Alternator Warning Light

[sj-bradley]

Put an ammeter in series with all these items and see what current they are drawing when on and off (or dim)

 

LEDs will draw up to 20 milliAmps when lit and < 1 mA if not lit (should be zero). Lamps will depend on the wattage, but dash lights of about 1.2 watts will draw around 100 mA.

 

If you get NEGATIVE current and the LED is lit - it's knackered as it should not be drawing anything. (It means that the voltage difference has changed direction and is forcing current back through the LED. I would be very surprised if it lit when back biased though)

 

You can test LEDs using the diode test on most meters. Connect the meter one way and you should see around 1.2 to 1.7 volts register one way only which is the typical voltage drop for a red LED. Other colours have different readings.

 

(I just tested several of my meters and one didn't register which surprised me, three others did though)

 

Simon.

Edited March 12, 2015 by Simon B

[Thrashed]

Hi guys, bit more testing tonight on the dash LED. With the ignition off the LED reading on the multi meter in diode test mode is 868. With the ignition on the reading starts jumping all over the place but mostly with a - at the front. I checked how the oil pressure light worked and this was giving the same readings, so i think the LED is working correct.

 

I am now at a total loss as to why the dash LED only dims and does not go out as a normal bulb or other LED bulb i have does.

 

I think the only solution is to get new non LED bulbs and replace the lot!

[sj-bradley]

868 sounds kind of odd. The meter must be getting confused with the liming resistor in the circuit - oh well.

 

As these LEDs need a 12 volt supply and ground, you can connect the battery direct across the dash LED and it should light up one way and not the other way. If it lights both ways, it is not a simple LED. It "might" be two LEDs wired internally in parallel, one forward biased and the other reversed so that only one ever lights up.

 

However, you should be able to measure the current through the LED. Just connect Battery+ -> Meter +ve , meter -ve to LED +ve and the other side of LED to chassis. If the LED doesn't light up, you may have the meter in "volts" mode - change to Amps mode and it should light and give you a reading in mAmps.

 

DON'T FORGET to reset the meter to volts mode after this test as connecting the meter in amps mode across the battery will produce a loud BANG and possible smoke ... The internal fuses should protect you at least (provided you haven't got an El Cheapo Chinese one though ...

 

This can be done with the engine NOT running as this may be producing an unstable voltage whereas the battery is free of this noise.

 

Simon.

[Thrashed]

868 sounds kind of odd. The meter must be getting confused with the liming resistor in the circuit - oh well.

 

As these LEDs need a 12 volt supply and ground, you can connect the battery direct across the dash LED and it should light up one way and not the other way. If it lights both ways, it is not a simple LED. It "might" be two LEDs wired internally in parallel, one forward biased and the other reversed so that only one ever lights up.

 

However, you should be able to measure the current through the LED. Just connect Battery+ -> Meter +ve , meter -ve to LED +ve and the other side of LED to chassis. If the LED doesn't light up, you may have the meter in "volts" mode - change to Amps mode and it should light and give you a reading in mAmps.

 

DON'T FORGET to reset the meter to volts mode after this test as connecting the meter in amps mode across the battery will produce a loud BANG and possible smoke ... The internal fuses should protect you at least (provided you haven't got an El Cheapo Chinese one though ...

 

This can be done with the engine NOT running as this may be producing an unstable voltage whereas the battery is free of this noise.

 

Simon.

 

Hi Simon

 

I have tried the led straight of the battery and it only lights up one way. I also tried swapping the loom wires on the back and it just did not light up so assum it is just a normal led and works correctly.

 

When I did the test I had the meter on the little diode picture setting? Is this right?

 

I will do the test later with the multi meter and let you know the result.

 

This is now well beyond my understanding of electrica as I know it should be working but isn't. Is it worth swapping the to one of the other leds to see if that works normally to rule out anything weird with the battery led?

Edited March 14, 2015 by Thrashed

[sj-bradley]

The diode test is just for out of circuit testing. So, no, it's not the one you use to measure current flow, you need the amps or milliamps setting (for LEDs, relay coils and sensors only - most everything else in a car is Amps and no meter can handle the amps through the starter cables)

 

If you want to measure current through it you need to move to the amps setting (DC). The meter then replaces a piece of wire which is why you have to "break" a circuit in order to measure current through it.

 

(Yes, I know that you can have clamp style meters but they are only good for large current flow and you can also measure current indirectly using another method, but I want to keep this simple.)

 

It doesn't matter which way round you connect a digital meter, it will just read negative if it's the wrong way round. A moving coil type needs to be the right way round though.

 

I think that the reason one of your LEDs works correctly is that it requires a higher current to light and the dash one lights on only a very small current (hence why it's dim)

 

This can be fixed by adding in the correct size resistor in series with the dash until it only lights when the engine is not running. I am trying to find out what the currents flowing are to give me some idea what size to recommend.

 

(Forgot to add - most meters have a different socket on the front which says "Amps" or such like. You need to move the red wire from the Volts socket to the Amps one for this to work, don't use the one called 10A as that is for big currents only)

 

Simon.

