car battery/alternator question

[Chataigner]

LOML's little Renault has the battery warning light on when driving. Fan belt OK, no slip. I've checked battery voltage, it is normal, that is 12 ish with the engine not running, drops a tad if you put the headlights on etc., but, and this is the bit I dont understand : rises exactly as one would expect to around 14v when the engine is running (revs dependant). That all seems to indicate that everything is working as it should. So why the warning light ?

She only told me about this a couple of days ago, it seems the light has been on for a week or more with the car in daily use (2 or 3 hrs of use par day). :roll:

Help !!!

 

[Gerry]

It's a faulty rectifier diode in the alternator. Your seeing 14v but it will be ac and there is leakage back through the rectifier for the warning light due to a faulty diode/diodes.

Gerry

 

[Chataigner]

It's a faulty rectifier diode in the alternator. Your seeing 14v but it will be ac and there is leakage back through the rectifier for the warning light due to a faulty diode/diodes.

Gerry

Thanks, that sound very likely. I seem to remember from another car that the rectifier block is a separate assembly inside the end of the alternator and therefore could be replaced without changing the alternator - is that right ?

 

[MMUK]

What year is the car? I'm assuming it's a Clio? If it's pre-1998 it will have a Lucas alternator which has a sealed diode pack. If it's the Phase 2/3 or 4 (1998 onwards) they have Bosch alternators and you can replace the diode pack on it's own, even though most garages will tell you otherwise.

 

[MickCheese]

If the system is the same for a car as a motorcycle then a duff rec/reg could ruin your battery.

Honda motorcycles are well known for failing reg/rec I only know about bikes.

The alternator produces 3 phase AC power. The reg/rec then converts it to DC power and regulates it to 12v.

The various parts can be checked quite easily with a multimeter but you need to be systematic.

Mick