Alternator failures

[dwestie]

7.4 mercuiser.
My alternator failed. Bad diodes and voltage regulator. Had it rebuilt and all was fine for the first hour or so. Then again voltage wouldn't go above abot 13 volts and current varied from about 3 amps into battery to 1 amp from battery while running. Once i shut down the engine had 3 amps flowing from battery. Disconnected alternator and no more 3 amp load. What would cause 2 alternators to fail?

 

[dwco5051]

7.4 mercuiser.
My alternator failed. Bad diodes and voltage regulator. Had it rebuilt and all was fine for the first hour or so. Then again voltage wouldn't go above abot 13 volts and current varied from about 3 amps into battery to 1 amp from battery while running. Once i shut down the engine had 3 amps flowing from battery. Disconnected alternator and no more 3 amp load. What would cause 2 alternators to fail?

Not saying this is your case but alternators can be damaged if the battery is disconnected while the engine is running. I would check the battery connections, cables, battery switch and the engine ground cable.

 

[sam am I]

I assume you have a clamp-on current meter or inline amp gauge with the battery?

While running those current readings sound a bit low somehow, esp right after starting up, I'd think, the charge current would jump higher then 3 amps..........

Also, to re-fry an alternator, normally it would usually take way higher than 3 amps to fry it.

Wondering if your re-build was done right.......

 

[dwestie]

I have a battery monitor that measures the current via a shunt. When i installed the rebuilt alternator everything was fine for about an hour. It was after that that i was only getting 3 amps and about 13 volt.

 

[dwestie]

What do the other 2 wires on the small connector do? The regulator is in the back of the alte

 

[sam am I]

What do the other 2 wires on the small connector do? The regulator is in the back of the alte

 

[sam am I]

everything was fine for about an hour.

Oh yeah, missed that part............

It was after that that i was only getting 3 amps and about 13 volt.

How much was it drawing during that hour? Also, what was voltage?

 

[dwco5051]

I just had to replace the alternator in one of my trucks after 175 thousand miles and this was packed with the replacement. I have no idea if your engine has an ECM but in my case the brown wire feeds the PCM and activates the warning light. The gray wire excites the field coils and duty cycles them up to about 550 cycles per second. The voltage to the field will stay right around 8.4 volts.