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Alternator Light did not illuminate when Alternator Failed

jboult

jboult

New member
I can only reply from basic knowledge (owning a Land Rover will do that for you...), but the warning light circuit is usually very simple. Ignition on feeds a +ve to one side of the light, the other is connected to the alternator output which, when not rotating, effectively gives a path to earth. Diodey stuff I guess. Anyway, this means the warning light comes on. Once the engine is started, the alternator is rotating and the output matches the battery voltage, so there is +ve voltage on both sides of the lamp and no earth - hence it goes out. I guess in a low alternator output situation it might still be enough to not have the lamp come on, although I've never had that happen to me. I'd expect it to glow a little, at least. I'm sure you've checked, but just in case... Does the lamp work at all? I don't know if it's necessary for the circuit in order to allow charging on a 993. I know it was required on my old Landy and without it, no charging. Easy enough to check as it will illuminate when you turn the ignition on. Did you check the output from the alternator before starting the rebuild? I don't know offhand what it should be, but I'd assume anything around the 14V mark would indicate the alternator was working fine. I'd have started by checking the alternator output (inc. at battery), battery earth, belt tension etc. before a rebuild. Caveat - I am not an auto-electrician! Cheers/John
 
Hi,
I am replacing the brushes in my Alternator as the existing brushes are badly worn. The car failed due to the Alternator not charging the battery. However, there was no warning given until the voltage levels were very low in some of the other systems. The Alternator light in the clock display did not illuminate.
I thought the process was that it should illuminate if the alternator is not charging. Does anyone know how the alternator light is supposed to work?

 
hello, after the car was returned to my address by the AA. I recharged the battery. The following day I turned on the ignition and prior to starting the engine it was shown that the Alternator lamp was lit. on starting the engine the light went out. the voltage across the battery was shown to about 14.3 volts. I drove the car for about 10 miles and no warnings shown. The voltage across the battery was now 12 volts, i.e. the charge system not operating. The engine was turned off. The ignition was turned on and the Alternator lamp was not lit. According to a independent Porsche garage they said the problem was probably heat related. I have removed the Alternator and the brushes need replacing. I will also check the wire from the Battery to the Alternator for any build up of Resistance. I suppose I am taking a risk by not replacing the Alternator as further problems could exist, but I will rebuild the Alternator, fan and belts and see if the problem is fixed.
 
I had an alternator failure a couple of years ago. It was making an awful noise and then the noise stopped. I hoped that whatever it was had sorted itself. Silly. I ran out of leccy a few miles later. The RAC chap turned up and fitted a booster battery to get me home. He escorted me all the way. I asked him why my warning light hadn't come on - he said if it's a catastrophic failure the light won't come on. A new alternator fixed things.
 
I've had 2 faults on alternators and both times the warning light remained off. This is what happened on my SAAB 900T. 1. the brushes were worn to the point where it wasn't producing enough juice to charge the battery but was still producing an output. 2. the battery wire to the field coils on the alternator had broken giving no juice at all but the light still remained off.
 
Hello

New Brushes in the Alternator, Alternator and Fan re installed in the car, Belts installed and adjusted and air con condenser bolted in and belt installed and adjusted. Will take the car for a good fast run tomorrow on the lovely North Wales roads and check the alternator is functioning correctly and that the belts have settled in OK.

 
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