Dash Battery Light Woes. Trying my best

[bmouthboyo]

I have read around this topic on the forum, wiki, archive, facebook group etc but I am hitting a bit of a brick wall.

Basically my dash battery light is very faintly on all the time. It was brighter but I managed to dim it after I replaced my alternator regulator. My alternator, even with this new regulator only seems to put out about 13.7 Volt

Facts so far
- Disconnecting the blue wire to the alternator when engine running makes the dash LED go out! (opposite to what I imagined should happen)
- Jumping the blue wire to battery positive with key position 1 turns the light out.
- Alternator is only putting out about 13.7 Volts even with new regulator (tested at alternator)
- All earths (engine to chassis, gearbox to chassis, batter negative to chassis, 2 earth crowns under dash, 1 earth point in engine bay) have been wire wheeled clean with a dremel.
- No leisure battery system


Questions

Could an old battery cause this issue? My battery charges OK and holds voltage but is old and when I put it onto my MXS 5.0 ever after a few days it never reached the fully charged / maintain state.

Any ideas how I can pin this issue? The light is ever so dim, but dim nonetheless.

Can anyone also explain how this light works i.e. what makes it light exactly? Is it when the voltage at dash is lower than that of the alternator? If so how does the bulb 'know' what the alternator output is? Blue wire?

Or is the bulb being lit after comparing the alternator output to a predefined threshold?

Sorry for all the questions, just want to understand whats going on.

[Oldiebut goodie]

Simply put. The light is glowing dimly because there is a very small voltage difference across the bulb.With an accurate voltmeter you can check this. It may be that the blue wire is carrying a slightly higher voltage than that at the dash circuit or it may be that it is slightly lower and the dash is higher. Without checking it is impossible to tell but my guess is that the blue wire is higher unless you are using it as a trigger for a split charge relay in which case the relay could knock the voltage down slightly.
Ignore it and only worry when it is on full whilst the engine is running.
I note that you haven't cleaned all positive connection at the same time - they can cause problems just the same as the earths.
What voltage regulator did you buy, 14v or 14.5v? My 14.5v one gives me just over 14.2v which will do me. If you bought a 14v one you may well be seeing the same 0.3v difference as mine.
13.7v using what as earth? Alternator or chassis? This can show up where there is a problem.
Your removing the blue wire and jumping it proves nothing - I don't know what you trying to achieve with that!

[bmouthboyo]

Thanks for the help oldie

First point is I have no leisure battery system yet so no split charge relay.

The regulator I replaced today is one from Brickwerks that is rated at 14.5v however my alternator is not outputting this. Does the output from the alternator vary? When would I expect to see the 14.5v? On startup? With all lights, fans etc on? Or when the battery has no load and engine warmed up?

I did the jump test as I read on the forum http://forum.club8090.co.uk/viewtopic.php?f=39&t=156389 that this is a test I should do, I guess to check the light goes out as it should?

If you was in my situation what would you test next? I know it is only dim but I would like to sort it before starting to build my leisure setup.

How can I measure the voltage difference across the bulb?

Thanks again for your help

[AngeloEvs]

The charge indicator is an LED with one side connected to the ignition live and the other side to a winding on the alternator. The ignition live will be whatever your battery voltage is with engine off approx (12.7V) or whatever the voltage across the battery is with engine running (in your case 13.8v). The LED lights up if the voltage on the ignition side of the LED is higher than the voltage on the other side of the LED that connects to the alternator. It only needs a small voltage difference across the LED to cause it to illuminate. For the LED to completely extinguish the voltage on both sides of the LED should be pretty much identical though, due to the circuit, there could be a slight difference of 0.6V due to a second protection diode being in series with the LED.

Firstly, I changed the regulator and brush pack on my alternator and, like yours, my charge voltage is 13.8v but I haven't bothered to replace it because it still still sufficient to charge the battery to a reasonable level for my needs (been like it for years). However, my charge LED does go out completely so I thnk it would be reasonable to say that the slightly lower output voltage is not the issue.

