on this particular stage of the project. If you want to see how to test an outboard stator with the engine running, watch the video below for a more comprehensive testing procedure.

Since the stator checked out, it’s time to test the regulator-rectifier next. If it checks out, then the next step is to fix the connections on the wires, which would save a lot of money by not having to replace the actual charging system components. 

Outboard Regulator-Rectifier Testing

The first test to perform is a diode test to see if the diodes can reverse bias and forward bias. To do this test, set up your multimeter by switching it over to the symbol circled in the picture below. 

NOTE: On our multimeter, we also needed to shift it to diode testing by pushing the yellow rectangular button on the upper left-hand side above the dial. 

Take the black lead on the multimeter and connect it to the red wire, then take the red lead and connect it to either one of the yellow wires. This is for forward biasing the diode, and it should read somewhere in between 0.4 and 0.5 volts, which is what we got.

The reverse bias test is to make sure that the diode is stopping voltage from passing through. To do the reverse bias test, you need to switch the polarity (red lead to red wire; black lead to yellow wires) and look for an open circuit (OL), which is also what we got. 

Next is the SCR voltage test, with one side grounded while the multimeter is still set to diode, and the red lead going to either one of the yellow wires. The reading on our multimeter should be close to 1.5, and the 1.9 reading we got is within range, so we know the regulator-rectifier is good.

NOTE: For a more comprehensive look at testing a regulator-rectifier, check out our How to Test a Regulator Rectifier on an Outboard video below.

Installing New Connectors

With the stator and regulator-rectifier checking out, it’s time to put new connectors on the wiring that connects both components together. 

NOTE: We used a replacement terminal kit from Mercruiser, but if you're in a pinch, you can use butt connectors and do a direct connection. However, if you’re going to use butt connectors, make sure they’re the type you would seal with a heat gun to get it to encapsulate the connection. If you use a regular blue butt connector that's open on either end, it's going to corrode, especially in salt water. 

While making connections on the stator wires, make sure you put on the insulator first, then use a quality terminal crimper to hold the butt connector in place while bringing it over the wire. Make sure you've got it at the right depth before crimping the connector to the wire. 

While making connections on the stator wires, make sure you put on the insulator first, then use a quality terminal crimper to hold the butt connector in place while bringing it over the wire. Make sure you've got it at the right depth before crimping the connector to the wire. 

After crimping the butt connector onto the wire, all you have to do is pull up the insulator over the connector and the connection is good to go. 

Repeat the same process for the other wire on the stator, and then again for the three wires on the regulator-rectifier and you’re done.

By testing the stator and the regulator-rectifier first, we saved a ton of money, as we determined we didn’t have to replace those components. If you have a burnt connection on your outboard’s charging system, test those components first and if they check out, replace the connectors on the wires to see if that solves your burnt connection problem.