My 2002 VW New Beetle has a sketchy (dis)charging issue. Specifically it appears that while the AC is running (though I'm not positive that's always the case) after about 15-20 minutes of driving the battery discharges enough that the engine/car goes all herky jerky and eventually the motor stalls out.

We suspected the alternator was the issue - O'Reilly's machine says it's producing enough juice to keep things going. With the engine off, measuring voltage across the terminals gives 12.9v. I thought there might be a parasitic draw, but if there is it's not a consistent one. I had it charged over a week ago, and it started OK today. I suspected the battery (Energizer, bought at Sam's Club) but they said it tested OK and just had a low charge.

I have a cheap-o amazon battery tester, and after topping it up last week I turned the headlights on for about 10-20s, hooked up the tester and it said the 650 CCA were fine.

The last day things went unfortunate -- a week ago Sunday -- I had driven 13 miles, about 20 minutes, mostly on the Interstate when it died on me. The engine was running but there was no throttle response at first, and then I jerked into a parking lot. From there I was able to jump start it with a jump pack and made it all of 20 feet before the engine lost power (though it was still running). I only had about a mile left to go so crossing my fingers, I wedged the jump pack in and left it hooked up and managed to get to my destination.

Later that day to get it back, I had my partner jump the car -- we left it hooked up for about 5-10m and then I took off again. I managed about 5 miles before the car... well, honestly it acts like it ran out of gas, but it's something with the electrical. Jumped the car again, left it hooked up for about 5 minutes, and made it about another 5 miles. Wash, rinse, repeat 2x and I was able to make it home.

To review the facts:

  • battery takes a charge, will hold it for over a week at >12v
  • after a full charge, turning headlights on for ~10-20s, the battery tested fine at 650 CCA
  • O'Reilly's alternator tester says it provides enough juice
  • Testing across the terminals while the engine is running reports ~13V. 12.9-13V. Definitely not 14+. Turning the AC on full blast doesn't appear to have a significant impact (maybe 0.1v?)
  • Jump starting will allow the car to run for a short period of time
  • When the battery voltage (?) gets low enough the car acts like it's out of gas.

What other tests can I run to figure out what's going on here? I have read that some of the wiring in the box on top of the battery is a bit undersized -- there's some evidence that one of the wires (black, furthest to the left) may have been a bit toasty at one point in its life. It's not completely burnt, but the metal is not shiny, and the jacket looks like it's been cooked.

What can I do to further track down this issue?

  • 1
    13V is not enough. The symptoms are screaming "bad alternator" no matter what O'Reilly's says, though it could be a bad tensioner causing belt slippage. You can diagnose the charging system with a clamp-on DC ammeter. Measure amps produced by the ammeter and amps flowing into or out of the battery at various engine speeds and with max electrical drain (lights, A/C max fan, seat heaters, rear window defogger). Depending on how you orient the clamp, + or - will tell you if the battery is charging or discharging. Let us know.
    – MTA
    Commented Jun 14, 2023 at 1:25
  • "15-20m"?! Metres? Or minutes? Or miles?
    – Chris H
    Commented Jun 14, 2023 at 13:08
  • @ChrisH - I took it to mean minutes, but could be miles. I'm thinking meters would be a bit outside, especially since nothing else is metric in the question. Commented Jun 14, 2023 at 14:10
  • @Pᴀᴜʟsᴛᴇʀ2 I think metres is unlikely too given the use of feet later on, but that is the unit most likely to be denoted by "m" ("min", and "mi" or "M" for the others). Even excluding metres, with both minutes and miles in the question, it's ambiguous, by enough to make a difference if driving slowly.
    – Chris H
    Commented Jun 14, 2023 at 14:17
  • 1
    @ChrisH - I completely understand. I'm very anal about such things as well. I guess we'll hope the OP comes back and clarifies so we can quit being triggered here :P Commented Jun 14, 2023 at 16:14

2 Answers 2


Agree with what MTA stated in comments. This is most likely a bad alternator. You state the voltage at the battery while running is ~12.9vdc. This isn't much more from what I'd expect to see a good battery at full charge without the engine running. Charging voltage while running should be in the 13.5-14.2vdc range. Maybe even a bit higher. Without it being this high, the battery will discharge during usage. Your diagnostics on the battery was spot on to show it is in good shape, so you can eliminate that. To me, I don't think you need further diagnostics. It seems pretty cut and dry the alternator, while producing some energy, isn't providing what is needed to charge or keep the battery charged.


So I think that the problem was simply due to this wire having gotten a bit on the toasty side (see the black wire, which is actually positive, from the alternator?)

fuse box on top of VW beetle battery

In a feat of interesting choices, VW made this wire connected to the positive output of the alternator black. These metal straps are actually fuses - the one on the black wire was labeled 110A, and apparently failed to do its job or something since the wire got baked/oxidized, increasing its resistance, making it get hotter and more oxidized, and... well, yeah.

Where it was connected to the alternator was "ouch heck that's hot!" degrees. I didn't try checking here, and it wasn't until after I followed some advice from similar-ish symptoms from Pine Hollow Auto Diagnostics on YouTube (titled: "Dealer Tells Customer: "JUNK YOUR CAR!" (VW Beetle)"), specifically pulling apart the crimp, cleaning up the wire with Deoxit & sandpaper, that I thought to take some thermal images. Here's the cleaned terminals, you can see the much brighter copper

cleaned up terminals, kinda

thermal image of wires, indicating the three leftmost wires are all around 180F

I know on PHAD's video he was seeing >200F almost immediately. This was the temperatures after driving around town in 90F weather in direct sun. I'm not sure if that kind of heat still indicates an issue, but we had the same engine code with the air leakage as in the video, and we're not seeing that anymore.

So... it appears that the alternator is just fine as O'Reilly's reported, and there were(?) other electrical issues at play here.

  • What are you using to take those thermal pictures? Looks useful.
    – gowenfawr
    Commented Jun 25, 2023 at 0:19
  • 1
    I used a Topdon USB-C thermal camera. It's worked well for the few times I've used it. Commented Oct 4, 2023 at 3:06

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