1984 Mercruiser lost battery charge. Bad alternator?

[kbmsound]

I was out on the boat today, beautiful day. Cruised on the Hillsborough River in Tampa, Fl for about 2 hours, at mostly idle speeds. Upon returning, i kicked reverse in to get close to another boat. Boat would not start after that. It would try to turn but it had no juice. Batteries were dead. I am guessing bad alternator. Other than taking to autoparts to test, can i do a test a home to check like disconnecting batteries while motor running?

Also, I have a problem with motor dying while back to neutral from reverse. Anyone have any pointers? Idle speed to low? Kill switch need adjustment? Many thanks in advance.
Les.

 

[Bt Doctur]

During your cruise what did the voltmeter gauge read? stalling while coming out of gear is usually a worn shift cable or corrosion in the shift mech from not taking the outdrive off for service.

 

[Scott Danforth]

Dying while shifting in reverse is most likely a bad shift cable. You can start motors and disconnect batteries, however it's not recommended for alternator longevity. Simply measuring alternator output is adequate

 

[Bt Doctur]

Dying while shifting in reverse is most likely a bad shift cable. You can start motors and disconnect batteries, however it's not recommended for alternator longevity. Simply measuring alternator output is adequate

DO NOT DIS-CONNECT A BATTERY CABLE WITH THE MOTOR RUNNING, that worked for generators but does not work well with alt systems.

 

[wrench 3]

Charge the battery. Then check the voltage at the battery with the engine running. Any cheap multi meter will do. You should have 13.5 to 14.5 volts. If you don't, rev the engine to about 1500 RPM and recheck the voltage. If it's still below 13.5 it's not charging. If you remove the alternator any auto electric shop can test it for you. Also many retail auto parts stores offer the same service.

 

[kbmsound]

I must admit my volt gauge is old, and might not be reading very accurate, but it read 12v. The shift cable i just had replaced 1 month ago, so i know that's not it. I have the batteries hooked up to a trickle charge. Tomorrow I will attempt to start, and will measure the volts at the batteries. I have 2 batteries with a switch. I am not very familiar with how these switches work. When the motor is on, does it charge both batteries at once? It has 2 positions, 1, or 1+2. Does the switch combine both batteries on the 1+2 setting?

During your cruise what did the voltmeter gauge read? stalling while coming out of gear is usually a worn shift cable or corrosion in the shift mech from not taking the outdrive off for service.

 

[kbmsound]

Charge the battery. Then check the voltage at the battery with the engine running. Any cheap multi meter will do. You should have 13.5 to 14.5 volts. If you don't, rev the engine to about 1500 RPM and recheck the voltage. If it's still below 13.5 it's not charging. If you remove the alternator any auto electric shop can test it for you. Also many retail auto parts stores offer the same service.

I will perform this test tomorrow, thanks.

 

[Scott Danforth]

your gauge on your dash is not a valid test instrument. go to harbor freight and spend $5 on a multi meter which will be more accurate than the gauge on your dash.

since your shift cable is new, the dying in reverse is still most likely shift cable related, as it pertains to the adjustment and the interupt switch.

 

[alldodge]

Driving along and all is good, shift into reverse, motor dies and when key is turned there is nothing.

Maybe a dumb question, but are you sure the batteries were dead?

 

[kbmsound]

Driving along and all is good, shift into reverse, motor dies and when key is turned there is nothing.

Maybe a dumb question, but are you sure the batteries were dead?

Motor does not die when shifting into reverse, it dies when shifting out of reverse. And it only does it a sometimes, sometimes it stays on like it should. It also does it when shifting out of forward, but much fewer times. I charged the batteries. Motor started right up. I turned off. Hooked the chargers again. Tested batteries while charging and readings were at 14.75v for both. Disconnected chargers, turned the motor on, tested batteries again, both batteries at 12.35v. Not getting charge I presume. Still will take them to get tested to see if they are holding charge.

 

[alldodge]

OK, my misunderstanding.
Battery is a bit low at 12.3V (12.7V), that is if your meter is accurate, my guess would be it is. May also need to adjust the shift interrupter

 

[flipbro]

Fire the engine up Rev to 1500 2000 rpm and check voltage at batteries. Should be 13.5 volts or so..

 

[iand464]

It sounds like the op did that and got 12.3 with the engine running and the external chargers off. This would indicate a faulty alternator or connection. An auto parts store can load test your batteries and alternator for free. It sounds like you may need an alternator though.

 

[flipbro]

I did catch that. Just don't state at what rpm he tested at. But yes 12.3 is low.