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battery/alternator no charge light ALWAYS ON


Ghost14
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I tightened up the belt on my alternator yesterday and now the no charge light is always on, it is ON when the engine is running, it is ON when the ignition is off and when the key is removed however it goes on when you turn the ignition to position 2.

I dont think the belt is too turn, can just turn it to 90 degrees with 2 fingers which is what i was told you should be able to do...... any ideas?? i've tested the voltage across it when the engine is running and it reads between 13-14 volts so i guess it;s on its way out. but why is the light on even when there are no keys in the ignition?

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I tightened up the belt on my alternator yesterday and now the no charge light is always on, it is ON when the engine is running, it is ON when the ignition is off and when the key is removed however it goes on when you turn the ignition to position 2.

So always on then? Why especially position 2?

but why is the light on even when there are no keys in the ignition?

This could could be because the alternator end of the thin wire off the back of the alternator is shorting on something or the alternator regulator pack has failed - 13V suggests that this might be true.

Does the warning light ever flicker?

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So always on then? Why especially position 2?

This could could be because the alternator end of the thin wire off the back of the alternator is shorting on something or the alternator regulator pack has failed - 13V suggests that this might be true.

Does the warning light ever flicker?

sorry i meant it goes off with position 2

low and behold this morning I had a flat battery.

UPDATE

anyways for most of today if i put my foot down the light gets brighter and let off it gets dimmer although at some point today the light went off, and now its off all the time, even when you turn the ignition to position 2 (which its meant to come on)

I tested the voltage across the alternator and i get 13.5 max and have got 12.5 across the battery. i then disconnected various things from the +ve feen and found that the main cable for the car electrics (ie not the cb, or winch or compressor etc) is drawing 3 Amps with the ignition off so i'm guessing it's something on that circuit that is draining the battery, although i'm not sure really what to do next to go about diagnosing exactly what it is........

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Usually it's a sign that the alternator diode pack has died. Disconnect the small brown/yellow wire from the alternator, if the light goes out then its a faulty diode pack. The diode pack is inside the alternator, it can be repaired/replaced but it's normally cheaper to get a replacement alternator.

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Usually it's a sign that the alternator diode pack has died. Disconnect the small brown/yellow wire from the alternator, if the light goes out then its a faulty diode pack. The diode pack is inside the alternator, it can be repaired/replaced but it's normally cheaper to get a replacement alternator.

Definitely agrre with the above its happenned to me before, new alternator was cheaper than a diode pack.

Keith

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Usually it's a sign that the alternator diode pack has died. Disconnect the small brown/yellow wire from the alternator, if the light goes out then its a faulty diode pack. The diode pack is inside the alternator, it can be repaired/replaced but it's normally cheaper to get a replacement alternator.

as i said in my previous post, now the light is permentally out... and low and behold again this morning.. flat battery!

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I'm a bit confused as to the comments saying that it might be the alternator diode pack, this is a common problem with alternators but I can't work out why this would cause this.

The warning light is wired into the ignition live, so with the ignition off that side of the bulb cannot be getting a positive unless there is a short, and on the alternator end when the engine is not running all it does is act as a earth, so with the engine still off how can the light turn on without the key then on with the key, as nothing is happening at the alternator end to change the bulb condition. But as the OP said, now it doesn't turn on at all, but as we know it did at the wrong time, means it's unlikely to be something on the alternator end

The voltage across the alternator wont be too relevant if the bulb is not working as it should as that's how it generates it's voltage regulation IIRC?

I would start by checking the wires on the starter motor and alternator to make sure they are sound then remove the no-charge wire off the alternator and putting a wire from that to the battery with a 100ohm resistor inline, then start the car and test the voltage across the alternator output. That would conclusively prove or disprove if the alternator is ok or not.

After that check the conductivity between the bulb and the alternator no-charge terminal, then the resistance across the bulb holder with the battery disconnected (should show around 100ohm) then the voltage across the other side of the bulb to a earth to find out what stage of the key turn gets 12V.

failing that disconnect all the wires and wire in a new circuit! lol

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I'm a bit confused as to the comments saying that it might be the alternator diode pack, this is a common problem with alternators but I can't work out why this would cause this.

