 | Home > Tdi > Battery light on after new alternator fitted? |
 
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| Ianh Member Since: 17 Sep 2018 Location: Essex Posts: 2287  
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What you are doing by blipping the throttle is exciting he alternator, which then causes the alternator to work.
I’m not familiar with the 300TDI alternator wiring, however alongside the heavy gauge wire going to the battery you should also have at least one small gauge wire. This wire should apply 12v to the alternator when the ignition is on and is used to excite the alternator without the need to blip the throttle. I would look at the wiring diagrams and check this circuit out as your first investigation. Making sure you have terminated this wire correctly when you replaced the alternator. Here is the diagram , the yellow / Brown wire from the instrument cluster goes to a diode (G126), a Brown/ yellow wire from the diode goes to the alternator. This should excite the alternator when ignition is switched on. So you need to test the 12v is getting to the alternator. If not check the wires and diode. https://www.lrworkshop.com/wiring-diagram/...ing/300tdi |
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27th Apr 2025 3:18pm |
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| GeorgeE Member Since: 10 Feb 2025 Location: Wanborough Posts: 22 
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Did you keep the same pulley? It isn't that uncommon for the Lucas alternators to need a bit of waking up, I wouldn;t worry about it. |
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28th Apr 2025 11:51am |
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| GeorgeE Member Since: 10 Feb 2025 Location: Wanborough Posts: 22
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If that wire wasn't connected properly the charge warning light wouldn't be coming on so I think that is all good. |
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28th Apr 2025 11:52am |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 8241  
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So alternator is only working when accelerator is pressed? Ie battery light goes out when pressed but comes back on my pedal released?
Ie light doesn't stay out after initial excitement and battery voltage drops to 12.8 from 14.4? Cheers James 110 2010 XS Utility 130 2011 M57 bespoke Camper 90 2010 Hardtop 90 M57 1988 Hardtop |
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28th Apr 2025 3:48pm |
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| KiwiNCFC Member Since: 14 Jul 2019 Location: East anglia Posts: 53 
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Thanks for all your help guys, appreciate it.
I checked for 12v at the alternator on that smaller gauge wire and I had no voltage (with the ignition on). I then turned the engine on and I only had 2.4v on that wire which was weird and the battery light was illuminated and battery voltage was 12.8v and not rising. I blipped the throttle and then I had 12v at that wire and then battery voltage was rising to 13.3v+. I then read some other throwaway comment on another post on this forum regarding alternator issues and someone suggested replacing the bulb for the battery light. Now, I had replaced all my bulbs for LED versions around a year ago so I bought a standard halogen bulb and fitted it and tested for voltage at that small wire with the ignition on and I now had 12v! I then started it up and battery light was not illuminated and I had 13.4v+ on the battery!! So it was down to an LED bulb! I suspect the load of the bulb needed to be seen by the alternator? (Specifically the little electronic module stuck on the back of the alternator?) Just thought it was worth updating for people in the future but still doesn't explain why it's only started having this problem since the new alternator?? I did replace it because I kept getting a flat battery so possibly the alternator was failing anyway and thus it wasn't really relying on that bulb in the first place. We live and learn! |
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29th Apr 2025 9:29pm |
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