Battery / alternator issue

[Ciaran]

Morning all

I'm having a problem with my 205 gti. The car wouldn't start a few days ago, the mechanic replaced the alternator and all seemed to be fine. Picked the car up and drove into work. About 3 hours later on my way home the car died and wouldn't start - I could see the dash lights fading so know it was about to conk out.

The battery was replaced in June but would the knackered alternator have damaged the battery meaning I need a new one?

Cheers

[Motown Junk]

Check that the wiring to the alternator isn't loose or broken (one of the old, age hardened wires to my alternator broke last Sunday night) and that belt isn't slipping. Depending on how long the old alternator was playing up the battery could've been overworking, but would've thought it was ok.

If nothing obv, shout at the mechanic.

[Ciaran]

The car had been playing up for a week or so to be honest, slow to start and I could see the lights weren't as bright as they aught to be.

[andy-xr]

POssible that after the alternator swap it's just been running on the battery alone. Depends on distance

[Cost Captain]

if its got a new alternator its probably related to the wiring. friend had a very similar issue with a citroen ax, 4 alternators, no solution.

[Ciaran]

andy-xr said:

POssible that after the alternator swap it's just been running on the battery alone. Depends on distance

My mechanic is only about 1-2 miles from my office so perhaps that's not long enough for a charge?

He is putting in a new one this morning and will get an auto electrician to check it out.

[wolf1]

If the engine was running then the alternator should have been charging the battery and providing power to the cars electrical systems regardless of battery condition. Sounds like either the alternator is goosed or the wiring for the charging circuit was the original fault.

You need to test the alternator output at the alternator to check it is working. (ie multimeter on the live output cable and alternator body)

Also does the battery light on the dash still work, on older cars it is part of the excitor circuit which initiates the alternator into charging. This can fail after an alternator failure as the alternator backfeeds too high a voltage through the circuit blowing the bulb (the single small wire to the alternator should be reading battery voltage with just the ignition on)

Connections to the alternator should be nice and shiney bare metal. Rust and contamination creates a voltage drop (bit like pinching a hose pipe) which prevents the current flow being sufficient.

Engine to chassis earth connection should be checked for security and clean conections

Check battery conections, if they are dirty or loose it cause another voltage drop

[Ciaran]

Thanks.

The alternator was a brand new one and the mechanic checked the voltage before and after fitting it.

The battery light on the dash doesn't work.

I'd say I could do with cleaning the connections on the battery, what would I use? WD40 and a wire brush?

My gut says electrical issue, the mechanic is leaving the car round to an car electrician to see what the problem may be.

Edited by Ciaran on Tuesday 9th November 12:26

[wolf1]

Ciaran said:

Thanks.

The battery light on the dash doesn't work.

Looks like that is your problem right there. It's a common fault when alternators fail and should be cured by replacing the bulb. Just before the alternator goes the light will have probably glowed quite brightly as the voltage from the alternator rose until the bulb blew.

[Ciaran]

wolf1 said:

Ciaran said:

Thanks.

The battery light on the dash doesn't work.

Looks like that is your problem right there. It's a common fault when alternators fail and should be cured by replacing the bulb. Just before the alternator goes the light will have probably glowed quite brightly as the voltage from the alternator rose until the bulb blew.

To be honest I don't remember the light ever working - I've only had the car for a month.

[wolf1]

Just to explain how it works. Modern alternators require a 12volt feed to excite them so that they can produce power. This part of the charging circuit is called the excitor. It is an ignition feed which runs through a bulb through to the alternator. With just the ignition on the power runs through the bulb to the alternator where it reaches earth (thus illuminating the battery/charging light). When the car is started the windings are excited and this sends power back down the wire towards the bulb causing it to extinguish (ie two positive supplies so the bulb no longer has a direct circuit to earth) If this bulb is blown the alternator will not charge as the circuit is broken.

Later alternators run a smart charge system which a different kettle of fish though. Your will be the former type though due to the age of the vehicle.

[Ciaran]

You sir, are a gent. Thanks!

[Ciaran]

Fixed!

For now I hope.

New battery and a wire replaced. Battery light now works.