Yamaha 9.9 four-stroke rectifier issue

[vizwhiz]

To start with, I have a '93 Yamaha 9.9 four-stroke, pull-start, tiller control, manual tilt, extra-long shaft outboard on a sailboat. The model number on the nameplate is not valid, after I compared it to the model/feature comparison for those motors...and the guy who sold it to me rebuilds motors from pieces and parts, so it's really a '49, '50, '51, '52, if you know what I mean.

I wanted to see if I could set up the charging circuit to provide an extra battery charging option if needed. It's only supposed to supply 6 Amps, and of course that would be at a healthy RPM, so I'm not going into this expecting much. But I checked the magneto and the wiring harnesses in the motor, which has the CDI box, and it turned out that it had the wiring for a charging circuit in-place and wired to the magneto. So I bought a used rectifier off the "e" website online...came from a marina or boat repair place down on the Gulf Coast somewhere... I figured they pulled it off a motor that HAD been working, and they were just parting it out.

I installed it according to manuals and parts lists I found online (boats-dot-net is a super-great resource for parts diagrams and such). Put the ears on the motor and ran it in the garage on the stand. With my voltmeter, I was getting voltage out of the red wire when checked to ground on the motor chassis. Voltage increased as I throttled up, so I gave it a thumbs-up and called it a success.

Due to life getting in the way, I didn't get to check the motor out for almost two years - this past weekend. (I would run it periodically to make sure things worked, oil circulated, etc.) I had finally put the motor on the boat and was going through making sure things worked. I ran a wire from the red output wire on the rectifier to a common positive connection so it would feed the positive battery circuit, and ran a wire from the common battery ground connection to the motor chassis. Then I cranked up the engine to check the voltage output and see if there was any current going to my already-charged-by-a-solar-panel house battery (~12.7+V). To make the long story short, it seems that the power back-fed somehow and before I could even get my voltmeter on the terminals, it burnt the wires to the rectifier up. Toasted. Roasted. Fried, crispy-like. I quickly shut everything off, but it was connected that way for several minutes, and it melted wire connectors and the little wire holder, and even part of the cover. I also noticed during the event that the battery voltage dropped to 10.5V (which is why I say it was backfeeding from the battery through those wires to ground). Once disconnected, the battery voltage went right back up to 12.7V.

So I'm wondering if a bad diode in the rectifier would have allowed the power to flow directly to ground? Was I wrong to connect the battery ground to the motor chassis? What else could have caused that much power to drain through those wires? Is there something else about a rectifier that I should know that I don't? (Not rocket science, I didn't think...) The magneto can't put that much power out... Any ideas?

 

[Brian D]

I am not sure if you have the service manual. If you do, that might help. If not, http://briandphoto.net/docs/LIT-18616-01-24.pdf is where you can get it.

Other than that, I am not a motor guy but when your battery took off like a bat out of hell, I could only thing direct short. Is the Magneto the way to go with this motor? I wonder if you needed an isolator between the motor and the battery. My '95 9.9 had an isolator on the output of the motor. But for the life of me, I cannot remember if that motor had an alternator.

Anyway, like I said, if you don't have the Service Manual, download it and check out how the charging circuit is wired and working.

Good luck

 

[Johnb]

If the battery was connected with the polarity reversed with respect to the diodes in the motor then what you described would happen. When you tested the motor in the garage and got voltage did you note the polarity of the voltage? This would have required paying particular attention to how the test leads were plugged into the meter and that little minus sign that precedes the voltage reading.

As an aside, this is a perfect demonstration of why all wires connected to a battery need to have a fuse, commensurate with the size of the wire, at the battery. The current that the charger can produce is unlikely to fry things but the battery can produce welding current.

If you plan to try again I would suggest both of the above.

 

[vizwhiz]

I appreciate the feedback. I've purchased a replacement rectifier, and plan to try again with a new one that I know is good.

Is there any reason I shouldn't run the battery ground to the motor chassis, where the motor electrical all grounds? I can't see how it would work as a charging circuit if the motor is not grounded to the battery also...

 

[Brian D]

Do you not have a wiring harness? If so, then the harness is already at a single ground point on the chassis. That should take care of any grounding needed between the battery and the motor. Just, this time, put a fuse at the battery so if you have the same issue again, it will take out the fuse.

