Alternator not charging whilst idle (MK1)

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Hi Gang,

Bit of a pickle with my MK1 2.0 Petrol. A few weeks back my JBL GTO amp started to intermittently cut out / in, which via testing turned out to be the protection mode. Puzzled I did some more testing and it was only happening when the engine was on, engine off and it works fine. Took it to a garage as I needed a new earth cable (mine had perished) and asked them to test the alternator and battery at the same time. Their conclusion was the battery was fine, alternator works but the alternator was not charging with full load, especially idle, however even with more revs it works when stationary but driving along the amp still cuts in / out and the lights will dim if in traffic corresponding with the amp in / out. The garage suggested a 'more powerful' alternator (if it's possible?), but the thing I don't get is how come I've been running these amps for 5 years on this battery / alternator setup with no issues.

So the garage is not 100% sure its the alternator, however I'm stumped what else it could be? I don't mind coughing to replace it, but then I could play the 'lets replace the part' game until I have a new car. I've checked around this forum and so far most things along this vein is to do with the smart charge, but that's not til MK3?

I guess what I'm asking is if the regulars here would bet on a new alternator or if there's something else it might be?

Thanks

[stef123]

What voltage are you getting from the alternator at idle and fast idle?

Also, are you getting the same voltage present at the terminals on the amp itself? 

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Started at 14.4 then kept dropping as lights, heaters etc went on, got down to 14.1 with front heated screen and revs dropped to around 650 ish.

[BOF]

I'm guessing hat you have probably lost an alternator phase and that there is a high AC component to your voltages and your amp is dropping out on the voltage troughs. This would also cause various lights to flicker.

A warning though: this can also be caused by poor wring such as poor earthing of the alternator/engine so you'd probably best check that out first.
 

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Thanks for some ideas, looks like a multi-meter weekend for me, will report back.

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Ok so I tested the amp when it cuts out and it's only getting 11v (hence protection mode). Spoke with an audio guy who is 99.9% sure it's the alternator, as he checked all the amp cables and the other garage tested the earths on the vehicle and alternator. Time to cough up I think, not bad, 94,000 miles and the first time it's being changed.