Battery light on, charging at 14.1

rhino89523

Member
Joined
Apr 28, 2020
Location
Reno
TDI
1997 Passat
Well just like the title says, my battery light is on, my battery shows like 12.6 just sitting there and 14.1 when the motor is running. revving it up does nothing to the light. New to this car, new to the TDI game, not new to VW or wrenches but this is my first water cooled VW. Bought it cheap (1997 Passat Wagon) looking forward to playing with it and getting that mileage. Just working out a few kinks before my timing belt kit shows up. I'll get a new TB in there and start driving it!!! Guess I should also add...I have a totally dead instrument cluster...no LED, No tach, no speedo, but I figured these lights were a seperate deal since they seem to work .
 
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Mozambiquer

Vendor , w/Business number
Joined
Mar 21, 2015
Location
Versailles Missouri
TDI
1998 VW Jetta TDI. 1982 VW Rabbit pickup, 2001 VW Jetta TDI, 2005 VW Passat wagon TDI X3, 2001 VW golf TDI, 1980 VW rabbit pickup,
One thing to check is what the ripple voltage is. That would be the AC voltage output from the alternator. It should be no more than 0.5 volts. If it's more than that, then you have a failed rectifier or stator. If so, you'll either need to replace the parts that have failed, or get a new OEM or OEM quality remanufactured one.
My recommendation based on my experience as an alternator rebuilder and as a VW nut, is that the parts store or Amazon varieties are not going to last very long.

Other possibilities could be the wiring from the alternator to the light. I know on my mk3 Jetta, the light would be off if the alternator plug was unplugged (learned that the hard way when the alternator output wire broke while the plug was unplugged, it drained the battery but didn't turn on the light. I just noticed the clock go dim.) But, if the wire is not getting correct voltage, it will turn the battery light on.

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Windex

Top Post Dawg
Joined
Apr 1, 2006
Location
Cambridge
TDI
05 B5V 01E FRF
^ this

The most common cause of an alternator light on yet still charging is a failed rectifier diode.
 

rhino89523

Member
Joined
Apr 28, 2020
Location
Reno
TDI
1997 Passat
Looks like it’s back to the google cave for me. I don’t understand ripple voltage or how to check for it. I can wire up a whole car and work the test light like a champ but the OHM meter I am not the greatest, but like I always say “I am the smartest guy In the world when I have my phone in my hand” so google cave it is. I tried running it with the plug unplugged and the battery light was still lit. I didn’t check if it was still charging with it unplugged. I’m guessing the rectifier is in the alternator somewhere. It is already a junkyard alt. I had the light on with no charging before, now that I am saying that I had a thought. The junkyard alt had one wire the one in the car had 2...maybe it has something to do with that? The plug fit. It is a 90 amp instead of the 120.
 

Mozambiquer

Vendor , w/Business number
Joined
Mar 21, 2015
Location
Versailles Missouri
TDI
1998 VW Jetta TDI. 1982 VW Rabbit pickup, 2001 VW Jetta TDI, 2005 VW Passat wagon TDI X3, 2001 VW golf TDI, 1980 VW rabbit pickup,
The two wire is for the tachometer, I believe the terminals would be the same. I'm guessing the one you got it from was a gasser? Yes, the rectifier is inside the alternator. It's what houses the diodes.
If you have an auto parts store which does free alternator testing, that'll tell you if the diodes are good or not.

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