The connection on your alternator (to which the LED is connected) may be lower than the Alternator main output so I would be measuring this and comparing it to the main output - should be identical. If the main output is 13.8V and the output thatconnects to the LED is lower by a Volt or two then the LED will illuminate but very dim.

The reason the LED comes on with engine off is simply that the alterntor output voltage drops to Zero (the alternator is not rotating and not producing a voltage) this allows current to flow from the ignition - through the LED and to earth (via the field winding).

[bmouthboyo]

The charge indicator is an LED with one side connected to the ignition live and the other side to a winding on the alternator. The ignition live will be whatever your battery voltage is with engine off approx (12.7V) or whatever the voltage across the battery is with engine running (in your case 13.8v). The LED lights up if the voltage on the ignition side of the LED is higher than the voltage than on the other side of the LED that connects to the alternator. It only needs a small voltage difference across the LED to cause it to illuminate. For the LED to completely extinguish the voltage on both sides of the LED should be pretty much identical though, due to the circuit, there could be a slight difference of 0.6V due to a second protection diode being in series with the LED.

Firstly, I changed the regulator and brush pack on my alternator and, like yours, my charge voltage is 13.8v but I haven't bothered to replace it because it still still sufficient to charge the battery to a reasonable level for my needs (been like it for years). However, my charge LED does go out completely so I thnk it would be reasonable to say that the slightly lower output voltage is not the issue.

The fconnection on your alternator ti which the LED is connected may be lower than the Alternator main output so I would be measuring this and comparing it to the main output - should be identical.

The reason the LED comes on with engine off is simply that the alterntor output voltage drops to Zero (the alternator is not rotating and not producing a voltage) this allows current to flow from the ignition - through the LED and to earth (via the field winding).

I just wanted to check your statement "The LED lights up if the voltage on the ignition side of the LED is higher than the voltage than on the other side of the LED that connects to the alternator. " is correct, and if so how does the other side of the LED that connects to the alternator (blue wire) become lower than the ignition live? Is the blue wire on the alternator giving the alternator current or receiving it, or both (I understand this blue wire is used to 'excite' the alternator, once this has occurred does is the wire then fed volts from the alternator?

Sorry I really am getting myself confused here. I understand the principle of the LED having 2 sides, one directly to alternator via blue wire, and one from battery / alternator via the ignition. Is it lighting because the battery is giving more volt than the alternator is producing?

[AngeloEvs]

The battery cannot give more voltage than the alternator since the output of the alternator is connected directly to the battery. If the alternator is outputting 13.8V then that is what will appear across the battery. The battery voltage could be lower (if there is a voltage loss en route from the alternator because of poor connections).

When the ignition is turned on 12 volts is applied to one side of the LED or bulb, the other side being earthed via the field windings. Hence the LED or bulb lights
showing no charge. Once the alternator self excites and starts charging the battery, the LED has 12 volts on both sides and It goes out indicating that the battery is being charged.

With engine running, you need to measure the voltage at the battery and then the voltage at the alternator connection to which the LED is connected. They should be the same. If the LED alternator connection reading is slightly lower then that would explain why the LED is illuminating.

[bmouthboyo]

The battery cannot give more voltage than the alternator since the output of the alternator is connected directly to the battery. If the alternator is outputting 13.8V then that is what will appear across the battery. The battery voltage could be lower (if there is a voltage loss en route from the alternator because of poor connections).

When the ignition is turned on 12 volts is applied to one side of the LED or bulb, the other side being earthed via the field windings. Hence the LED or bulb lights
showing no charge. Once the alternator self excites and starts charging the battery, the LED has 12 volts on both sides and It goes out indicating that the battery is being charged.

With engine running, you need to measure the voltage at the battery and then the voltage at the alternator connection to which the LED is connected. They should be the same. If the LED alternator connection reading is slightly lower then that would explain why the LED is illuminating.

This is brilliant thankyou so much for the advice.

So I measure voltage at battery but using the 2 poles on it? And then how is best to test 'the alternator connection to which the LED is connected'? Is that just the blue wire on the D+ alternator post? So alternator chassis for - and D+ for the +?

If the LED connection is lower, what would cause that?

