Mercury rectifier test?

[wiscoboater]

Hi Guys,
Restoring a Merc Mark 55. It has the 3 wire rectifier and I’m wondering how to test it to see if it is good. Anyone know how to go about testing this?

Thank you,
Wannabe

[crosbyman]

you should have continuity one way only from both yellows (AC ) to the red power out DC

if you need to repair and are not a purist a small square full wave rectifier will work nicely
just google full wave rectifier 50 amps 50 volts or more…… should cost you about $5-$10

try electronics junk shops

yellows to sine wave symbol on the rectifier (either pin is ok being an AC voltage)
+ post to battery (RED)
– post to frame ground (black) on the engine frame

mount (bolt on) with a small aluminum plate as backing and heat sink

Joining AOMCI has priviledges 🙂

[frankr]

Not a mercury man, but shouldn’t you add continuity one way only between each yellow and ground? Disconnect leads from rectifier before testing. Best use an analog multimeter or even a simple continuity light. Some digital meters ok if you know how to use it. Some will confuse the socks off of you.

[crosbyman]

true but while continuity is detected…. yellow to red No continuity should be seen to ground

testing yellows to ground continuity only….. will need to have probes reversed and NO continuity should be detected to RED while testing ground continuity

lots of info on the web . Wannabe does not say why he thinks it is bad… no symptoms listed… rectifier does not look toast but those old selenium rectifiers did not work very hard as you surely know. That engine looks like a purist engine…IF the rectifier is bad I would leave it in for looks and hook up and hide the little modern wonder behind somewhere to do all the work 🙂

I have left the dark side with my 75 hp ETEC and my many OMC oldies 🙂

Joining AOMCI has priviledges 🙂

[wiscoboater]

Thank you Crosby and Frank.
Electrical systems on these old outboards is still a mystery to me and I try to take every chance I can to learn about them. From my layman’s understanding of what you have described, I would take my test light clip to pos.+ on battery, battery neg- to ground on motor, then check for continuity at the yellow terminals on rectifier? Like I said, I’m not that familiar with electronics. If this is incorrect, please advise.

Thanks,
Jeff

[Michael]

Why not just start it and see if battery voltage increases as engine is run. Best to get reading on battery voltage before you start it. Leave volt meter attached to battery. Then start it and rev engine a little, if voltage increases above voltage before starting I would say the rectifier is good. If no change in voltage, then test rectifier as suggested above

[wiscoboater]

The motor is in the process of being restored. No where near ready to run yet. That and it’s the middle of winter here so test barrel is hibernating out in the garage. That’s why I was looking for a "bench test" procedure that I can perform in the basement.

[frankr]

quote wannabe outboard guy:

Thank you Crosby and Frank.
Electrical systems on these old outboards is still a mystery to me and I try to take every chance I can to learn about them. From my layman’s understanding of what you have described, I would take my test light clip to pos.+ on battery, battery neg- to ground on motor, then check for continuity at the yellow terminals on rectifier? Like I said, I’m not that familiar with electronics. If this is incorrect, please advise.

Thanks,
Jeff

Well then pay attention and learn something. But first, like I said, I don’t know about Mercury, but I presume that is a full wave bridge selenium rectifier. Nevertheless, the basics apply.

Picture 1 shows what is called a Full Wave Bridge Rectifier, Negative Ground. Very common. Understand this one and you can understand the variations. Note there are four diodes. The four diodes are connected to ground and battery + as shown. We are going to focus on only one diode at a time, first one marked D1 (by me, not on the actual unit)

Those four diodes must be tested one at a time. Any diode is tested the same way, regardless of where it is being used. So, look at picture 2 and learn how a diode is tested. Note the multimeter + and – leads are connected to each end of the diode. You will get either a high or low reading on the meter. Then the + and – leads are reversed in picture 2 (actually the diode is turned end for end). Again, you will get either a high or low reading on the meter. Conclusion: If you get a high reading on one test and a low reading on the second test, the diode is good..

Again, there are two tests for each diode and four diodes. You must test all four diodes. That is eight tests. It must pass all eight or the rectifier is junk. If you have not done all eight tests, you have not tested it at all.

And remember, disconnect the leads from the rectifier before beginning the tests. Otherwise you will get false results.

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[wiscoboater]

Thanks Frank!
That helps greatly. I will give it a try. I appreciate you taking the time to do the drawing. Visuals are what seems to click in my head.

Thanks again!

[frankr]

Followup: It would be possible to modify the circuit and use only two diodes. In that case, the two between AC and ground would be missing and you would only have the other two to test.

Furthermore, it is also possible to use only one diode. That is called a half wave rectifier. Often done in lawn mowers.

Anything to save a buck.

Just so you know.

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