Thread: 98 110 Battery warning light

If my understanding of how the battery warning light works is correct, then I think it's very unlikely that a led globe will work as it can effectively "see" 12 volts on both sides of it. Have you tried a normal known good bulb in its place?

Originally Posted by 101FC

If my understanding of how the battery warning light works is correct, then I think it's very unlikely that a led globe will work as it can effectively "see" 12 volts on both sides of it. Have you tried a normal known good bulb in its place?

Yes but I will triple check. All other dash warning lights are working with led replacement globes

An LED will certainly work in that position (and mine does). The only position where an LED will not function with the stock wiring is the indicator dash light, as the current is expected to flow through the bulb in both directions. Biasing an LED to 12V on both sides (which this circuit does do) has the same effect on all bulbs, it stops current flowing through it and puts it out.

Is it in the right way around? LEDs are polarity sensitive, unlike normal bulbs. Sorry, I have to ask

Originally Posted by Dervish

An LED will certainly work in that position (and mine does). The only position where an LED will not function with the stock wiring is the indicator dash light, as the current is expected to flow through the bulb in both directions. Biasing an LED to 12V on both sides (which this circuit does do) has the same effect on all bulbs, it stops current flowing through it and puts it out.

Is it in the right way around? LEDs are polarity sensitive, unlike normal bulbs. Sorry, I have to ask

I did not know they were polarity sensitive, thanks. But I did remove it and spin around. No good.

btw ALL my other led warning/dash lights, except the trailer, work. That includes the idicator lights working.

so where do I start checking?

I'm not 100% sure if the 300tdi has a dumb style warning light like the county, but if it does, AFAIK the way they work is to have +ve from the ignition on one side of the bulb, and the other side goes to a warning light wire from the alternator.
My understanding is that when there is no alternator output that wire provides a ground for the light, and as the alternator wire voltage comes up to the same as the battery there is no longer any current flow, so the light goes out. Someone please correct me if that's wrong.

Assuming that's correct, then with IGN on you should have ~12v on one side of the light, and ground on the other. If you're getting ~12v on both sides of the light then I'd be looking at the wire back to the alternator, or if no voltage look at the wire back to the IGN.

Its not just a blown fuse is it? Not sure if its on a separate one to the other lights...

Steve