[AngeloEvs]

Measure across the battery terminals and then the D+ to alternator body (engine running).

Take the readings first. It could be alternator or some sort of leakage, poor connection somewhere between Alt D+ and Dash. Readings will help to narrow down potential causes.

[Oldiebut goodie]

Far simpler to do as I suggested at the start - just measure the voltage either side of the bulb/led! This is where the voltage difference is. Then you can move on to tracing (quite frankly I don't see that it is worthwhile putting in all that effort).
If this had been done we would be half way to solving it by now.

[marlinowner]

There is a diode in the warning light circuit, presumably to prevent current flow and hence glowing bulb due to voltage drop at the battery compared to the alternator, so for the bulb to glow either that diode is shorted or the voltage at the alternator is lower at the r terminal than the main one.
Don't think a duff battery can cause the problem but you can disconnect the battery earth terminal once the engine is running, if it is the battery then the light will go out. A poor contact in the warning light circuit can't cause the problem either.

[AngeloEvs]

Have to disagree regards simplicity. It takes no more than a minute or so to switch the engine on, measure the battery voltage then the D+ to ground voltage. If those match then you can proceed to, what for some, could be a little more tricky in trying to access the LED connections wth the pod in place and placing meter probes.

[bmouthboyo]

OK so I had some time to do a little more digging on this.

I started her up from cold and battery was reading 13.34v at idle but after 5 minutes this was at 12.15v.

Alternator after starting her up was reading 13.40 from B+ to alternator body and 13.38 from B+ to engine earth point.

The blue wire from dash I disconnected and coming from dash end was reading 12.72 and from alternator it jumped about from 8.98 to 9.04v.

No idea what all this points to? I have replaced the regulator with a BW one so expected that to sort this.

No idea why battery showing lower volt after 5 mins?

[AngeloEvs]

The blue wire from dash I disconnected and coming from dash end was reading 12.72 and from alternator it jumped about from 8.98 to 9.04v.

This confuses me regards the 'blue wire coming from the dash'. Is this the wire that connects to the D+ on the alternator?

The reading of 8.98v - 9.4v is potentially the cause of your LED illuminating slightly. As I said in a previous post you need to measure the voltage at the alternator D+ terminal. It should be the same as the B+ terminal.

Once you have established the D+ voltage is ok then this would eliminate the alternator as the problem and leaves a bad connection somewhere from alternator D+ to the LED as the most likely cause.

The fact that the LED is illuminating tells you that there is a voltage across the LED that should not be there.

[bmouthboyo]

The blue wire from dash I disconnected and coming from dash end was reading 12.72 and from alternator it jumped about from 8.98 to 9.04v.

This confuses me regards the 'blue wire coming from the dash'. Is this the wire that connects to the D+ on the alternator?

The reading of 8.98v - 9.4v is potentially the cause of your LED illuminating slightly. As I said in a previous post you need to measure the voltage at the alternator D+ terminal. It should be the same as the B+ terminal.

Once you have established the D+ voltage is ok then this would eliminate the alternator as the problem and leaves a bad connection somewhere from alternator D+ to the LED as the most likely cause.

The fact that the LED is illuminating tells you that there is a voltage across the LED that should not be there.

Sorry I wasn't sure what side of the blue wire to measure when I disconnect it so measured both the wire coming from dash end to earth which was 12.72 volt and coming from alternator D+ post which was jumping around 8.98v.

From this it looks like an alternator issue? Not sure why the blue wire wire from dash is showing almost half a volt less than the alternator output?

[AngeloEvs]

Given your measurements I would do the following test to establish if it is a alternator issue.

1. Make or obtain a small 12v bulb with two leads, a low wattage one similar to something fitted for instrument lighting.

2 Disconnect the wire to the D+ terminal on the alternator.

3 Connect the bulb with one wire on the B+ and the other to the vacant D+ terminal.

You now have a charge warning light circuit in its simplest form. If the light comes on then it seems likely that the alternator is not providing the full output at the D + terminal.

The final check would be to measure the voltage either side of the bulb to earth. An alternator has two main component sections the regulator with brush pack and the diode rectifier assy.