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    VWVortex


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    Thread: Battery light flickering on acceleration

    1. 01-21-2010 02:26 AM #26
      The light comes on redlining in neutral.

    2. 01-21-2010 04:16 PM #27
      time to do some voltage tests again.
      you said before that you tested the battery with the car on and off and it was 13.5 volts across the board. you should be seeing a gain of around 2volts when the car is running. a normal reading would be 12volts with car off, and 13.5 with the car running. check this again and make sure you can confirm the alternator output with your meter.
      next you need to rev the car to get the battery light to come on and observe the voltage at the battery now. if the voltage is indeed dropping below 12volts I'd say the alternator is to blame even though you already replaced it. is the new alternator OEM? I suspect its having trouble at higher rpms.

    3. 01-21-2010 04:28 PM #28
      Ok. I'm just not confident that I can read the meter that I have precisely enough. That minute of a change is hard to read off that meter. I'm going to try to make it to a Radio Shack in the next day or so and pick up a digital reader so that I will know for a fact that I'm reporting back accurate readings to you guys.

    4. 01-22-2010 08:28 PM #29
      Ok, the battery AND the alternator test at 11.4volts with the car off.
      When the car is cranked, both tested at around 10.34volts.
      It's like the alternator is backwards and sucking power.
      The front fans were not coming on either, but they were the other day. But I'm thinking there was just not enough juice to run them. Because every once in awhile it was like they would start to kick on and then cut themselves back off.
      In addition to this I got some super loud hissing from a vacuum line now that is obviously cracked coming off the intake manifold. Those lines are under pressure, so now I'm thinking that the rough idle is definitely from that and not the power issue.
      But I'm just stumped on the alternator thing. Could it be pulling juice out of the battery? It was not OEM, it was AutoZone.
      It just seems too convenient that I replace the alternator and the exact same thing is going on with the new alternator. Is there anything in the loop we haven't checked out besides the fuse box, battery, battery to alternator cable, and alternator?

    5. 01-22-2010 11:14 PM #30
      the battery and alternator are always going to measure almost the exact same. they are directly connected with a big wire. you should be checking the alternator with the car RUNNING, not cranking. the battery voltage will be low when you are cranking because a lot of its power is going to the starter.

    6. Member BassNotes's Avatar
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      01-23-2010 02:16 AM #31
      Quote, originally posted by beaker420 »
      you should be checking the alternator with the car RUNNING, not cranking. the battery voltage will be low when you are cranking because a lot of its power is going to the starter.

      +1 That's an important point (I thought it was obvious).
      Bob Lee
      2002 GTI 1.8t 249,000+ miles
      VCDS 14.10.2 with KEY-USB interface

    7. 01-23-2010 03:38 PM #32
      Quote, originally posted by BassNotes »
      +1 That's an important point (I thought it was obvious).

      Those tests were with the car running. Cranking is an action. Cranked is past tense meaning the action has already occurred.
      Those tests were with the car off, and then with the car running. Talked to a tech at VW today and he seems to think we either got a faulty alternator or a faulty alternator-battery cable. Said they received faulty brand new parts all the time and this is the only two things it could be.
      So he gave us a test I could run to bypass the cable. Putting the positive of the multimeter on the alternator and the negative of the multimeter on the battery with the car running should result in 14.5volts if the alternator is good. So gonna try that shortly.

    8. Member BassNotes's Avatar
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      01-23-2010 05:16 PM #33
      Why did you say "cranked" if you meant "running"?
      Bob Lee
      2002 GTI 1.8t 249,000+ miles
      VCDS 14.10.2 with KEY-USB interface

    9. 01-23-2010 05:24 PM #34
      Quote, originally posted by BassNotes »
      Why did you say "cranked" if you meant "running"?

      I was saying the car was already cranked, i.e. it was running. Not that I was testing while cranking.

    10. Member BassNotes's Avatar
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      01-23-2010 05:48 PM #35
      Quote, originally posted by RoofusX »
      I was saying the car was already cranked, i.e. it was running. Not that I was testing while cranking.

      Okay, well, "cranked" doesn't come across as "running," it comes across as a process of cranking via the starter.
      Clarity helps.
      Good luck.
      Bob Lee
      2002 GTI 1.8t 249,000+ miles
      VCDS 14.10.2 with KEY-USB interface

    11. 01-23-2010 07:37 PM #36
      if you are only seeing below 13volts with the car running then the battery light should be on all the time. your test results don't seem to make much sense.

    12. 01-27-2010 01:40 AM #37
      Got it solved. First off, thanks so much for the help gents.
      To review for those searching for this solution, car was turning itself off driving down the highway, at high rpm battery light was flickering. I replaced the battery, alternator, and alternator to battery cable.
      Upon doing so car was still dying and was still getting the flickering light. Tech at VW advised that I run a single jumper cable from the alternator (where the alternator to battery cable attaches) to the positive battery terminal without the positive battery connection attached. This would essentially allow power to run through the jumper cable and bypass the battery to alternator cable. Assuming your battery tests as functioning properly if the battery does not indicate roughly 13v when the car is off and then 14.5v (or at least a 1.5v increase as suggested in this thread) then the alternator was bad. If a test did indicate the increase in voltage then the cable was bad.
      Upon using this test the multimeter indicated no voltage increase when the car was running, indicating that the brand new alternator was bad. I took the alternator into AutoZone tonight and they hooked it up and it failed the regulator test in about 3 seconds.
      My brand new alternator was bad... Brought a new one home and it cranked up fine, no battery light, no flickering battery light on redlining, and multimeter test was over 14.5v when the car was running.
      The good side I guess was that installing it the second time with experience was a cinch. Took about 20min to pull the bad alternator and about 40min to install the new one.
      I guess the really good thing is that this wasn't a VR6 and I didn't have to pull the front end off again to replace a brand new alternator.
      Other than now trying to figure out which tube off the intake manifold is leaking and giving me a rough idle, it looks like I'm good to go.
      Solved. Again, thanks for the help!

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