Alternator only starts charging when taken over 3000rpm

11 May 2013
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United Kingdom
Anyone understand alternators and their control systems?

I have a Peugeot 306 LX Auto 1600cc reg 2000. It failed while driving at night - all the electrics failed slowly until it wouldn't run. Clearly it wasn't charging and just went until the battery died.

RAC man tested the battery and declared it to be fine (though now flat). He jump started it and then metered the output and it was not charging. Next he put a wire between one terminal of the alternator and... another point, don't know where, could have been the battery? When this extra wire was connected it started charging.

Next he took off the extra wire, turned the engine off, restarted it, REVVED IT TO approx 3000RPM and it started charging AND KEPT CHARGING EVEN WHEN IDLING.

This is how I've been running it for about 4 months. Every time I start the engine I blip the throttle to a good 3000rpm - at this point THE BATTERY WARNING LIGHT COMES ON which now indicates it's charging. It stays charging until the engine is turned off. I've tested this many times with the meter, it works every time, NOT INTERMITTENT, and it's been fine for 4 months. The battery has never been charged other than by the alternator.

So what's the problem? Well I don't want to keep revving it hard from cold, and I don't like driving with a red warning light permanently on. I'd like to fix it rather than just get round it.

My local garage insist that I need a new alternator and/or battery. I don't see this when the battery behaves perfectly and the alternator consistently puts out 14.5v - when it's told to. The bit I need help with is how to instruct the alternator to start charging other than revving the engine.

My impression is that there's a broken connection somewhere between the alternator and the battery warning light - but where?

Thanks for reading this far! Got any ideas?
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Earth the warning light wire going to the alternator (not the larger + wire or you will have smoke) and see if the light comes on with the ignition key? Then start the engine and see if it still comes on at 3000RPM with the warning light disconnected?

If it doesn't you have either a broken connection between the alternator and light or between the light and ignition switch live.

Is the oil pressure warning light working? This might tell if its the power side of the light.

All the fuses OK?

You could put your own light in connected to the WL terminal on the alternator and the + terminal on the battery. To ensure the alternator internals are OK

If all else fails you will have to a visual check around the engine for broken corroded connection block

Very simple charging light circuit
Thanks for that :) Now I know where to start looking I shall have a tinker and see what I learn.
If you are charging at a normal idle (>13.3v in summer) then you are probably overcharging your battery at a fast idle and above. Normal range is 13.3v to 15.3v (winter charging voltage). A cold battery can stand a higher charging voltage.
17v in summer will vigorously boil your electrolyte and shorten the life of your battery.

Can you post a legible schematic of your charging system?

It sounds like a bad regulator, or you'd have to replace the alternator if the regulator is inside the alternator.
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If this alternator was stripped down, I'd be very surprised if the slip ring and brushes weren't completely worn away.
John :)
Okay, don't know whether to feel embarrassed, vindicated or just happy, but I seem to have fixed it!

I metered it again and it was charging perfectly within range, just not being excited on starting. And so...

even though I was sure I checked all the fuses before, I checked again and found a blown 15A fuse. I replaced this and, sure enough, it's working just fine. Doh!

Not sure exactly what this fuse does as none of the 3 possible versions shown in the manual bear any obvious relation to the one actually in the vehicle. Also the clock and the electric windows are now working, so apparently part of the same circuit. Don't know what caused it to blow in the first place either, as all seems to be well now.

Anyway, thanks all for your input - if nothing else it encouraged me to go and get my hands dirty rather than just believe the guys garage who had a look and told me I definitely needed a new alternator - sounds like that would have been an expensive way not to solve the problem!

I'll keep an eye on it just in case some kind of alternator issue caused the blown fuse in the first place, but I'm seeing 14.5v at fast idle, dropping to about 14.3 with all the lights and A/C on - that's about right isn't it? That's from cold this morning and it's about 12C here.

If the fuse element was splattered it means a severe overload, a hairline crack may mean it failed from metal fatique or a mild overload.

Since the voltage hardly changed with increased load (and I assume it won't change much with engine speed) your alternator is being regulated. Good.
Wait and see what happens, intermittents are pretty awful to troubleshoot.
yep, from that I'd say it's definitely being regulated as there's minimal voltage change with load or speed.

The fuse looked like it had burned rather than just cracked, but not one of those dramatic spatters so I'm guessing a mild overload.

I'm holding out for it just being one of those random things - if it blows the fuse again I may have to admit it as intermittent, but not yet ;)
If it failed every x days/weeks/months you need it not to fail in 3x days/weeks/months to be virtually certain it is fixed.
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