Alternator Overcharging

[joshstan1978]

I replaced an alternator on a 4.3 mercruiser about a year ago because the gauge on the dash was reading low and I had to use a jumper pack a couple times. Got a new alternator online and everything was good for the season. I tried to get one that matched the output of the original. Can’t remember the specs. This season the battery (was very old) stopped holding a charge. Had it tested and bad battery. Got a marine battery from a buddy and put it on a trickle charge and seemed ok. Threw it in the boat and seems fine EXCEPT when cruising at higher rpm’s. The gauge on the dash will be all the way up at approximately 80 percent or in the red higher end. The radio cuts out too. If I put on the blower motor, lights and trim up and down the radio will kick in because the charge is back down to normal. I don’t know much about electrical systems yet. What should I test and try next ??

 

[Bt Doctur]

disconnecting the battery cable lead can do it causing the alt to run wild. dirty connection ,fittings, corrosion,battery switch and connections neg cable at engine loose ,corroded, etc Also a open winding in the alt can do the same.

 

[Lou C]

You need to get a Merc wiring schematic for your engine and find out where the S (sense, which reads system voltage) terminal on the alternator connects to the harness. If that connection is corroded or loose, the alt won't properly sense battery voltage and it will tend to overcharge. If it is an old harness with splices buried etc, one thing you can do easily is disconnect the sense wire at the alt and tape it up in the harness, then run a short jumper wire from the alt B+ terminal to the sense terminal. Since the B+ terminal is connected to the starter solenoid which in turn is fed by the battery +, it will read battery voltage pretty accurately.
Some Volvos were wired that way from the factory and, my OMC had the same problem and I fixed it the same way.

 

[dwco5051]

disconnecting the battery cable lead can do it causing the alt to run wild. dirty connection ,fittings, corrosion,battery switch and connections neg cable at engine loose ,corroded, etc Also a open winding in the alt can do the same.

Check, clean, tighten all your battery connections especially all the ground (negative) ones.

 

[airshot]

If you have a starter/ generator rebuilding service near you, take your alternator and battery to them to be checked out.