Engine warning lights

[harvey38]

In the process of updating the instrumentation, I want to update the oil pressure and alternator charging warning lights from filament bulbs to LEDs on my antiquated Ford Sabres so no ECUs etc.

I seem to recall that on these older engines, they rely on the resistance of the bulb in the circuit for them to function correctly so LEDs won't work, is this correct, twaddle or could I put in a resistor?

 

[Big John]

You will have no problem using a LED for the oil pressure.
Alternator is a whole different story. When I changed my ignition filament bulb to a LED I used a 22R resistor and a relay to fool the alternator. Sorry it was a long time ago and I can't remember how I did it. Suggest you experiment if you are that way inclined.

 

[Portofino]

Your recollections are correct .

You can get LED bulbs with built in resistance to compensate.Try Halfords .

 

[Boater Sam]

On a 12v system you need a 56 ohm 3 watt resistor in parallel with a 12v LED for the ignition lamp to excite the alternator.

22 ohms is too low.

The LED has to be connected the right way round too.

 

[VicS]

Your recollections are correct .

You can get LED bulbs with built in resistance to compensate.Try Halfords .

Are you sure that is a resistance in parallel to increase the current , or is it a ballast resistor in series to reduce the voltage for the LED

 

[VicS]

On a 12v system you need a 56 ohm 3 watt resistor in parallel with a 12v LED for the ignition lamp to excite the alternator.

22 ohms is too low.

The LED has to be connected the right way round too.

Some later VP engines have a 33 ohm resistor from ignition switch to warning light connection on the alternator.

Some Hitachi alternators have two connections , one from the warning light and one direct from the switch with a 50 ohm resistor within the regulator.

 

[Boater Sam]

22 ohms will pass approximately over half an amp on 12v and dissipate over 6 watts, getting quite and unnecessarily hot.

 

[William_H]

The resistor described sounds OK 33 ohms might be good. However the regulator and field coil will have a resistance probably around 12 ohms or 1 amp max field current. (might be more). But what that means is that with the current flowing and hopefully light showing you will only get about 7 or 8 volts dropped across the resistor. Not very bright for a LED with internal resistor designed for 12v. I would suggest a bare LED (diode) with a series resistor of about 47 ohms giving perhaps 100 ma bulb current. (this may be too much current for chosen bulb. OP can select the series resistance to suit the bulb. (guessing about 6volts dropped so current equals 6v divided by resistance. To get max lamp brightness. (that 6v is 3v dropped in LED and 3v dropped in filed coil. ) Of course the bulb current will add to current to field coil or reduce effective resistance of the shunt resistor. So it is all a bit variable. Too low shunt resistor might overheat regulator too high a resistor might leave alternator unable to kick start charging. ol'will

 

[Big John]

This is probably a bit late to help harvey 28 but as it was a nice sunny day I went to the boat and low and behold whilst looking for something unrelated I found the circuit diagram of my set-up of using a 12v RED led assembly for the ignition light. For all those who said 22R was not correct I can only say that for the last 15 years my set-up has worked and the resistor does not get warm, it is a wire wound ceramic probably 10 watts. I don't think the circuit needs any explanation but if you need any advice post on the forum.