The diode pack is a pack of four diodes that rectify the AC voltage from the generator to convert it to DC for the vehicle and battery. The battery is permanently connected to one side of the rectifier and the exciter and regulator are connected to the other. (For anyone that doesn't know, a Diode is like a one way valve, it will only let current flow through it in one direction.)

The problem occurs when one of the diodes in the pack goes short circuit allowing current to flow back through the alternator from the battery lead. The charge/exciter light lights up because it's now effectively connected full time to the battery +ve at the alternator and will then find an earth through something else that is usually ignition switched. Depending on the vehicle wiring it's not uncommon to see other lights on too, oil warning light, for example, often makes a nice easy rout to earth.

When you turn the ignition on you now have 12v at both sides of the lamp so it normally goes out or dims when the ignition is turned on.

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Yeah, I understand mostly how the diode pack works but the other end of the circuit is connected to live not earth, the only way it could possibly find a earth is if something behind the dash was shorted as otherwise another light would need to turn on too no?

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Yeah, I understand mostly how the diode pack works but the other end of the circuit is connected to live not earth, the only way it could possibly find a earth is if something behind the dash was shorted

Lots of things are connected to earth from the ignition live, the oil pressure warning light, any number of relays... basically anything that normally turns on when you turn the ignition on is capable of providing a route to earth.

Here's a simplified version using the oil light...

post-2025-127471386394_thumb.jpg

If the diode goes short circuit current can flow from the battery through the charge light to the ignition circuit. From there it can flow through the oil light (and it's switch) to earth. With just these two lights in circuit each bulb will have 6v across it. As you add more circuits in parallel with the oil warning light the overall resistance of that part of the circuit reduces increasing the voltage drop across the charge light. Add, for example, the handbrake warning light which is in parallel with the oil light and those two lights now only have 3v across them and the charge light has 9v.

If you sit down and consider just how many items are connected between the ignition circuit and earth, all of them in parallel with each other, the combined resistance soon drops quite dramatically when compared to a low power charge light bulb.

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so going back then...... what's my next step?

Notwithstanding the foregoing, there are two bits of elastic-trickery magic inside an alternator. One is the much discussed diode or rectifier pack, which turns wiggly AC into nice smooth DC, but there is also a regulator pack, which controls the output voltage by twiddling the field winding strength. (circuit diagram of alternator for you, if it helps) If your charge light gets dimmer / brighter / stays on / won't come on and your alternator isn't chucking out 14V+ and your battery is going flat, then I think it is the regulator circuit myself.

Overall, have the bugger out and get yourself to an auto-electrical specialist place - they can test it, fix it and replace it if needs be. You can self-repair alternators, but I (speaking only fot me) wouldn't bother.

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Right ok, I see where you are coming from now.

On his car the other bulb circuits I can think of on the same route that would provide a earth are the oil switch, fuel low light, brake differential switch and diff lock as I don't suspect he has the hand brake switch or seat belt circuits. But the only one I can think of that is normally earthed is the oil switch, so wouldn't that also be lit at the same time and both dimmed? On that theory there should be at least something else proportionally lit up for the alternator light to be fully lit up.

I'm not saying it's not the alternator as that would be my first point of call, the symptoms just don't seem to add up.

I can't see the resistor playing a part either. hmmm

guess then theres the 3amp load, which is far more than the light and resistor could generate..stumped lol

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if you disconnect the wiring on the back of the alternator (insulate it when you do) then try the meter in line again to see if the current drain has stopped and the charge light has gone out, try starting the engine (with the battery charged) if it stays out then it would definately point at the alternator, then leave it overnight to see if the battery holds it's charge, if that solves all your mentioned problems then either fix the regulator/rectifier/diode pack or replace the alternator.

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if you disconnect the wiring on the back of the alternator (insulate it when you do) then try the meter in line again to see if the current drain has stopped and the charge light has gone out, try starting the engine (with the battery charged)

IANAVS* but I was told in no uncertain terms by my old boss that running the engine with the alternator wiring not connected was a guaranteed way to cook the diode/regulator pack! :blink:

Doubtless someone more knowledgeable than me will correct my somewhat flimsy grasp of electrical fault diagnosis... :)

*I am not a vehicle sparkie

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