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I left my A3 for an hour after driving it normally, and when I returned I found the battery was almost completely dead. The car wouldn't start, and even the hazard lights were having trouble staying on.

I used jump leads from a friends car to get it started, and drove for about 30 minutes. The dashboard lights and dials would occasionally cut out while I was driving! Fortunately the headlights stayed on the whole time.

As soon as I turned the engine off the battery was dead again and the car wouldn't come close to starting.

Clearly an electrical problem, but where so I begin? Is the battery at fault, or the alternator which is meant to charge it? Some people have suggested a faulty starter motor might be the issue. I'd like to know the best way to diagnose the problem. It will be hard to get the car to a garage from its current resting place in the countryside! Thanks.

UPDATES

I did some simple tests to narrow down the fault, between the alternator and battery. Perhaps someone could help me draw a conclusion from the results:

  • The battery was taken out and charged. The car started (somewhat reluctantly) and ran for 30 minutes until parked, but after leaving the car for 2 hours after that short journey, the battery was dead.

  • With the charged battery connected, revving the engine did not affect the headlight brightness.

  • Disconnecting the battery while the engine was running did not stall the engine or effect the electrics.

  • The starter motor makes a short, loud whirring noise just after the engine starts. This has happened before though, with no noticeable effect on performance.

So... it needs a new battery, right? Unfortunately I do not have a voltmeter, and I'm stuck in the countryside!

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    It sounds like your battery was marginal, and the event of leaving the lights on caused it to go toes up. Can you connect it to a charger, let it fully charge, and then read the voltage? – mongo Jun 3 '17 at 12:44
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    Could be the alternator or the battery, but would have nothing to do with the starter. The car started fine when you had enough juice applies (during the jump start). – Pᴀᴜʟsᴛᴇʀ2 Jun 3 '17 at 13:02
  • The starter motor sound is likely not connected but a different issue where the pinion cog doesnt retract quickly enough after starting and needs the shaft/bush greasing. – Digital Lightcraft Jun 5 '17 at 14:32
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Sounds like the alternator isn't charging seeing as dials and lights were cutting out when driving.

You need to jump start it with a known good battery. When it's running check the voltage across the battery (If you've used another running vehicle to jump start you must disconnect it before measuring).

The voltage when running should be 13.8-14.4v.

If there is less than this you need to investigate why.

Check the voltage directly at the alternator to rule out the cable.

You haven't mentioned what fuel type or engine but its very common on the A3 for some wires to break on the front of the gearbox. If you follow wiring loom along you'll find some black plastic conduit bolted to the lower front of the gearbox with a connector on one end. I've seen on numerous occasions where the wiring has rubbed through or the connector has corroded. Check this and repair if necessary, if its the connector just cut the connector out and use heatsink joiners to repair with a lot of insulation tape and conduit to protect.

Check resistance between the alternator main lead and the battery positive post. Should be as close to 0 as possible.

If its not that check pin 2 on the 2 pin connector usually the blue wire. It should be 12v when ignition on and 0v when running. If it isn't, disconnect the connector and hook up a test light (an old fashioned type with a filament bulb) between pin2 and battery positive and then see if it charges.

If none of the above is successful you can be pretty certain the alternator is at fault.

Since the update:

Seeing as disconnecting the battery didn't affect the electrics the alternator seems to be working to an extent, it doesn't rule it out completely though. It could be putting out less than 13.8 volts, enough to power to electrics but not enough to charge the battery.

I would recommend charging the battery and then taking it to a place that can test it, a good motor factors/parts supplier will usually do this for free. Its more than just testing the voltage, a battery tester will apply a big load usually well over 100amps and then measure voltage drop and current.

If the battery is good it is most likely the regulator/brush pack on the back of the alternator. Dealers usually don't sell these (I don't know about VAG) as they want to sell you the whole alternator but a good parts supplier or auto electrical place will be able to.

  • If I ever get back to civilisation, I'm buying a multimeter :( – Leon Jun 5 '17 at 10:30
  • I took your advice and removed the battery, and took it to the nearest car parts dealer. It hadn't occurred to me that these places can test batteries! Although the battery appeared to be fine (it was quite new and had a green light on top) it was in fact in need of replacing, which I have done. I bought a multimeter and will test the alternator output myself. Fingers crossed that the battery was the only thing at fault. Either way, I'd have needed a new one. Thanks for taking me through it step by step. – Leon Jun 5 '17 at 17:32
  • The alternator was putting out too low a voltage, which wasn't good for the battery and over time, killed. The Battery and alternator needed to be replaced. Thanks again. – Leon Jun 23 '17 at 15:15
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This is a fairly common problem with most VW and Skoda manufactured cars. It's the Alternator.

How long have you had the Audi? When was the last time it was serviced at the dealership (and how frequently do you send it there?)

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