Oh dear...

[wasted]

Following its resounding success with ramming Ford Focuses, my old Rocco has just died. I noticed a while ago that the stereo was cutting out at higher volumes, but didn't really suspect that my battery would totally go so suddenly - it's now totally flat, and I'm suspecting a bad alternator.

Does anyone know whether a bad alternator could cause the engine not to idle when warm? No idea why, but when I successfully bumped it, it died again about two minutes later... then again it's always been a bit rough when hot, so maybe two problems.

Anyway, it looks like I'm now in need of an alternator - anyone local to Bristol got one available? Thanks!

[GT Matt]

Hi

Yes, as you need electricity to power your spark, if your alternator fails, you have until the battery depleats until your car dies.

The battery is actually only used for starting the car, storing charge and completing the circuit. It also supplies electrical power when the engine isn't running. When the engine is running however, the battery simply receives 'charge' until it potential difference (voltage) is equal to that of the alternator, it's the alternator that supplies all the electrical current.

First thing to check is whether you have a battery warning lamp? If you haven't it will never ever charge. If you don't have a light, it can be caused by bulb failure (led on rocco?), broken exicter wire or internal failure within the alternator.

I assume the drive belt is also ok?

Next is to test the output from the alternator (should be between 13.7v & 14.2v with the engine running (assuming you have a charge light) and the light should go out once the engine is running.

You also need to check your earth strap between engine/gearbox and the chassis. This completes the circuit and is often overlooked by many people. It could have been damaged in your accident?

It's really hard to say what's at fault with yours without being able to test it but don't just assume alternator, the wiring up to it could have been damaged in your crash.

Matt

[Nate]

i'm just the other side of cirencester and i've got a 90A one clicky but you may need to swap the pulley from your rocco one

[wasted]

Matt - cheers for the advice on this. I definitely didn't get a battery warning lamp - just the usual oil light when the car was off but ignition on. The headlights are dim, hazards are dim too, so it's pretty much guaranteed to be a flat battery I reckon.

The drive belt is fine - checked that the alternator turned freely (which it does), and when turning the key the engine did turn just once before I got the good old click of doom with no turning. So the solenoid on the starter is fine too.

It all does point to a faulty alternator, but I will check all the things you recommend - do you think if I get someone to jump the car with leads it'll make it home (about 4 miles or so)? Or will the battery need a full charge first? I'm guessing the latter, but I stupidly left the battery in the car when I left it parked up so I can't charge it now Pretty dim really.

Nate - thanks for the offer mate, and I might take you up on it depending on how checks go - I'm hoping I can get it back home to give it a proper look, so I'll let you know.

[Nate]

if i'm reading this right, when you turn the key but not start it, the battery light doesn't come on? if so, the bulb has probably blown. this qwill stop the alternator "turning on" when the engine starts. as a get-you-home bodge, disconnect the small blue wire from the alternator. make up a wire with an appropriate end (spade or ring terminal, depending which alternator yours has) that goes to a small bulb (the wing indicators should work ok) and the other side of the bulb connect to battery +ve. the bulb should light up, then go out when the engine starts

[wasted]

I'm actually not sure now. I guess I'll have to check! Why on earth did VW design the circuit so that a bulb completes it?!

[Nate]

that's just how alternators work. it's not a vw thing, it's standard across all makes#

http://www.alternatorparts.com/understa ... nators.htm

[wasted]

Aha, I see - that's a very handy link, cheers for that. I think my only option will be to take a fully charged battery out to it along with some tools and some wire... should hopefully get me home at least!

[Funky Diver]

The other point to think about, is that a battery will semi rejuvinate itself... enough to hold a charge to drive plugs, but die rapidly under use, as Matt has identified above regarding residual charge. Not enough to start the engine, but enough to keep it going.

Definitely a charging circuit issue, IMHO.

[wasted]

No worries - I'm going to go and collect the battery tomorrow, and leave it on a trickle charge overnight. Hopefully when I take it back I'll be able to get it started and driven back home so I can diagnose the fault.

Going by all the various possibilities here, I reckon the most difficult to sort is going to be the blown bulb in the dash - how easy is it to replace that? I assume the clocks have to come out, so does that mean the dash has to come out as well?

[GT Matt]

Just because your bulb isn't illuminated doesn't mean the bulb is blown.

It still could be an internal failure of the alternator. It could also be a broken wire between the bulb and the alternator.

You could try removing the plug from the rear of the alternator and, with the ignition on (and a charged battery), try earthing out the thin blue wire in the alternator plug. If the battery light comes on then it's more than likely the alternator. If it doesn't come on, could be bulb or wiring fault.

[wasted]

Nice one, just what I need. I'll give that a go - thanks for the advice!

[wasted]

Update on this:

Returned to the car this evening and noted that there was indeed an ignition light when the ignition was on (without the car running), so I decided to try jumping it from a friendly local bloke with a Passat. After a couple of minutes of charge, I fired it up first turn of the key, and the ignition light went off - so far so good. I took the leads off and left it idling for about five minutes, and it was still going, so I decided to have a shot at getting it home. And amazingly, it made it - and it now turns over with no problems following a drive of about 5 miles.

Sooo, it looks like the stall when idling was just the old problem of running from a hot start, which I'm hoping will be resolved by replacing my AAV at some point; and the actual issue is being caused by either a dying battery or a voltage drain somewhere in the car (I'm kind of suspecting the stereo, since it's a bit bodged on the wiring). The acid test will be tomorrow - I've removed the stereo for now, so if it starts and seems to have a good charge I'll drive it for a bit and then sort that wiring out. If it dies again, it's new battery time.

Thanks for all your help, guys! It's good to have it home again :D

[Nate]

if you've got access to a volt meter, take a reading with the engine running and the headlights on. looking for around 13.5v

[wasted]

Hmm. A week or two later, and it's done the exact same thing again. Thankfully managed to get it home this time. The symptoms seem to be that the engine starts idling at about 500rpm (but it does idle) - it does rev happily enough. When the engine is stopped, it then won't start again - battery is totally dead. When I try to start it, the clock resets.

Current theories are:

1. Intermittent alternator fault (possibly heat related?)
2. Coil problem maybe?
3. Faulty battery

I replaced the distributor cap today, as an aside... I wonder if this dislodged a loose wire?