battery light on, RPMs dip and int. ligths dim once a second - regulator or battery?

verylongdrive

Veteran Member
Joined
Feb 10, 2005
Location
HSV
TDI
Golf,2001
Help:

01 Golf sat for some time, dead battery, jump-started, drove some, let it sit running, drove some more, turned off.
Tried to start; battery dead again.

Jump-started it again, noticed odd rhythmic engine noise, and looked at the instruments:
The battery light is on. With a frequency of about 1 Hz, the RPMs briefly dip a tiny bit, and the dash and interior (when on) lights dim a little bit.
If I rev it up some, the same dipping continues.
What does that rhythm indicate? What is it trying to do but then giving up on?
Bad battery? Bad regulator?

Thanks.
 

verylongdrive

Veteran Member
Joined
Feb 10, 2005
Location
HSV
TDI
Golf,2001
My voltmeter doesn't update quick enough to really measure this, but the voltage at the battery is
-during at least 13.7V (the meter tends to catch 13.6 to 13.7 if it updates near the dip)
-as low as 12.0 away from the dip

Does that indicate a bad battery because a bad regulator couldn't go that high?

Or does it indicate a bad regulator because a good one wouldn't give up? (WHAT is giving up, anyway?)
 

verylongdrive

Veteran Member
Joined
Feb 10, 2005
Location
HSV
TDI
Golf,2001
Also, to be clear, when I rev it to constant 2.5k rpm, nothing changes: the batt. light stays on, the rhythm (rpm dip and dimming) doesn't change.
 

4ME+N02

Active member
Joined
Apr 22, 2007
Location
NM
TDI
2002 JETTA
First, I think you should get a full charge on your battery and then have it tested.

If you can drive it to a parts store, most can test your battery and charging system at the same time.

Hope this helps!
 

Corsair

Veteran Member
Joined
Aug 17, 2003
Location
Weedsport, New York
TDI
2002 Jetta GLS TDI 5M
Agree in general with above. And...
If you can't or don't want to risk driving it, you can disconnect one lead from the battery to isolate battery from car. Then charge the battery to a full charge. then remove the charger and let the battery sit for a day or two or three. Measure the battery Voltage. If the battery goes flat by itself, it's defective.

I don't recall reading of this symptom anywhere else- maybe someone else will recognize it. Sounds sorta like something in the system is trying to come online and putting a huge load on the battery & charging system, then maybe a current or thermal breaker is opening, then resetting itself. Or maybe the regulator is bad and the fault is internal to the alternator. One of the big fusible links atop the battery is for the alternator feed into the rest of the system- it can be disconnected (I think the nuts are 10mm). You could start and run the car with that disconnected. Of course, the alternator won't charge the battery in that configuration, but it would allow you to determine whether the fault lies within the alternator or somewhere else.

Wish you all the best in tracking it down. Please do post what you find.

Edit PS: Does the car have any aftermarket electronics installed, such as maybe big audio amplifier and subwoofer etc. ?? HAM radio ??
 

verylongdrive

Veteran Member
Joined
Feb 10, 2005
Location
HSV
TDI
Golf,2001
No additional electrics or electronics; DRLs disabled.
I remember turning the fan down after I started it this morning, but I don't remember whether I turned it (and therefore the AC) all the way off at the time. If I hadn't, would the AC go on/off like that due to low voltage? (This RPM dip seemed to be much less - though quite audible - than what happens when the AC comes on at idle.)
 

993er

Veteran Member
Joined
Oct 10, 2013
Location
Canada
TDI
None
Bad battery? Bad regulator?
My motto: less guessing, more measuring/testing.

As already mentioned, charge the battery using a battery charger. It should reach 14.4V at the end of charge. Then with the battery sitting overnight not connected to anything, it should measure at least 12.6 V the next morning. But that does not prove that the battery is healthy. A load test is the next test. Lastly, but not all that important, is a capacity test which is time consuming and never done in the automotive world.

With a battery known to be good in your car, you should have a charge voltage of at least 14V but no higher than 14.4V at 25C or 77F.

Having an on-board voltmeter in a car is more useful than most of the bling installed these days.

 

JB05

Top Post Dawg
Joined
Oct 20, 2005
Location
Il.USA
TDI
Golf,2005,anthracite blue
993er, where did you get the volt meter? I have one plugged into the accessory outlet, but yours looks neater.
 

993er

Veteran Member
Joined
Oct 10, 2013
Location
Canada
TDI
None
993er, where did you get the volt meter? I have one plugged into the accessory outlet, but yours looks neater.
I made my voltmeter from parts. Parts and sources are listed along with the pics. Here is my Surface Mount Datel LCD Voltmeter Install how-to site. Cost? About $40 in parts. I know you can get one that plugs into the cigarette lighter outlet and those are fine IF the voltage there is battery voltage. Usually though, voltage at some point in a circuit is lower than battery voltage.

There isn't much available for motorcycles that is small and draws no current, which is why I went the route I did. Originally, I made two of them, one for each of my BMW R1150 GS Adventures. But one of them ended up in my 911 instead after some kid on a cell phone took me out and wrote off my BMW motorcycle...so I had a spare. I could have used it when my alternator belt went 100 miles from home...good thing I just installed a new battery in the 911, so my RC of 120 minutes got me home.

I just bought two more; one for my new 2013 Jetta and another for my other motorcycle. Too bad the Jetta doesn't have a knock-out panel on the dash, or I would mount it there.
 

AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
What was the issue? I'm having this behavior.

Based on the comments of the OP, I'd say the battery was the culprit. I am assuming his explanation was based on the engine at idle. If the battery light was coming on at higher RPM, chances are that the clutched pulley could have been slipping.

All the advice relative to a healthy battery is good. However, my experience is if the battery voltage drops below 12.5 volts in less than 12 hours it is dying. If that battery is used daily, it will hang on for a bit.
 
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