Alternator Warning Light

[nigelt]

OK, so in that case you need a resistor in series with the LED to make it look like a bulb and then a resistor in parallel in case the LED fails

 

Not quite Alan, If you put an LED straight across a battery then it will go pop! So the resistor in series with the LED is to limit how much current it can draw and LED's don't need very much to light up. However, the LED and Resistor are now in series with the coil of the alternator and this limits the current in the coil to that of the current passing through the LED. Which is now not enough to energise the Alternator. So the Resistor in parallel With the LED and its serial resistor are to give more current to the coil. Or in other words all three components make the LED look like a bulb... Hmmm perhaps I should have started with statement. :-)

[alanrichey]

OK, that makes sense. I think

[sj-bradley]

I understand the need for the extra 1k resistor now. It is in parallel with the LED on the dash and it allows ~ 12 milliAmps excitation current for the alternator to work as the LED will (as has been said) only allow current to flow one way.

 

The dash lights already have the series resistor built in, so you don't need an extra series resistor although I can't remember wiring in a parallel resistor in my kit, yet it works. Might be the type of alternator ...

 

Of course you "could" wire this up

 

(ignore the resistor listed - it's already present inside the dash light). You can buy ready built full wave rectifier blocks with the 4 pins which are ~ + - ~ for (~ AC input) and +- the outputs. So you don't need to use 4 diodes - although that would work as well.

 

And then you could supply 12 volts and ground to either of the inputs (and vice versa) and it will look like an ordinary lamp. It doesn't care which way round the power / ground is attached.

 

Simon.

Edited March 10, 2015 by Simon B

[Thrashed]

So... the Alternator is working fine and it must be the way I have it wired. I bypassed the loom wiring and tested with a bulb holder as per longboarders diagram and it worked as expected. I put the Diode in the wiring as Alan described and this made no difference to the LED bulb.

 

Next will be to remove the diode and resistor and try with just a bulb holder and then trace back the wiring from that.

 

Will keep you posted

[alanrichey]

I put the Diode in the wiring as Alan described and this made no difference to the LED bulb.

I need to apologise for that. Realised too late that I am using a bulb as my auxiliary warning light (can't see the one in the dash) which does need a diode and that an LED is already a diode (the clue is in the name ) so adding one is pointless. Bring it to Stoneleigh and I'll buy it off you as a spare.

[ac01]

Steve go down to Droitwich auto electrics they will test the alternator on or off the car and tell you what's wrong for free. I used them a couple of weeks ago Great service they charged me £35 to refurbish the alternator.

[Thrashed]

Adrian. I think the Alt is OK. The test this evening worked the bulb perfectly and the output at the battery is 14.3v and the volt meter at the dash is 14v. think it is something to do with the wiring, so going to work some process of elimination and see how I get on. Good to know of a local company though for the future.

 

Let m know when the next Shelsley event if on. Missed the last few!

[Thrashed]

Not a worry Alan, it was only pennies and I am sure it will come in useful to someone at some point.

[nigelt]

LED's only need about 1.2v across them (depending on the type) to illuminate. It might be worth measuring the voltage across the LED, you may find that the voltage generated out of the excitation part of the Alternator is not as high as the voltage supplying the LED. So it won't go out.

 

Nige

[nigelt]

Did you say that you used the bulb as well as the LED at the same time? and the bulb went out but the LED did not? Or did you try the bulb on its own and that worked, then tried the LED and that did not?. If it was the first then check the voltage across the bulb or LED as I mention earlier. If it was the latter then the 1K resistor is probably too high. Try putting two more 1k resistor across the 1st one (i.e. in parallel), makes it about 333 ohms.

 

Nige

[Longboarder]

Nice to know the alternator is OK. Now just a matter of chasing down the wiring fault. LEDs do introduce wiring changes that can be a puzzle to make them simulate a tungsten filament.

Remove the LED and any associated gubbins of resistors and make a 'hole' in the wiring. One side of the 'hole should have 12.5v from an ignition live. The other side of the hole should be the wire that simply goes to terminal 3 of the alternator. A tungsten bulb filling said hole should now work to confirm all is well. Then just remake the package of resistors/diodes and any other magic bits to fill said hole.

