Mercury 125EXLPTO Charging Question

lsyrkin

Recruit
Joined
Oct 19, 2012
Messages
5
Hi,

Can someone explain how Mercury 125 charging system is designed? My uncle asked me to help him with his Mercury 125 125EXLPTO outboard. He noticed that voltage on running motor is about 12V and it used to be 13.5V. I have some experience with the car electrical but not with outboards. I suspected that generator does not work. We disconnected the battery while running the motor and it did not die. It makes me think that either generator is OK or ignition and charging are separate systems. Does it mean that when the voltage regulator is broken the motor still can run?

Thank you in advance,
Leonid
 

Barnacle_Bill

Admiral
Joined
Feb 8, 2004
Messages
6,469
Re: Mercury 125EXLPTO Charging Question

Test the rectifier/regulator. If it wasn't bad it most likely is now. Never run an outboard with the battery disconnected. It is not an old car with a generator system. There are coils that is part of the stator that provides AC to the rectifier which changes the AC to DC. The systems now days have a regulator that is part of the rectifier. When you run the motor with the battery disconnected you blow diodes in the rectifier. Also if you connect the battery up backwards even for a split second it will blow diodes.
 

wired247

Lieutenant Commander
Joined
Oct 8, 2011
Messages
1,557
Re: Mercury 125EXLPTO Charging Question

Yeah. Don't do that. You dont have a separate alternator or generator aside from the stator.

Boat motor companies are finally starting to pull their collective heads out of their butts and they are putting normal automotive style alternators on newer model boat motors. That isnt what you have though. The stator under the flywheel acts as a generator to power and time the spark and to create current to power the battery. Disconnect the battery from the motor for any amount of time and the fragile regulators they put in these things fry pretty quick. You may have had a bad one already though. If you are handy you can pick up a cheap bridge rectifier from radio shack and run the charging system through it for an unregulated system like boat motors did for decades or you can buy a new rectifier/regulator set for 30X the price. Let ebay be your friend and don't get too hung up on part numbers.
 

oldman570

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Joined
Mar 25, 2011
Messages
1,615
Re: Mercury 125EXLPTO Charging Question

Test the rectifier/regulator. If it wasn't bad it most likely is now. Never run an outboard with the battery disconnected. It is not an old car with a generator system. There are coils that is part of the stator that provides AC to the rectifier which changes the AC to DC. The systems now days have a regulator that is part of the rectifier. When you run the motor with the battery disconnected you blow diodes in the rectifier. Also if you connect the battery up backwards even for a split second it will blow diodes.

+1^ on bhiles advise! If the rectifier diodes were not bad before, and the motor was run with the battery disconnected, they probably are now. The CDI website has a section that tells how to test the stator and the rectifier, or a manual for the motor will tell how to do the test. The motor ignition and charging should be treated as different system on most motors. JMO
Oldman570
 

lsyrkin

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Joined
Oct 19, 2012
Messages
5
Re: Mercury 125EXLPTO Charging Question

Thanks to all who answered. The boat was purchased used - no any documentation or manuals. So we are learning sometimes hard way. I ordered the voltage regulator from Amazon. Will update with the results.
 

carholme

Rear Admiral
Joined
Sep 4, 2010
Messages
4,845
Re: Mercury 125EXLPTO Charging Question

Leonid;

I am pretty sure I have the service manual. If you post your engine s/n and PM me with an email address, I will send you a copy of the manual.

Gerry
 

lsyrkin

Recruit
Joined
Oct 19, 2012
Messages
5
Re: Mercury 125EXLPTO Charging Question

Yeah. Don't do that. You dont have a separate alternator or generator aside from the stator.

Boat motor companies are finally starting to pull their collective heads out of their butts and they are putting normal automotive style alternators on newer model boat motors. That isnt what you have though. The stator under the flywheel acts as a generator to power and time the spark and to create current to power the battery. Disconnect the battery from the motor for any amount of time and the fragile regulators they put in these things fry pretty quick. You may have had a bad one already though. If you are handy you can pick up a cheap bridge rectifier from radio shack and run the charging system through it for an unregulated system like boat motors did for decades or you can buy a new rectifier/regulator set for 30X the price. Let ebay be your friend and don't get too hung up on part numbers.

Voltage regulator has additional red and grey wires. With the bridge replacement they will not be connected. Can the motor work without them?
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Re: Mercury 125EXLPTO Charging Question

If you post the serial number of the motor, we can tell exactly what components are in your charging system. Some of the mercs used dual VRs some used a single VR with 4 yellow stator wires and 2 red wires. Grey is for tach signal

any idea of the part number of the regulator you bought?
 

wired247

Lieutenant Commander
Joined
Oct 8, 2011
Messages
1,557
Re: Mercury 125EXLPTO Charging Question

Voltage regulator has additional red and grey wires. With the bridge replacement they will not be connected. Can the motor work without them?

Grey goes to the tach. Either yellow can signal that. Red goes to the battery which you will be tapping one line off of the bridge rectifier for.
 

lsyrkin

Recruit
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Oct 19, 2012
Messages
5
Re: Mercury 125EXLPTO Charging Question

Thanks to all who answered. The boat was purchased used - no any documentation or manuals. So we are learning sometimes hard way. I ordered the voltage regulator from Amazon. Will update with the results.

I received the regulator and installed it. I measured up to 14.5 volts as expected for good VR. A couple of issues I noticed after that. Tach readings got too high. It jumped to 2000 rpm on idle and all the way up over 6000 rpm on 20 mph. Another thing that sounds not normal was motor noise on the radio. I am not sure if it was the same before the replacement. I just noticed it and it does not seem to be right for me. One more thing I noticed - when press transmission button on the radio tach reading jumped up. Looks like interference from the radio to the tachometer and the opposite. Thanks to everyone who already answered and to those who will do.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Re: Mercury 125EXLPTO Charging Question

Sounds like the tach wires are too close to the antenna of the radio. What is the Tach set on? 12P is usually correct.
 
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