My '87 Citroen AX 1.4 lost power yesterday, battery warning light getting brighter, headlights dimmed, indicators stopped flashing. Got it home and after a rest, alternator belt is tight, engine started fine and alternator span even under full load (heated screen, full beam, heater fan on, stereo on).

Multimeter readings from the battery terminals; not running 12.8V, cranking over ~10V, started ~14V but then rapidly dropping below 12.5V, with the battery warning light getting brighter.

So, I don't think it's the alternator not working at all, as voltage did increase from 10V back up to 14V. Something is draining the battery whilst it is running, but the battery retains charge and recovers when rest. Someone at work has suggested a ruptured cell in the battery, which is only causing a drain under load.

Any theories?

Thanks

Rick

Could be either battery or alternator, by the sounds of it though I would suggest the regulator pacy inside the alternator is faulty!

Sounds like the alternator is FUBAR to me.

Had a hire car once that conked out in bad weather – we restrictd the use of the electrics – turned the fan down, rear heated screen off, etc and it ran fine. Maybe the battery capacity just ain't upto the strain.

How old is the battery? My '95 Astra wouldn't start the other morning. Off I went to the local motor factors for a charger. He had one left! But he did tell me that 9 out of 10 people who had been in for a charger had been back the next day for a battery and their's were fubar. The extremely low temperatures we have experienced have screwed a load of batteries. Changed mine and all in now well with the world.

Wouldn't a corrupted earth wire from the battery cause these symptoms too? Might be worth checking…..

get another battery in there and test again. alternator o/p @ 3000rpm should be about 13.8V, but your battery might be pulling it down.

Thanks for the help folks!

The battery is 2 1/2 years old, but seems to hold charge well enough. The car isn't used much, but always starts well enough considering. I was running it on bare minimum load for the drive, just lights.

user-removed – where would I prod with a multimeter to check an earth wire – my guess is resistance between the body and the battery should be zero if the wire is fine?

If your battery warning light is on the alternator is more than likely goosed. The battery light works by the alternator taking away the earth of the warning light circuit when the fields excite within the alternator to start charging. The fact that the battery light is getting brighter would lead me to belive the wiring to the alternator is fine as the circuit is working. You may just have a some brush prblems within the alternator but not something worth pulling apart unles you know what your looking for.

I would suggest all cars will seriously drop voltage when cranking as the starter motor draws huge current and is the main reason why car batteries are so big.

To accurately test the battery you need to perform a 'drop test' which means putting a large current across the battery and making sure it holds a good voltage and then recovers back when current draw is removed.

In conclusion it sounds to me like you have an alternator problem either brushes or voltage regulator.

There is a "known" issue with the AX, the wires leading to the alternator
can become damaged due to the insulation being rubbed through.

May be worth checking

Sunday wobbler – great info, much appreciated. My main headache is why the alternator appears to kick out lots of volts initially, but then drops away – would a knackered voltage regulator be a cause of that?

My (limited) understanding is that the regulator keeps the output voltage above that of the battery, but decreases amperage as the battery recharges. If it was faulty, could it somehow drop the voltage as well, which would drain the battery, and cause the light to come one as well?

finishthat – thanks – that's one I'll check later!

The voltage reg keeps the output voltage below a certain level so as not to toast the battery, the current output depends on the resistance of the load (battery, wiring etc). Sounds as though the alt, or the wiring from the alt to the battery, is failing.

Coffeeking is right the regulator keeps the voltage at a level the battery can handle and the alternator gives as much current as the battery/operating systems of the car need and then drops off when the systems are switched off/battery charged.

Within the voltage regulator will be a system of diodes which convert AC waveform into DC waveform via full wave rectification. Also within this will be a zener diode which switches the charging on and off when the voltage reaches the specified level (around 14volts). You may find that this is not switching back on when the voltage drops back below 14v causing the symptoms you describe.

Admittedly this is all coming via the internet without seeing anything but may give you a little bit of background and some general reasoning behind stuff.

HTH

This happened to me in my Fiesta and turned out to be a knackered alternator.

You can get your alternator reconned for a lot less than a new one (or buy the parts and do it yourself if possible). The reg failed on mine and it was a £20 part and an hours strip and rebuild, against a £180 alternator.

Whatever you do, don't try to disconnect the battery whilst engine is running so you can check the alternator voltage…the alternator will almost certainly go pop.

Some alternators have built in protection for this, but I suspect not for an old AX.

Thanks for all the advice. Going to wait until the weekend and some daylight to go checking above stuff. Luckily I have another car to get to work in (injured hand stops me from riding to work 🙁 )