I'm working on my friend's '89 Firebird (V6, auto, with AC). It coded 53. The problem is intermittent: sometimes the alternator charges at 14.6V, and sometimes 18V+. I've replaced the alternator twice and tried a new battery -> no change.

I tested the voltages at the 4-pin connector on the alternator (disconnected, wire side, ignition on, engine not running):

battery = 12.8V
S (red wire) = wavering voltage between 10.7 and 11.7
F (tan/white wire) = 12.7V
L (brown wire) = 12.4V

Note: the readings for the tan/white wire and brown wire could be reversed - I don't remember.

I disassembled the battery terminal block located by the air filter, sanded all 5 of the eye terminals, smeared dielectric grease on the them, and tightened them down better than they had been. Now my reading at the S terminal (red wire at alternator connector) is wavering between 2.5V and 9V. I am now convinced this is my problem wire.

I need to know where this wire is routed, or where this wire is known to fail. I've been using the shop manual for an '88 Pontiac 6000 because my '85 Firebird manual shows the old si alternator. But the diagram for '88 6000 doesn't match what I have in the '89 Bird. Could somebody please send me the wiring diagram from the charging system page (V6, without digital cluster which means it has a battery idiot light instead of a voltmeter).