[Vwdiesel] Moving on and alternators

Doyt W. Echelberger doyt at buckeye-express.com
Thu Nov 17 10:28:04 EST 2005

At 11:54 PM 11/16/2005, you wrote:
>..........Alternator doesn't seem to be charging, after several days the 
>battery won't start the
>my 91 jetta td, i've swapped out several battery cables, changed a 
>battery, thought i found 12+ vlt
>coming from the alternator though still the battery doesn't charge. This 
>alt has a heavy cable which goes to the starter then from the starter to 
>the positive side of the batt. It also has a smaller gauge
>wire, excitor?, should this show 12 volts. I don't currently have a manual 
>so please forgive
>any improper term here. BTW the dash light does go out after first start up.
>I expect i will need to see my mech for a more accurate charging 
>determination though i had
>thought an aging ground was the culprit
>thanks,   ray
>thanks for all your help loren,
Ray........Listen to what those other guys said....all sounded good to me.

I'll just comment on my own experience.  Maybe that will help a little.

Intuitively, it doesn't make sense to have the charging light go off and 
find a low reading such as 12v + at the same time, as you indicate.....so I 
am trying to figure out why you got that low reading, and where that 
electrical energy is going during engine down time. The fact that your 
charging light goes off after start-up suggests to me that your battery is 
probably getting charged while running, and probably draining down between 
start-ups, while the car is sitting. Or maybe you have two problems....

Two comments about my own experience with what might be similar symptoms in 
my 85 Jetta TD, in which the battery ran down between trips:

Point #1........I found an electrical leak with the engine off......circuit 
to the (aftermarket) radio/CD player, clock, and associated electrical 
clients was draining a measurable but small current, so I took out that 
fuse and the battery lost its pattern of showing up dead every few weeks. 
Never did find the specific drain, and never fixed it. I just live without 
that circuit. I wonder if you did a circuit test for electrical leakage, 
with the engine off.

Point #2......The alternator on my 85 Jetta TD doesn't start charging the 
battery right away after I start the car. The red charging light stays on 
until I rev up the engine. Then it goes off and stays off. (Once I rev the 
engine and the light goes off, it doesn't come back on during that 
trip.)  Measured at the alternator output and after the charging light goes 
off, it charges at about 13.5 volts.

Assuming your 91 Jetta TD electrical system works like my old one in the 85 
Jetta (dangerous assumption), I wonder if you are testing your voltage at a 
normal idle and getting the low reading before your alternator has made the 
switch to "charging the battery."

So, I suggest that you check those voltage readings again, and make sure 
you take readings when somebody is reving the engine somewhere above 
whatever it normally is at idle.  And look for electrical drainage when the 
engine is off.

Doyt Echelberger 

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