Ford Focus 1.8 battery warning light lit

[nicolaobee]

We have a Ford Focus 1.8 16 valve, S reg, Battery light comes on whilst car is running. When reved to 3.5-4,000 revs, the light goes out. When engine goes back to idle, the light comes on again.
We have already changed the battery for a new one and had a new alternator fitted. The new battery has now died!
What could be wrong?

[NORMANTHEDOG]

seems a tricky one as both main reasons,the battery and alternator have been replaced,take it to a auto electrician,he should find the fault

[mr rusty]

it's not unknown for a recon alternator to have had nothing more than a clean up and a blow over of new paint....in fact it's pretty common.....

There's a couple of simple chacks you can make yourself before shelling out to someone else- firstly, ensure that the alternators drive belt ('fan' belt) that goes round the pulley on the end of the alternator is correctly tightened- you should be able just to deflect it at the mid point of its longest run under firm thumb pressure. If it's flapping about loose then it won't drive the alternator fast enough to keep the battery charged.

If you have a multimeter (you can get 'em from maplins for about a fiver..)and you can use it to check the alternator's output- set it to the 0-20v range and put it on the battery, and with the engine not running the battery should read about 12 volt or just above. Start the engine- the reading should now be somewhere between 14 and 15 volts, varying only slightly with revving, if the alternator is charging correctly. If it stays around the 12v mark, then it's a fair bet that the alternator isn't charging, in which case take it back where you got it from!!

[Bazza2764]

I remember once having a faulty ignition light bulb and apparently that was breaking the curcuit, wasnt expensive to replace (about 18p) was just a pig to do having to remove the dash to get at it.....Mr rusty gives good advice, most motor spare shops will test this for you free of charge