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Battery lamp ON until revving

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Battery lamp ON until revving

Postby Sinisha » Mon Jan 16, 2023 12:36 pm

I've noticed on a lot of old cars that the battery light doesn't go out until the revs are raised, so on the Transits as well. If it annoys you like it does me, you can easily fix that problem. It is enough to connect one 47 Ohm resistor (I put 5W) between some 50 (live when ignition is ON) and D+ (from alternator to the battery lamp, Excitement) terminals. Both wires are available on the multi-connector on the back of the instruments.
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Re: Battery lamp ON until revving

Postby meerkatmag » Mon Jan 16, 2023 2:40 pm

Interesting, can you explain how this is fixing it, what's the resistor doing?
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Re: Battery lamp ON until revving

Postby Sinisha » Tue Jan 17, 2023 9:58 am

meerkatmag wrote:Interesting, can you explain how this is fixing it, what's the resistor doing?

The role of a battery lamp is not only to inform us about problems in charging. Without it, the alternator cannot start to generate current. Battery lamp is a load and excites the alternator. Sometimes this load is not enough to excite. D+ terminal is normally "-", ground, so current can flow between some live terminal (in this case 50) and D+. Result of lamp and resistor parallel connection is resistance smaller than only lamp resistance. With smaller resist - current rising. This current excites the alternator. When the alternator starts to generate current D+ is live, it is not ground any more. Because there is no more potential differential between terminals 50 and D+, the lamp goes OFF.
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Re: Battery lamp ON until revving

Postby cherniy_chack » Sun Feb 19, 2023 6:23 pm

I installed a 15 ohm 1 watt resistor in parallel with the test light a few days ago.
The lamp goes out instantly.
What is the extra resistor for?
To increase the initial excitation current of the generator.
With an increase in the initial excitation current, the speed of the generator's transition to the self-excitation mode decreases.
A typical excitation/pilot lamp is 1 watt, giving an initial excitation current of less than 0.1 amps.
I just calculated using Ohm's law the resistance of a 1 watt glowing filament of a test lamp, it is about 150 ohms.
My friend from Belarus Sasha "Murzik" talked about installing an additional resistance of 100 ohms 2 watts.
My 15 ohm resistor is too low, I need to replace it with another one with a higher resistance.
To install the resistor, I stripped the conductive tracks on the prior panel and soldered a resistor to them.
Then I attached the resistor to the board with hot glue from a hot glue gun.
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Re: Battery lamp ON until revving

Postby cherniy_chack » Sun Feb 26, 2023 7:48 pm

Yesterday I bought a baseless 2 watt bulb and installed it in a plastic dashboard bulb holder.
I bought another 3 watt lamp, but for now I keep it in reserve.
I hope this lamp will provide enough initial drive current without an additional resistor.
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Re: Battery lamp ON until revving

Postby Sinisha » Tue Feb 28, 2023 11:55 am

cherniy_chack wrote:Yesterday I bought a baseless 2 watt bulb and installed it in a plastic dashboard bulb holder.
I bought another 3 watt lamp, but for now I keep it in reserve.
I hope this lamp will provide enough initial drive current without an additional resistor.


You should not be afraid of an additional parallel resistor. Bulb, in fact, is nothing else but resistor. Remember that in parallel connection the total resistance is less than the smallest individual resistance. Lower resistance -> higher current.
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