|
Good advice re: checking the alternator excitation wire function, but the idiot lights work a little differently.
When the key is in position II but the engine is not running, 12 volts is of course supplied to the instrument cluster (as it is to all other parts of the car), but it's not really distributed from here. The idiot lights that illumnate at this point - bulb failure, brake failure, parking brake, charge - are due to the 12 volts being supplied to one end of the these bulbs (due to the ignition switch) and the ground path provided to the other end of these bulbs via the wire between the charge idiot light and the alternator. When the engine starts, this wire provides "excitation" current to kick-start the charging cycle. If the alternator is working properly, it immediately puts 12 volts on this line. With 12 volts now on both sides of these four idiot lights, they extinguish.
The bulb failure, brake failure, and parking brake idiot lights are diode isolated from the alternator idiot light wire, so that when the alternator is charging, they can still be illuminated via other means (ie. their intended function - true bulb failure, brake failure, or parking brake engaged). The charge idiot light on the other hand continues to perform the same duty of sensing a difference in voltage between the battery (which is directly supplying voltage to one side of most idiot lights) and the alternator output (which is driving the other side of the charge idiot light). Slight differences in these voltages (ie. when the alternator is not charging and is therefore at a slightly lower voltage than the battery) will cause a dim glow of the charge idiot light.
I would have thought that even with no brushes contacting the slip rings in the alternator, there should still be a path to ground inside the alternator for the four idiot lights when the engine isn't running. The fact that Doug is seeing the charge light dimly lit during driving appears to indicate that the wire between the alternator and the charge idiot light is in place, so maybe no idiot lights with the engine not running is the expected indication when brushes have failed. A good check would be to disconnect this wire from the alternator and ground it with the ignition on, to verify the cluster function. If all four idiot lights don't illuminate, then maybe the flex circuit you mentioned has a problem.
The oil pressure idiot light by the way is completely independant of the above functions. I simply gets 12 volts from the cluster on one side when the key is turned on, and the pressure sensor provides the ground path to illuminate it. Once the oil pump provides pressure to the sensor, the sensor switch turns off and the idiot light goes out.
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto
|
