Quick alternator question

[steer from the rear]

Just a quick one the alternator charging light works by putting a voltage to both sides of the bulb? So when the ignitions on the voltage at the key side of the bulb is greater than at the alternator so the light is on but when running the voltages are equal so the light doesn't illuminate........ So what is in the alternator light side of things? As its really hurting my head at the moment as I've the same voltage at both sides all the time on or off

I turn it off I loose both feeds on the bulb, I turn it on I get the same voltage both sides, I run the car I get the same voltage both sides..... One wire never overpowers the other to put the light on :mad:

Why please :confused:

[Os8472]

Because the light takes two positive feeds, 1 from the output of the alt and 1 indirectly from the battery so the light stays off, the light will only come on when the alt isn't charging meaning the light then has a positive feed from the battery and a negative path to the alt

[steer from the rear]

So we're saying inside my alternator is damaged? As its feeding my dash light and won't light my batt light when the ignitions on and car not running

[Os8472]

Not necessarily, start with the basics

The two wires on the alt the right way round?
The lamp in the dash ok? Swap with a different known lamp

[steer from the rear]

I've tested both sides of the bulb in the clocks, there ignition switched

If wires are swapped alternator doesn't charge bulb is ok also so as regards as feed from battery to dash bulb ok and wire from other bulb terminal to alternator ok

[steer from the rear]

I just don't understand what the terminal on the alternator is meant to do? It grounds the wire before you start yes?

It should equal the battery voltage while the engines running yes hence no light you only get a light while one is greater than the other

[Dave Reed]

Have a read of this ;


http://auto.howstuffworks.com/alternator.htm

[Scoff]

Russell mate, think about this: If you have a bulb that has +12v connected to one side and nothing connected to the other then you will have 12v registering on both sides of the bulb. (the filiment in the bulb is a conductor, so 12v passes right through.) With the alternator unplugged and the bulb removed then you should only have 12v on ONE side of the bulb holder. That will be ignition live. It stands to reason that the other side of the bulb should be connected to the alternator plug (purple wire).

The job of the alternator is to switch that purple wire down to ground whenever it isn't charging. So with the key on but engine not running the alternator should be switching to ground and the bulb illuminates. Once the alt is running it should disconnect that wire from ground and you will be left reading 12v on both sides of the bulb again, and the bulb will go out.

[steer from the rear]

Well I just checked both spades out of my alternator and non of them tested for continuity to ground?

Scoff we had a issue with this said alternator when we mapped it last year you said then what the hell.......I've not been bothered about the light till now when I've redone my hole loom

I've also got a diagram of every pin on the back of the clio f7p clocks to add to the boards:agree:

[Scoff]

Yeah, but it won't read continuity mate, it'll be a transistor junction that's doing the switching, it doesn't quite work like a relay. You need to wire the bulb up as per normal and then measure the volts on that pin of the alternator. In normal operation you'd see 12v with the engine running (light off) and something close to 0v with the engine off but ignition on (light on). If the alt isn't doing that then it's fubar :D