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It may be charging fine but if a diode packs up will cause battery to lose its charge.Would go for that.or else battery is faulty?
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I dont think that you say how quickly it goes flat. This could give a good clue. If it is less than 2 hours say, you are looking at a very significant current drain! I would expect to find some localised heating if it an alternator fault. I cannot imagine that a semiconductor diode would exhibit an intermittent short circiut fault, however maybe when the fault occurs could it be that it has not been charging poperly for a period just before the problem?
I think that the electric radiator fan on this model can run on after ignition off, have you checked that this is not happening?
Can you swap batteries to make sure it is not an intermittent faulty connection between cells? What open circuit voltage does the battery show when fault has happened?
Earth strap connections on this model are known trouble spots.
pmh (was peter)
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Clean the battery terminals & cable conexions.
For the battery to discharge in 3 hours would need a big drain, like headlights left on.
So suspect the battery is not discharged but just a disconnection.
As stated before, internal battery disconnections are not unknown & has happened to me with a brand new battery.
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I doudt it is the alternator, and with a 3 hour 9am to maidday total discharge then I would suspect the Fan thermal switch is faulty. For a while disconnect the switch when you leave the car. However if this is, and it could be a faulty link inside the battery next time it happens and you jump start it, immediatley stop the engine and see it if will restart, if it does, and several times, then the battery is at fauly. Regards Peter
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Thanks for the good ideas, when it happens again I will feel for localised heating in case the alternator is the problem and will get the voltmeter out on the battery. The fan is an area that I will look at, also thought that maybe a central locking actuator may not be closing and may be draining current, but there is certainly no chattering of the actuator.
The problem had been apparent with 2 batteries, the last one was only 18 months old and the current battery was brand new 2 months ago.
Keep the ideas coming and I will report any news when the problem next arrives!
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Sorry, don't know VWs at all so what follows might be sheer nonsense! As another poster said, it sounds like a big (intermittent) drain, so possibly you should be looking at something with high consumption. My first thought might have been a boot light not going out - but that's nothing like a big enough drain. How about heated rear window? Any chance it's coming on because of a wiring fault / short? Have you checked the starter motor / solonoid? I can't think how, but if a solonoid remained stuck on that would drain a battery quickly and possible clear itself when car restarts.Or could be a short in that area (possibly within the motor) or a lead chafing now and again with movement of the engine. Do check earth straps for security and clean connection. If the battery had been going down consistently after standing, you could have disconnected it overnight and reconnected prior to restarting - if it kept the charge the drain was caused by faulty equipment - if it went flat when disconnected the battery was shorting internally or otherwise dud. This sounds unlikely in your case.Are you sure the alternator is charging all the time? (Does the charge warning light come on briefly at startup? If not, the bulb might be gone? Have you tested the voltage at battery with engine running? Should be around 13 volts. Hopefully, your alternator is not overcharging otherwise you could be cooking batteries - buckling plates - shorting out - flattening.....etc etc! Are you sure there is no corrosion between the connectors and terminals of battery? Have you tried removing non-essential fuses selectively in an effort to isolate the fault? Does the horn work? A duff horn might use a fair whack of power.
Good luck - ignore if I'm talking rubbish! (highly likely!)
Graeme
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Thanks for the info Graeme, I have tried 2 batteries so am sure it is not a battery problem, the alternator charges at idle at 14.1 volts with no load (about right), the alternator charge light works - it comes on at idle then goes off as soon as the throttle is blipped and if it was not charging the lights etc would dim during a nightime journey so I am sure it is not a charging problem, the heated window and most circuits (starter etc) are ignition live so assuming the ignition switch works these should not be the problem with the key removed, and unfortunately the problem is so intermittent it is tricky to use the fuse elimination trick. So still open to suggestions!
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Ian,
I had a similar problem on my old Rover 214 SEi and on a Volvo 360GLT. The battery discharged itself. I initially thought the battery needed replacing due to its age. After replacing it with two batteries. I realised that it must be the alternator. When I placed a voltmeter across the battery terminals it read 14.1V as expected. But occasionally it read 18V!! The short was in the voltage regulator on the alternator. I had to change the batteries twice as it discharged both of them. The voltage regulator is most likely a Zener diode which has probably failed. I don't know too much about VW's, but I suggest you check where the voltage regulator is fitted and how easy it is to relace. It's usualy bolted on to the alternator.
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306 2.0 SE Cabriolet
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