[Vwdiesel] 89 Jetta D phantom voltage update

[Erik Lane]

On the matter of the 11.85V that you were seeing - you weren't entirely
clear about where it was coming from. Between the red wires that you
disconnected and the positive battery post, or somewhere else?

That voltage is suspiciously close to the voltage of a diode drop below
battery voltage, so possibly it has something to do with the charging
circuit of the alternator after all. You could check to see if pulling the
wires off of the back of the alternator and testing with them off makes any
difference. It could also be from something on the dash. Having the key on
or off while testing might also show some differences.

I still think that the alternator is not right if it lets the car discharge
while driving like it did for you, but it's possible that there are multiple
things going on. It's also possible that the 11.85V that you saw is normal
for these cars. If you describe exactly where each probe was then maybe
someone else can check their car. I'm perfectly willing to check on my car,
but it might be a little different, since mine is a 1981 A1 Jetta. (In fact,
I would like to check, out of curiosity, once you say where it was. I'm just
not sure that it will help with your situation.)

Erik

On Sat, May 28, 2011 at 5:39 PM, Erik Lane <eriklane at gmail.com> wrote:

> Disconnecting the blue wire from the alternator would just show if the
> alternator was draining the battery through that. They can fail internally
> and have various problems.
>
> But the car running low on battery power towards the end of a 2 hour trip
> points right at the alternator not charging like it should. If it isn't
> charging then whatever is causing that could very easily cause all kinds of
> problems, electrically. Are you sure that the alternator belt is tight
> enough?
>
> If the problem wasn't in the alternator, but somewhere else, it would have
> to be drawing more current than the alternator could put out, plus a little
> to drain the battery. (Well, I guess it could just be about what the
> alternator was putting out and the accessories/regular draws of a running
> vehicle could account for the rest.) But these alternators are rated at
> something like 60-80 amps for the smallest of them. That kind of power isn't
> going to disappear without leaving evidence, and you aren't seeing that kind
> of draw with it turned off.
>
> So I would start looking at the alternator very carefully. It's probably on
> its last legs if it can't keep up with the car and a small drain.
>
> Erik
>
>
> On Sat, May 28, 2011 at 4:56 PM, Doyt W. Echelberger <
> doyt at buckeye-express.com> wrote:
>
>> In early May I asked for help understanding an apparent  battery drain,
>> and several of you replied with advice. Here is an update:
>>
>> I made sure I didn't park it with the key in the ignition, Loren.
>>
>> It is a base model and doesn't even have a trunk light, Sandy.
>>
>> Like Scott Kair, I have no idea about how to boost the charge rate, which
>> seems functionally inadequate because at times the battery ends up unable to
>> crank over the engine.
>>
>> Someone suggested disconnecting the blue wire from the alternator, but
>> that didn't seem to change anything so I reconnected it. I really didn't
>> understand how that was going to make the problem go away, but am
>> interested.
>>
>> OK, here are some facts: With the engine idling, the alternator starts
>> charging with a voltage of 12.7 and this slowly climbs to 13.7 over about 20
>> minutes with the car just sitting there idling. That is a test situation,
>> and I usually just get in the car and drive it. It doesn't idle much except
>> during the test.
>>
>> With the engine off, the key out, and all other wires (2) disconnected
>> from the positive battery terminal, I got an 11.85 constant voltage by
>> inserting a volt meter between the red wire that serves the headlights,
>> brake lights, cabin lights and radio.
>>
>> I then sequentially pulled and replaced every fuse and relay in the fuse
>> panel, with no flicker of the 11.85 voltage.
>>
>> When I applied the brakes, the volt meter jumped to 12.80 (max battery
>> voltage). Got the same voltage jump when I switch on the headlights or the
>> radio.
>>
>> I then replaced the volt meter with a 10 watt DC bulb (out of the dome
>> light) and it didn't glow. I assumed the volt meter would have still read
>> 11.85 volts but I didn't have enough hands and alligator clips to verify it.
>>
>> All I can think of that would behave that way (voltage but no current) is
>> a circuit that is charging a condenser.
>>
>> -----------------][-----------------  and then I don't know why it isn't
>> full battery voltage. 11.85 is about a volt less than full battery voltage.
>>
>> Also, it is possible that all the above information has nothing to do with
>> the battery ending up not charged.
>>
>> OK, I got practical and used a charger and brought the battery to full
>> charge, and then drove the car mostly on short trips around town for about
>> 10 days. It started fine and ran the lights and radio and wipers a few
>> times, and then I drove the car for two hours on a long trip,  and near the
>> end of the journey the red dash battery warning light came on, and when I
>> tried to start it after a layover, it didn't turn over the engine. Had to
>> jump it....then it ran OK. (I now carry a jumper battery in the trunk.)  So,
>> back on the big wheeled fleet charger and ready for another cycle.
>>
>> Also, a few hours ago I wired a Sears 1.5 amp  battery maintenance charger
>> (from another Jetta now deceased) into the 89's battery circuit and plugged
>> it into the wall. The green "charging" light came on and the "fully charged"
>> light isn't on. So, that's where things are at the moment.
>>
>> Maybe this story will strike a familiar note with someone and will point
>> out the magic button that makes it all work reliably for at least a few
>> months at a time. Until then, the phantom electrical consumer (?) will have
>> to deal with the Sears charger, and perhaps the emergency jump start battery
>> in the trunk. Diesel on.....   Doyt
>>
>>
>>
>>
>>
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