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1983 Johnson 90 v4 rectifier diagnosis and replacement

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Uploaded on Sep 8, 2009

My boat was reading 12.2 V at all speeds on the battery, a good sign that something is wrong, not charging. This is a 10AMP regulated system, after looking into specifics, charging should be around 13-13.5V. For motors without regulators 14+Volts is typical. Here is a test to try on a 3 wire rectifier, pull the negative from the battery to perform:

Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray,
then the red wire (some rectifiers may also have a fourth yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction.

Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again.
Once more, you should get a reading in one direction and none in the other.

Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires.

Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier. Note that a rectifier will not tolerate reverse polarity.


After I replaced it with an $8 rectifier it charges at 13.2 Volts. OMC wanted $60 for a new one, my way is cheaper but not tried and true. I may add a small heatsink later.

Update sept 16th, it charged tonight over 14V.

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