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I have a 2008 VW Jetta 2.5L Automatic , Recently the battery was dead and my car would not start. I charged it up and got it to turn over and let it run for about 15 min on its own. and everything went dim inside. I turned it off and attempted to start it to nothing again. I did a volt test on my battery that came up 7.76 from positive to negative terminal I also tested from the Alternator to the ground and got a 7.43. is this a sign my alternator is out or do I need to do a parasitic draw test on the entire fuse system. Whats confusing was I drove the car fine just the day before so all of this had to happen in less than a 24 hr time span. Before this the only issue i had was my stero wouldn't come on when everyhthing was turned on but then about after 2 to 3 minutes on the road it would kick in. No dim lights no flickering. went out in the morning to start it and it was dead. No its so bad the doors barely unlock.

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    I'm with PeteCon, I'm surprised that battery started you car, any DCV less than 10 volts is a bad battery, either the plates are covered in residue or you've blown a cell or two, either way it needs replaced, you test the alternator with the Car running, not by just putting a multimeter terminal on the alternator. – hello moto Sep 30 '19 at 4:56
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    @hellomoto, the key datapoint is that after a charged battery, the car died after 15 minutes or so. That is typical when the alternator can't supply the car's electrical needs. The battery gets worn down and then things die. If the alternator was working, once started, the engine would have run fine, until shutdown. – mongo Sep 30 '19 at 13:30
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Your battery should be around 12V with the engine off (11V upwards usually still works). With the engine running, a test at the battery should show 13.5-14V.

My guess would be that your battery is dead - you'll need a new one. Once you get it in, drive to a car spares store and have them test the battery and alternator. I'm amazed that the car did anything at all at 7V...

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    Put in a new battery and then determine if there is a problem . – blacksmith37 Sep 30 '19 at 14:02
  • Got a new Battery and I cranked everything on the car on and we were at 13.6. I turned a few thinigs off and it went to 14.02. I also found a old wire from someone who had a system in the car. I'm assuming its a power wire for a amp. not sure if that had anything to do with it but I taped it up with electrical tape. – Christopher Byndom Oct 10 '19 at 23:05
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Don't rule out the alternator! If the car is running, and the lights go dim, it means that you are not getting the power you need, which comes from the alternator.

Many of the VW alternators from the 2000's had overrunning clutches on them. IMO, this just created another failure point. So much so that every alternator I have replaced (more than 20 on the family cars) failed because of the clutch. Symptoms include intermittent power, as the clutch starts slipping and grabbing. Eventually it just fails so that little or no power is transferred to the alternator.

You can diagnose easily with a tool like VDCS or a VagCom tool, just read the current out of the alternator.

Throw your battery on a charger, and then you can take it to the car parts store for testing. If it tests OK, you might want to pick up an alternator.

Given your symptoms, I would pick up an alternator. After you charge your battery, and get the engine started, put a voltmeter on the B+ out of the alternator. It's usually a 13mm nut on a stud, which is covered with a plastic cover that fits over the nut. With load, you should be seeing 13 to 14V at that point, even with a battery which is marginal. If you do, and the voltage at the battery is much lower, then you might have a wiring problem.

Also, FWIW, I have seen the ground to the bell housing get loose on some VW. I doubt this in your case, but that too will be a 13mm nut.

All that said, I would be getting an alternator, because based upon what you said, that is the highest probability failure, not the battery.

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