 

[vizwhiz]

I do not have a wiring harness... As I mentioned, this is sort of a Frankenmotor, reduced down to a pull-start no-electric motor. It has the kill switch citcuits and the oil pressure sensor, and the wiring from the magneto for the charging circuit was all there and landed. But there was no rectifier or wiring harness. So it appeared to be simple - four-wire rectifier and a duplex wiring harness, green wires from the magneto to the two rectifier inputs, positive from red output to battery positive, negative from battery negative to the ground lug on the chassis where everything else was grounded (including the rectifier). It is that simple, isn't it?

 

[justsomeguy]

positive from red output to battery positive, negative from battery negative to the ground lug on the chassis where everything else was grounded (including the rectifier). It is that simple, isn't it?

On my 4 horse 4 stroke, there's an in-line fuse on the hot (red) wire just after the rectifier. HTH

 

[walt]

So it appeared to be simple - four-wire rectifier and a duplex wiring harness, green wires from the magneto to the two rectifier inputs, positive from red output to battery positive, negative from battery negative to the ground lug on the chassis where everything else was grounded (including the rectifier). It is that simple, isn't it?

I would think so.. there would usually be about a 10 or 20 (?) amp fuse in the +12 volt red wire run. The black wire would run back to the battery negative. I dont think it hurts anything to also have the black wire grounded to the outboard - but it also should not need to be grounded to the outboard for charging the battery.

If you first run that setup without a battery, the voltages will seem screwy but you can get an idea if its working correctly. Using a DVM, measure the voltage on the red and black wires (which will eventually go to the battery). I think you said you did this in your first post but didnt see what voltages you measured. Im just going by memory but at low throttle, you might see voltages well under 12 volts and at high throttle you might see voltages up near 20 volts. At least.. that is about what I saw on a small outboard I had in the past.. The voltage seems screwy because the motor is putting out pulses each time the coil moves past a magnet and the DVM will do some sort of averaging (not true RMS). So even though the "pulse" at low RPM is large enough to dump current into the 12 volt battery, the meter averaging might show it well under 12 volts because there are fewer pulses. At high RPM, seeing voltages well above 12 volts also doesnt mean anything bad.. there are just lots of pulses and the averaging of the meter shows a higher voltage.

The above test should be safe to do - no load so no current - so no big rectifier damaging voltage spikes. I would do it with mine but be careful. Accidental transient shorting the wires will likely blow the rectifier even if you have a fuse in the line. The very bad thing to do is to have that setup dumping charge into a battery and then disconnect it from the battery while the engine is running. In this case, the coils are pulsing current to the battery and when that gets interrupted, the current (magnetic field) in the coil has no where to go and creates a BIG voltage spike that can back bias and breakdown the diodes.

 

[vizwhiz]

Just for the record, i have a fuse to install in the positive wire run. I just didn't put it in at first because I wanted to be sure everything was working correctly before makinb things permanent. I will install the fuse right away this time.

 

[vizwhiz]

Fixed the rectifier issue. I bought a new one, generic, made for motorcycles, ATVs, and such. Works great.
I checked the output of the magneto as AC Volts, and at idle-ish it was generating about 24V. When I revved up it would go up as high as 45V, and I'm sure it would have gone farther, but I had no need to prove it beyond that.
Checked the new rectifiier output, and at rest (no load) it was putting out about 3.3VDC. It didn't change as I revved the motor.
Hooked up the output from the rectifier to a 12VDC car light bulb, and it lit up. The voltage across the light bulb jumped up to 6-ish volts, climbing to 7.2 or so as I revved the motor. The load made the difference.
I grabbed a 12VDC inflator blower that I had, and temporarily wired the output from the rectifier to the cigarette lighter plug on the blower. 3.3V before turning the blower on. When I turned it on, the meter ran right up to 12.3V and the blower ran like a champ. Sped up the engine and the voltage cranked up to 13.4 and the blower sped up at the same time.
Perfect.
Wired in the fuse, connected everything to the battery, turned the switch back on, and...held my breath. That and checked the voltage, the wires, etc. Everything was fine.
Problem solved.
Thanks to everyone for the input!

 

[Brian D]

Bravo Zuzu!