Bad alternator

[bob daily]

At the end of last season my tachometer would stop working and the battery light would flicker, initially I thought the key switch or a ground wire. After some research I found that the RPM's are delivered via the alternator, I removed the alternator and it was tested and found to be working, I tightened up everything tested and boat was working as normal. This year after fishing for three days same problem occurred, day four I had two dead batteries, replaced both starter and house boat would run fine for 30-45 minutes then the alternator would quit. Removed alternator and had it tested again they said it was fine, I thought it might be a heat problem as it will put out 13.5 volts when first started and then fail after running for awhile so I fished with the motor box propped open it still failed. I ordered a new alternator but would not get it for 5-6 days, so since I now had extra batteries I charged and changed out with fresh batteries for 5 days and kept fishing. Received new alternator and swapped out and everything is working great. I will have old alternator tested again (under load and for at least a hour) and rebuilt for a backup.
My question for those electrical knowledgeable is why would it keep testing good and then fail?
Needless to say the fishing was good and returned home with the freezer full ! Bob

 

[Martin610]

I would try to get it tested at a different shop if you can. When it comes to testing not all shops are equal.

I didn't notice how old your boat is. If the alternator bearing is showing signs of wear, or corrosion, that may take some running time to show up. If the testers just put it on the bench, spin it up, and check electrical output, that may not be enough time for a mechanical issue like a bearing problem to show up.

Are you sure the belt tension is correct?

You'll see some other ideas posted here I'm sure.

-martin610

 

[BB marine]

Bob it could be heat related. Electrical component troubleshooting can be difficult when it is intermittent. Many times there is a poor contact point that cause the connection to make or break. By simply removing a component and reinstalling it can temporarily fix the issue. When the vibrations from the engine influence the connection the electrical devise fails again. This can also happen when heat is applied. Windings in motors or generators can corrode, crack or short. When the component is cool all is good and the connection is not compromised. When the component heats up there is expansion and an open connection or short occurs. I worked on outboards for years. The stators under the flywheel would generate AC electricity for charging the batteries ( rectifier converted AC to DC) The stator was also used for ignition, generating the primary voltage for the ignition system. When inspecting the stator it would test OK, matching factory resistance and voltage out specifications. The engine would start up. The customer would have a complaint the the engine would not start. Stators generate heat when operating. I found that using a heat gun for troubleshooting electrical components was a good tool. The heat gun would simulate the higher temperatures that occur when the engine is running and the stator or alternator are generating electricity. Alternators run hot, most operate at 190 to 200F when generating charge voltage. An alternator that is cold may test good but after it comes up to temperature the resistances may get high enough to fail if there is a issue with it.


My advise inspect every connection and connector that is related to the charging system. When you take the alternator to the repair shop explain to them you think the failure is do to heat or vibration. It may test ok on the bench test but warrants a full inspection of internal components to find the issue. Testing it with a heat gun may prove that it has a failed component.