[Vwdiesel] Moving on and alternators

[Roger Brown]

Doyt W. Echelberger wrote:
> 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.

Actually the charge light can go off if the alternator is able to put out some voltage about equal to the battery.  The way the 
charge light works is that it is wired to 12V on one side and the other to the excitation terminal.  Once the alternator output 
gets close the the battery voltage, the LED goes off, since the voltage difference goes to near zero and thus no current can 
flow.

On a completely dead alternator, then yes, the LED should not go out.

-- 
    Roger