LEDs in older cars...watch out!

[Anth73]

Just a heads up regarding an unusual problem I had with an LED fitted into the alternator warning lamp on my GTV6.

Having finally got the car back on the road fitted with a 120A alternator, I found the alternator warning lamp would stay on for quite a while on startup, even with minimal electrical load. Once headlights were on and only one of my cooling fans running (drawing 13A), the motor would get a little rough. A check with the multimeter showed only 12.5V at idle and barely more than 11V under load.

It looked like I would need to get the new alternator tested as something was causing the diodes to remain inactive and not charge. Then I stumbled across an AlfaBB post describing similar symptoms to mine in a 105 Series running LEDs in the dash for all lighting and warning lamps. The issue is that the alternator warning lamp forms part of the current sensing circuit. The alternator needs the resistance of the incandescent globe in your alternantor warning lamp to wake up the diodes and start charging properly. An LED doesn't have sufficient resistance, requiring a resistor to be fitted in parallel or do as I did, remove the LED, refit the old incandescent globe and presto...13.9V at idle!

[MD]

If you take a small portable AM transistor radio and tune it between two stations, then walk around the front of the car fan banks, the engine bay and finally wave it around inside the cabin especially around your instrumentation, you will be amazed how much static the radio produces.Expecially around digital equipment
that includes leds in gauges screens and other lighting. The engine should be running of course.

This what is known as RF Noise and is really radio frequency (spurious) energy. Good for bugger all.

[shiny_car]

Thanks for the heads-up, Anth.

How many of your lights were LEDs?  I wonder what the threshold might be (ie: how many LEDs I can get away with, before there's an issue  ).

[bazzbazz]

The problem wasn't that he was using LEDs in the dash, the problem was he replaced the alternator bulb with an LED, the rest are fine.

The Alternator Bulb is part of the igniter circuit of the alternator.

[Anth73]

Spot on Baz, the alternator warning lamp is the one to keep as an incandescent globe. I'm not sure if the other warning lamps matter, though decided to swap those LEDs outtoo. I have kept all the dash lights as LEDs.

[GTVeloce]

Interestingly, the 75 uses the ARC module and the lights for that (including Alt warning lamp) are LEDs. I'm guessing they must use a resister inline to solve the issue Anth has raised.

[shiny_car]

The problem wasn't that he was using LEDs in the dash, the problem was he replaced the alternator bulb with an LED, the rest are fine.

The Alternator Bulb is part of the igniter circuit of the alternator.

Aah, now I get it (I might have been half asleep or had an extra glass of wine, or both, when I first read it).

Thanks for clarifying.