How to test rectifier or charging circuit?

[jbjennings]

Howdy fellers,
It's been a while since I've been on the merc forum. I've got a '73 merc 65hp that's still going strong with jbweld holding the water jackets together after probably 15 6 gallon tanks of gas. I've got myself another boat/motor/trailer cheap with a '75 merc 65hp on it to take it's place though, and hopefully have enough parts to keep this one running for a long time. I've got it running but while replacing the battery cables and repairing the wiring harness, I've boogered up the rectifier on the "newest" motor. I'm going to take the rectifier off my pontoon barge 65hp and use it momentarily. However, I don't think the motor on the barge is charging.
QUESTION: How do I check the rectifier?
How do I check to see if the charging circuit is working?

Also, I just wanted to mention that my newest merc has 150psi on the middle and bottom cylinders and 145 on top. I'm really happy about the compressioin #'s. I've had it running and it sounds good, but am replacing the wiring and am going to rebuild the carbs just in case they're dirty . This motor hasn't been used very much at all in the last 10 or 15 years. The impeller is working good, and there's an old one on the splash well so I'm going to test it out before tearing the lower unit off to check the impellers since it's working good and I don't think it's that old of an impeller. The oil in the lower unit is black but no water. I'm very pleased so far since it wasn't running when I bought the old boat and therefore got a good deal on it. I had in running good in about 10 minutes--it was missing a fuel pump bolt and the fuel was a little stale.
Thanks,
JBJ

 

[Laddies]

Re: How to test rectifier or charging circuit?

You can see if it's working by just hooking a voltmeter to the battery and starting the engine at high idle the voltage will climb. Heres a rectifier test.

 

[jbjennings]

Re: How to test rectifier or charging circuit?

Laddies,
I had my fingers crossed that you were still on the merc forum!
Will test it out and see what happens. I'm finding these 3 cylinder 65's aren't too bad. Of course, I've never ran the 4 cylinder 65!
The diagrams were Super.
Thanks for the help,
JBJ​

 

[Craigcat]

Re: How to test rectifier or charging circuit?

If you've rebuilt the carbs, replaced needle & seat, and then did a leak-down pressure test, I suggest you change out the fuel lines.

Do the purge/leak test for fuel routing, and replace the rectifier if it fails
that test. Check the voltage before you start the engine, and compare
after its started. It can have melted the internal diodes if its excessive.
The electric system is DC voltage, and the purpose of the rectifier is to
change the A/C waveform to Direct Current (DC). The ECU relys on DC.

Once you first notice that the motor is not performing as it should, immediatly park it and find out what is wrong. Do not keep trying to ride it. That is how motors get destroyed for no good reason other than it needed fixing before the damage is done. Now you need to do a compression test to see if you damaged your pistons. Hopefully, the rectifier has gone south and that is why it will not rev out properly, a very common cure for this type problem.

Also, knowing your carbs and fuel system is clean and flowing properly is very important on old motors. A clean fuel system and carbs will go along way to keep you from needing a new motor premateurly.

Hope this helps

 

[Craigcat]

Re: How to test rectifier or charging circuit?

Also I would like to add that TODAY's gas is very corrosive. These older motors were not designed to perform well with the gas that is available
today, so to protect the internals We suggest stabilizer as gas will go bad
in a few short weeks. If the pistons tops are black you should de-carbonize
them as well. Seafoam has a new airosol called Deep Creap you can just
spray into the carbs while running or you can use the old school method
of a small container of gas and Seafoam mix to de-carb your motor.