 

(Mini rant)

The joys of modern electrics. My daughter has my polo 1.9tdi up in Cheltenham and the indicator relay has just gone. After talking to me she took it to the VW dealer and told him she needed a new flasher relay. However he scanned the car, said 'you need a new flasher relay' and that will be £145 for the scan. VW in their wisdom have made the flasher relay part of the hazard switch which can only be removed out the back of the dash so there is some dismantling to do to get at it. They had to order the part (4 days) and were proposing £260! She picked it up this morning and was relieved they only wanted £170 total. Ridiculous! For the last 50 years a hazard relay has been a standard small black relay, in the relay board, costing a tenner and easy for a five year old to change.

And breath!

 

Nigel

[Thrashed]

Did you say that you used the bulb as well as the LED at the same time? and the bulb went out but the LED did not? Or did you try the bulb on its own and that worked, then tried the LED and that did not?. If it was the first then check the voltage across the bulb or LED as I mention earlier. If it was the latter then the 1K resistor is probably too high. Try putting two more 1k resistor across the 1st one (i.e. in parallel), makes it about 333 ohms.

 

Nige

 

Hi Nige. Option 2. Used a bulb in its own with new wiring and it worked. Reconnected the loom and tried with the led and not working. Think I will start with continuity testing all the wiring as at the moment it goes from the alt to a connector block under the dash and then from the connector block to the dash. I will also repeat the bulb test with that wiring and see if I can get the led to work correctly. At that stage I shoild then know if it is the wiring or the led that is causing the issue.

[sj-bradley]

LED's only need about 1.2v across them (depending on the type) to illuminate. It might be worth measuring the voltage across the LED, you may find that the voltage generated out of the excitation part of the Alternator is not as high as the voltage supplying the LED. So it won't go out.

 

Nige

 

Almost correct. LEDs do drop a small voltage across them, but that is not what kills them - current does.

If you have an LED without the current limiting resistor and it works at 1.2 volts, you could kill it with 1.3 volts.

 

You will find that the specified current is what you are trying to achieve. LEDs typically work from 3 to 20 mAmps which is not very much. If you have a 1k resistor across a 12 volt supply, it will only allow 12 mAmps to flow, which is why LEDs are fitted with these resistors, you can increase to applied voltage to 20 volts and the LED will still only draw 20 mAmps through the resistor.

 

You have to imagine that the LED is an open circuit one way and a short circuit the other way so that is why these resistors are required.

 

Having said all that, most LEDs in the dash instruments and add-on indicators already have these "current limiting" resistors built into them which is why you can just connect them across the 12 volt supply and not worry about blowing them up. All the mechanics really need to know is that they "only work one way round"

 

So, you may not measure the voltage drop on the dash LED as 1.2 volts but the normal 12 volts.

 

If you break the wire feeding the LED and connect the meter in AMPS mode (Do NOT try and measure voltage when in AMPS mode - you will blow the fuse in the meter), it will act like a wire and measure the current flowing through the LED.

 

if there is any current between 3 -> 20 mAmps, the LED will be lit.

 

Hope that helps (a little )

 

Simon.

[ac01]

Steve Next breckfast club meet at Shelsley walsh hill on 12/4 there is a breakfast club meet at Prescott hill climb on 19/4. First race meeting at Prescott on 25/4, first race meeting at Shelsley on 2/5. We are testing the car for the last pre season shake down last weekend in March. Tests so far car is much quicker than last season few engine management gremlins been sorted, now delivering over 450bhp. Trouble is everyone else is reporting the same with their cars, loads of sand bagging!!

[Thrashed]

Hi all. Latest update on the testing tonight. Repeated my test from the other night by disconnecting the loom and running a new wire from pin 1 to bulb holder and then to the battery and a new wire from pin 3 to the battery. Connect to the battery and the bulb lights up. start the engine and the bulb went out, all as expected.

 

Next i replaced the bulb with a spare LED bulb i have, same wiring and all works perfectly.

 

Then, i moved to the dash and unplugged the dash from the loom and connected the spare LED to the 12v and battery light, ignition on LED lights, engine running LED goes out. So the wiring from the Alt to the dash is fine. Then, i connected the loom back to the dash and removed the wires to the dash LED, connected these to the spare LED, ignition on the LED light, engine running LED out.

 

So everything so far is working as expected, so connected the wiring back to the new dash LED and ignition on dash LED lights, engine running it is still on but much dimmer, about half as bright!

 

So, tried to put the diode in the system, no different, just brightly lite with ignition on and dim when engine running. did the same with the resistor and same results.

 

My conclusion is that all my wiring and the Alternatior is fine, but the dash LED does not want to work correctly.

 

Any ideas based on this?

 

Thanks guys

Edited March 12, 2015 by Thrashed