Alternator Light Stays On

Bad alternator you are likely running off the battery and that will not last long. 

 

 

Check (off and on ad off and on a bunch of times) the small wire going to the top of the alternator.  Then follow that wire to the Tach and do the same off and on and off and on to clean up the contacts on both ends of that wire (very important).

Remove the 12V BIG wire at the starter and clean all of the terminals attached there.  Find the battery ground wire and clean both the cable lug AND the place on the chassis it mates to, then put it back and tighten it all until it seems tight.

If THAT doesn't make it right, then it's time to go to someplace where they can test your charging circuit under load (they put a Big, Honkin resistor on it) to see if your alternator is putting out.  If not, it's time for a new one

11.8v then drops to 11v  you're running on battery voltage ( should show approx 13.8v) and when you get to 10.5 the ignition will begin to stumble.  You should have 12v - 12.5v at the battery with the motor off and 13.8v with the motor running. If the voltage remains at battery reading and does not increase with motor running, other than a bad / loose connection,  the alt. is suspect.

 

Chiming in from the peanut gallery; could it be a loose/slipping belt maybe getting ready to go?
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The fan belt uses a tensioner system, but I'll double check. 

Assuming I have a bad alternator, is one replacement just as good as another? Or are there good, better and best?

Get a Bosch alternator.  They are made in either Brazil or Mexico.  The other option is made in either China or Taiwan.  Your third option if you have a Bosch unit is to have yours rebuilt..message me if you any difficultly getting one.

Thanks Anthony. I see a Bosch 75 amp that's says it has been rewound in the US. What's that about?

The rewound Bosch alternators are ok. The higher amp alternators are great  for the increase amperage. One issue with them is they don't start charging until the rpms are above 1700. When you start your car with a standard 55 amp the light goes out almost immediately and unit begins to charge.  I have a 75 amp unit on my daily driver and no issues 

OK, now this is really odd.

I went to the Speedster today to measure the voltage with the car off, then measure voltage with the car running. 

Voltage on the battery was good and when I started the engine the alternator light went off and it was charging the battery at 14V.

What could've changed with the car simply sitting for two days? 

Now I don't trust it! 

Internal voltage regulator going bad
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Repair or replace alternator?

If replace, would you do it now or wait for it to fail?

Cole Thompson- posted:

 

...What could've changed with the car simply sitting for two days?...

 You might check the connections again.

It's not enough that the battery (and other) connections are tight. The contact surfaces can corrode just sitting for long periods, especially over damp winter months (ask any boat owner).

Pull the wires, then sand, scrape, or file down to clean metal, and reconnect. A simple (and cheap) place to start before ripping out and replacing things.

Contrary to popular belief, electrical stuff is seldom possessed by goblins and demons. There are usually pretty simple reasons for why it doesn't work.

 

I had the same problem and it turned out to be a loose ground from the battery to the chassis.

Cole Thompson- posted:

Repair or replace alternator?

If replace, would you do it now or wait for it to fail?

If you wait for it to fail it will fail when you are on the side of the road and a freak rain storm will hit and your "emergency top" will be in the garage because you took it out of the car to give yourself more storage on the luggage shelf.

As a habit, I install internal-tooth star washers on both sides of heavy current lug connections to get a good "bite" into the lug (both positive and ground connections).  I do a single internal tooth star washer on any ring connections.  I don't necessarily solder all connections, but solder whenever it's practical.  Every little bit helps.

Sacto Mitch posted:

Cole Thompson- posted:

 

...What could've changed with the car simply sitting for two days?...

 You might check the connections again.

It's not enough that the battery (and other) connections are tight. The contact surfaces can corrode just sitting for long periods, especially over damp winter months (ask any boat owner).

Pull the wires, then sand, scrape, or file down to clean metal, and reconnect. A simple (and cheap) place to start before ripping out and replacing things.

Contrary to popular belief, electrical stuff is seldom possessed by goblins and demons. There are usually pretty simple reasons for why it doesn't work.

 

the exception is Lucas wiring/items..