Help wanted in solving Battery Light Problem

I have been having an intermitant problem with my E46 318. Now and again the engine will lose power and the battery light will come on and then as quick as it came will go off and all will be normal again. I have taken the car to my local BMW garage but they suggested replacing the alternator as they told me that they had detected some fault codes which suggested that the alternator was on its way out (which I thought as strange as alternators don't normally fail in this way) the car has done 80.000 miles. Anyway I enquired as to how much a new one would be and they told me £300 +. Not willing to part with such a large sum of money I decided to source another alternator (second hand) £50.00 (more like it). I have replaced the alternator but the problem still exists. I feel it unlikely that both alternators have the same problem. Anyone have any ideas as to what else may be wrong?

If the rev's drop a lot then the alternator may not produce enough juice and its light will come on. So your alternator is unlikely to be your problem.

Why does it nearly stall - you want to find out the answer to that.

It will almost stall if stationary but this is not the issue I am sure, as it will normally happen when travelling at high speeds (usually with the cruise on) as it will disengage the cruise also.

I'd get a DVM - if you don't have one - and rig it up to an accessory plug which fits the cigar lighter. Set it to 0-20 volts and observe the reading when the fault light comes on. It should normally read a near steady 13.8 volts with the engine running and the battery in good condition - and if this changes drastically when the light comes on should give you a clue. If the voltage falls to about 13 instantly it may well be the alternator - or charging circuit wiring, although this shouldn't cause the engine to lose power. If it goes up considerably, this might cause the engine electronics to shut down for self protection.

Thanks all but I have now discovered what the problem was, after it broke down completely and had to have it diagnosed again (an independant this time)!!. It turns out that it was the crank shaft sensor (a real pig to get to) had failed causing the revs to drop as the EMC had no idea where TDC was. I got a new sensor quite easily (BMW obviously have these on standby). But me and my mate set about replacing it on saturday gone. It involves removing the inlet manifold just to get to the sensor it took us about 3 hours in total to replace it but the car works fine now.

Thanks all.

Stu

Wonder why the no charge light came on?

I had the same failure on my 528 and although slightly tricky to change could be done from underneath without removing the manifold - it was the plug which was difficult to get at. The fan and fan shroud had to be removed, though.