1980s 50hp Mercury Not Recharging Battery

Can anybody help? I have a 1980s 50 hp Merc that will start fine after battery is charged, i can go off and fish at 1 or two locations and then the engine progressively becomes harder and harder to start until its dead. Yesterday i was able to pull start it to get back to the ramp, but once back at the ramp i tried to fire her again and she wouldnt even pull start. There is enough power in the battery to easily operate the tilt/trim (even when i was back at home) but the fish finder was turning itself off when i was in strife out on the water. This is the second time this has happened. The first time i put it down to a weak battery ( the one im using has only had 3 trips out on it in 7 weeks and has been fully recharged after the first non starting incident) and a couple of slightly loose nuts under the cowling, but that clearly is not the case and i guess im now stumped. Any help or advice would be greatly appreciated. I should probably add to this that im a complete novice and am only just finding my way with boats engines etc.

Re: 1980s 50hp Mercury Not Recharging Battery

Time to pick up a factory service manual! You're gonna need to troubleshoot the charging system; most likely the problem is a bad rectifier.

If you check the resistance of the stator windings (two yellow wires connecting to the square-ish rectifier which will also have a red wire connected), it'll be a very low resistance. If you don't have a meter, Sears has them on sale all the time. One in the $20 range is perfectly adequate for the type of troubleshooting you'll do to your Merc.

If the stator has some low value of resistance the rectifier is most always bad. If the stator has infinite resistance it's 'open' and requires replacement.

You can also check the rectifier with your meter. Use this forum's search feature for "rectifier test" "rectifier testing" or similar and you'll find instructions for doing so.

Other things you can do with your meter is to check charging voltage at the battery; it'll be close to battery voltage at idle and should show a rise as soon as you increase engine speed. You can also measure AC output at the (2) yellow stator wires, which will tell you if everything under the flywheel is working as it should.

Rectifiers are quite simple to replace, a couple of mounting screws and (3) wires; a bad stator is a bit more "fun" since you'd need to remove the flywheel to get at the stator. Let's just assume the rectifier's shot and take it from there.

BTW a manual will really come in handy if you have to pop that flywheel !

You can look up parts for your motor at this site in the Store, or other places such as www.iboats.com or www.boats.net

This is a great place to find parts diagrams, you'll need your motor's serial number to get to the right diag's:

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

HTH............ed

Re: 1980s 50hp Mercury Not Recharging Battery

2819Con said:

Can anybody help? I have a 1980s 50 hp Merc that will start fine after battery is charged, i can go off and fish at 1 or two locations and then the engine progressively becomes harder and harder to start until its dead. Yesterday i was able to pull start it to get back to the ramp, but once back at the ramp i tried to fire her again and she wouldnt even pull start. There is enough power in the battery to easily operate the tilt/trim (even when i was back at home) but the fish finder was turning itself off when i was in strife out on the water. This is the second time this has happened. The first time i put it down to a weak battery ( the one im using has only had 3 trips out on it in 7 weeks and has been fully recharged after the first non starting incident) and a couple of slightly loose nuts under the cowling, but that clearly is not the case and i guess im now stumped. Any help or advice would be greatly appreciated. I should probably add to this that im a complete novice and am only just finding my way with boats engines etc.

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If the battery has enough charge to operate the trim/tilt motor then it has enough to run the fishfinder. I think you have a shorted wire or bad connection somewhere.

Start at your battery terminal connections; remove them and inspect. Use a wire brush to clean off the cable ends and the terminals on the battery.

Follow the cables from the battery to the motor and examine the connections there. Same deal, disconnect, clean and reattach tight.

Once clean, reattach the battery cables using HEX nuts, not wing nuts. Tighten them down with a wrench.

Get a voltmeter and check the battery voltage with the motor off. Should read 12.6 to 12.8 on a fully charged battery.

Start the motor and increase RPMS to 1000 and check voltage at the battery again. Should show SOME increase in voltage from what you saw with the motor off. This will tell you if your rectifier is putting out any current at all.

Re: 1980s 50hp Mercury Not Recharging Battery

Thanks Heaps Fellas, i know exactly what your talking about. The loose nuts i mentioned earlier were on the yellow stator wires. Ill give your advice a shot and keep you posted. Here in Australia there are no bloody outboard mechanics working until Jan 11 so i have plenty of time on my side before i concede defeat and bite the bullet with a mechanic. Your input is much appreciated.

Re: 1980s 50hp Mercury Not Recharging Battery

Here is how to check the rectifier.

1. Disconnect the battery leads.
2. Disconnect all wires on the rectifier.
3. Connect the red test lead to ground and the black lead alternately to terminals A and C.
The meter should show continuity.
4. Connect the black lead to gound and the red lead alternately to terminals A and C.
The meter should not show continuity.
5. Connect the black lead to terminal B and the red lead alternately to terminals A and C.
The meter should show continuity.
6. Connect the red lead to terminal B and the black lead alternately to terminals A and C.
The meter should not show continuity.
7. Replace the rectifier if you do not get correct readings.

Re: 1980s 50hp Mercury Not Recharging Battery

Turns out my problem was a combination of 2 things
1. Spark was jumping down the side of plug on 2 plugs therefor was only getting kick to 1 cylinder. 3 new spark plugs and boots. cost about $35 AUS or $30 US
2. There is a problem with the stator it wont recharge my battery beyond 12/12.5 but it consistently seems to always reach that level so i guess thats not too bad but well worth keeping an eye on, i was advised to change it at the cost of about $600, but since my battery isnt taking a pounding of me fruitlessly trying to crank my engine i have decided against it and just bought another battery which i now run all my accessories off.
Now she starts first time everytime and runs a like dream. Thanks to all for your advice even if it wasnt necessarily the right diagnosis i have still learned a heap of stuff that i had no idea about previously.