alternator advice please

When you say the bulb and earth are OK.... that may give a clue as there isn't an earth as such...

The bulb uses the alternator as an "earth" normally. A +ve feed goes from the ignition to the bulb then onto the terminal on the back of the alternator (often via a diode these days). When the alternator isn't running it effectively provides an earth, as it gets up to voltage the potential difference evens out and the bulb goes out.

The voltage supplied via the bulb is used by the regulator on the alternator to adjust it's output which is why a lot of modern systems have a resistor in parallel with the bulb, otherwise if the bulb blows, the alternator may not charge.

Your bulb seems to be working backwards which suggests:

a) there is no diode between the bulb and the terminal b) there may be no +ve feed to the other side of the bulb.

cheers

Dave W.

|| || When you say the bulb and earth are OK.... that may give a clue as || there isn't an earth as such...

I knew this was going to be complicated...

|| || The bulb uses the alternator as an "earth" normally. A +ve feed goes || from the ignition to the bulb then onto the terminal on the back of || the alternator (often via a diode these days). When the alternator || isn't running it effectively provides an earth, as it gets up to || voltage the potential difference evens out and the bulb goes out.

Thanks for this. I thought it was the other way round, ie the alt earthed through the bulb.

|| || || || Your bulb seems to be working backwards which suggests: || || a) there is no diode between the bulb and the terminal || b) there may be no +ve feed to the other side of the bulb. ||

So my first check is that there is a +ve feed to the bulb with ignition on. If there is, does that mean there's something wrong with the alternator? There's no separate diode pack that I can see - the wire goes straight from the bulb to the terminal block on the back of the alternator. Presumably the diodes are built in to the alternator. Problem is, the old alternator was tested when I exchanged it for the new one, and the guy said that it was working OK, but at reduced output, and he tested the new one before I bought it and it checked out OK in the shop.

Thanks for the help.

The easy way to check is to disconnect the wire from the back of the alternator and, with the ignition on, measure the voltage between the wire and earth. It should read battery voltage. If it doesn't then check the voltage at the bulb and so on back to the ignition switch. (You can also connect the wire to earth and make sure the light comes on...)

The diode I am referring to is fitted to later models between the bulb and the alternator - probably part of the dash PCB. One of it's uses is to prevent power being applied to that wire in turn providing power to the vehicles ignition system, bypassing the ignition switch... You may well not have one. Earlier versions normally use some sort of load relay/ resistor to earth the ignition circuit when it is turned off.

If you want to check the alternator independantly just connect a bulb between the terminal and the +ve battery terminal. It should light up when the alternator/engine isn't running and go out when it is running.

cheers

Dave W.

|| The easy way to check is to disconnect the wire from the back of the || alternator

|| cheers || || Dave W.

Dave, many thanks. It turned out that the ignition bulb had been wired to earth instead of to +ve. I connected it to a switched +ve feed and it all now works perfectly. All working and, better still, I understand why. Every day's a day at school.

Cheers