Alternator wiring

[Mark Murdoch]

I have a 71 IIA 109 Euro 6 cyl negative earth.  When I connect the lead from the voltage regulator to the back of the alternator the charge light on the dash comes on with the key in the off position.  

Any ideas?

Thanks, Mark

 

[Snagger]

Unless the dash side of the warning lamp has been incorrectly connected to a permanent live instead of a switched live in the fuse box, then you have a faulty alternator.  

The lamp works as follows:  with the ignition off, the alternator is providing no power and the W post is dead, and the feed to the lamp should be isolated by the ign switch from the battery.  With the ign switch on but engine not started, the lamp is connected to the battery but the alternator is still not producing power, so battery current will earth through the alternator and illuminate the lamp.  With the engine running, the battery power to the lamp will be opposed by alternator power, giving no current through the lamp and it'll go out. 

So, to check quickly if you have a wiring fault, remove the W wire from the alternator and earth it to the engine or chassis with the ign switch off.  If the bulb stays out, then battery power was going via the thick main charge cables to the alternator and through the failed alternator to back power the bulb.  If the light illuminates when disconnected from the alternator but earthed elsewhere, the power can only be coming from the dash end and you have a short or incorrect connection.

[Mark Murdoch]

Oops. Thank you. Traced the errant wire back and found it to be a previous owner's cobble. All well now.