picked up a 1997 blazer 4.3 vortec from a friend who had it in the garage for 2 years. sweet deal,free.supposely heater core problem. jumped it off on my way battery light comes on. check voltage i determined bad alternator. pick up a used one from a friend, battery light still on. did think we were that close of friend to get a good one.lol. got a new one, installed, battery light goes out. good to go for at least an hour. fryed new alternator. ok 4 times a charm or was it three. buy another new one(ok i lied, to them the sold me a crappy alternator got a new one)put it in, got the voltmeter on the battery im putting out almost 18v. ok while truck was running disconnected both battery terminals still pushing 18v and rising. so i unpluged the only input plug and the voltage drops to normal. im thinking either ignition switch or instrument cluster. oh i also tryed grounding the alt. no dice.HHHHHEEEEEELLLLLPPPPPP

 

Do you have battery voltage present on the wire in the 4-pin connector? If not, then you either have a blown GAUGE fuse, have a break in that wire, or a fault in the ignition switch that is not supplying voltage to the GAUGE (#4 - 10A) fuse. This wire is what the alternator uses to regulate voltage.

 

i checked the fuse its good, i have power on it in the start and run position. i started the truck with the four plug disconnected i have battery voltage on it. i plug it in the alternator the plug voltage drops to 2 to 3 volts. i check the output bolt on the alternator i get 0.80 volts. now i unplug the four plug connector and the alternator starts charging fine between 13.9 to 14.2. my question is on the 4 plug connector shouldnt there be voltage for start up to get the feild coils to start charging then voltage stops. if the voltage gets to high or low then the alternator sends a signal back to the 4 plug connector (which only has one brown wire in it) back to the instrument panel which would turn on the battery light indicating a charging problem. does the brown wire have continious power at all times? if not then maybe i should investigate the ignition switch. thanks

 

The way that circuit works is that voltage is present on that wire through the fuse. The alternator when not charging completes the ground. Once the alternator is charging, that terminal in the alternator should have + system voltage which would make your light go out (no ground or potential difference across the bulb).

 

is it possiable that i could have a bad instrument cluster? i took the cluster out an notice that in the battery light circuit there is a diode in the circuit, which in the wiring diagram i have there is no mention or symbol indicating one,or should i be checking the ignition switch? what about a ground?