alternator light

In a nutshell the lamp is powered when the ignition is turned on, the neutral side of the bulb goes to the electronics in the alternator and tells the alt. there is power, the alt. connects to the earth of the car and completes the circuit making the lamp work, so you can jury rig a test lead with a bulb to prove that the alternator connection will make the light work. the foregoing applies to most common alternator types, but not all.

MrCheerful

so if my battery lights not comming on would this mean a fault with the alternator?

tim

message

Or a blown bulb.

Not necessarily, but the easy way to check is to disconnect the small wire to the alternator, turn the ignition on and connect this lead to an earth (either the battery negative or clean metal, try several places to eliminate just having a bad place you are trying to earth to), the warning light should come on. If it does then reconnect it to the alternator, if the light then does not come on, then either the alternator is faulty or it is not earthed. If the light fails to come on when the little cable is earthed then probably the bulb is faulty or there is a wiring fault (usually near the alternator)

MrCheerful

This is al very interesting to me, the vehicle is a 1984 bedford cf, the bulb just stopped comming on on one day and soon after the battery tottaly discharged, the engine carried on running but it would being a diesel, the lights grew dim and eventually went so low i could not see the road, this was the state of play when i last drove it 2 weeks ago, i had gone for a drive at night to try to charge the battery i was lucky to get home that night, we drove some 30 miles and when i got home and turned the engine off and tried to restart it the thing was flat as a pancake, remember this is a diesel and it has glow plugs that suck juice, but it should have held a charge after running for over an hour, i note with interest that the last poster says the light forms part of a field cicuit, i have recharged the battery now and intend to refit it tommorow so what should i now look for, i will check the small wire if i can find it, space is packed on this one as its a big engine in a small space.

It's worth remembering that some alternators use the connection through the ignition bulb to provide the initial field current - the consequence of this design being that a blown bulb will cause the alternator to produce no output at all. Every alternator requires a current to power the field coil initially, so if the battery is missing or completely flat, this can also cause zero output.

If you can get the connector block off the alternator, you will find three connections - one big wire to earth (may be a braid strap outside the connector), another big wire going to the battery, and a small wire which provides 12v via the ignition switch and warning light. If you have a meter, you can check that you get 12v on the small wire when the ignition is on, and if you earth this wire the dashboard light should come on. If these tests work, the problem is with the alternator/earth, if they don't the problem is with the switch/bulb/wiring. BTW you only need to supply 12v to the small terminal for a moment when the engine is running, as soon as the alternator produces output it's entirely self-powered (which is why the bulb goes out.)