Edited March 14, 2015 by Simon B

[Thrashed]

Thanks Simon. Will give that a try later when I get back from work and keep you posted

[Thrashed]

Hi, so i have a gunson pocketmeter 2.

 

 

Put the black lead in the com hole and and red lead in the middle hole, volts, milliamps (middle hole). connected the leads to the wires after removing from the LED bulb and set the meter to 200m (7oclock on the meter). No reading, tried all the other from 2000 to 10 and still no reading even if the leads are reversed.

 

Any help on what i am doing wrong in a real idoits guide, i i am so far out of my depth i am thinking of just spending £120 and buying 6 new savage switches that i know work!

 

Thanks in advance

[Thrashed]

Further head scratching and i have no idea if this is relevant but the 12v on the battery LED is on the other side to the other LEDS that need a 12v (oil and brake).

 

Not sure if this means anything or not!

 

Thanks

[Thrashed]

Hi all, good news. I have made some progress. I decided to add in the spare red led to the loom wiring basically piggy backing it to the new dash LED. The new LED works!!! Horray !!!

 

The next step is going to be checking the resistance of the spare red LED and then fitting the appropriate resistor into the loom. I am guessing I will need something different to the 1k resistor I already tried.

 

Looks like I have a solutions, so thanks to everyone for the help and guidance.

 

Steve

Edited March 15, 2015 by Thrashed

[fry61]

Just add an ordinary 5watt tungsten filament lamp/bulb in parallel with your dash LED -- leave it some where behind the dash -- maybe so it throws some illumination down into the foot-well. Gives the necessary start current to the alternator & acts as a back-up to the LED ( although they are very un-likely to fail )

[sj-bradley]

Here is as simple as I can make it.

 

 

The Ammeter shows about 9.5 millamps flowing through the resistor and LED combination. This is what you should be seeing with the meter in Amps mode. The ammeter has to be IN the circuit to measure current.

 

Voltages shown are measured ACROSS a component. You are unlikely to see the 2.4 volts in this picture as the resistor will be combined into the dash LED. Resistors are used to limit the maximum current so that the LEDs don't "blow".

 

With Ohm's Law, you can work out what current there will be in any given resistor by the following

 

Volts = Resistance x Current.

 

So, for a 12 volt battery and a 1k resistor you would get 12 volts / 1000 = 12 milliamps or 0.012 amps.

And as the circuit above shows 9.58 mA through the resistor drops 9.58 volts + the 2.41 across the LED and you get the 12 volts (ok 11.99 here - this simulator is only 2 d.p. accurate) from the battery. It all down to this guy http://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws

 

If your meter is not showing ANY reading comparable to that (and by the way, the 10 scale only works when the wire is plugged into the 10 Amp socket and even then the meter is usually rated for no more than 10 seconds so I would advise NOT to use that connection - there is rarely any need to use that socket as the volts drop and a calculator is much safer)

 

It is possible that you may have blown the internal fuses inside the meter. You can test these by taking them out of the meter, setting the meter to the lowest ohms setting or continuity setting and testing them - they are not required for this measurement, only to protect you and the meter from careless short circuits.

 

I wanted to know what the current (milliamps) reading was for the LED that you found working and the one which wasn't working correctly. I could then advise what size resistor you would need to add to get the dash light working correctly.

 

Firstly, check your meter fuses. You may have to take the meter apart to get to them as not many make them easily accessible. If they ARE blown, make sure to replace like for like as they may be HRC type fuses (High Rupture Capacity). If in doubt, ask and post a picture.

 

Simon.

Edited March 15, 2015 by Simon B

[Thrashed]

Hi Simon

 

I measures the resistance of the spare red led i have and this was 0026. I also measure the resistance of the dash led and this flashed 735 and then zero. Not sure what this means but piggybacking the red led everything works correctly.

 

Does this mean that I need a resistor in the wire for the both the light feed and the 12v side? i would imagine a 1k resistor is enough?

 

I am so out of my league with this and it just does not make any sense to me.

 

Failing everything else, i will follow Bob's suggestion and just leave the red lead under the dash, but i would prefer to have it work properly.

 

Thanks for your patience.

[theduck]

The led is doing the job you need. Leave it in and move on!

[sj-bradley]

You don't measure the LED resistance as that will be almost zero one way and OL or Meg Ohms the other way.

 

Can you draw the circuit you are making as this will help more than words can. I am trying to help, but the answers you give me aren't making much sense at the moment so I can't make any sensible answers yet.

 

If you meant 26 milliamps, then that is a little high, but your comment about 735 and then zero would have destroyed most LEDs if that was milliamps. If that LED still works, then the reading makes no sense.

 

Are you going to the Stoneleigh Show?

 

Simon.

[Thrashed]

Thanks Simon. I think I give up as I am so far out of my depth is it untrue! I will be going to Stoneleigh, so would be good to catch up. In the meantime I am just going to leave the red led in place and piggyback off that. At least it works and I can then deal with it as my next winter project after doing some reading up.

 

Thanks for your help.