i drove it around a while with no problems at all until one day my guage to drop to 8v

here is whats been done

New alt to bat wire
new ground, ground to body and another ground
new alternator, after i turned in another one that passed a test but they exchanged it anyways (tested in another car to be sure it was working!!)
new alternator belt nice and snug on the pully
all connections appear to be good.
battery tested good

now i tested the back of the alt at the single post,engine running and got 8v. i thought it would be at least twelve.

what else could it be?
i figure it cant be the alternator is passed and brand new! although its only reading 8v. help!

 

i should put up this thread were i had a prob with voltage @ Wot. Fixed the problem

https://www.thirdgen.org/forums/tech...s-voltage.html

 

What do the three wires that run to the back of the alt do exactly?

 

What year, car and engine do you have?

 

o shoot sorry. 1987 firebird TPI 305 stock except cold air intake. No check engine light.

 

Here is a wiring diagram for the alternator. The 3 wires on the back of the alternator break down to this. Terminal S red wire is a consant 12v, terminal F brown wire is 12v with only the ignition turned on, terminal L brown wire is for the charge light indicator on the dash. Black/red wire is the bigger one on the back of the alternator which is the charging wire for the battery. This wire should read 12 volts with ignition off and with the engine running, it should be about 14v. Use a DVOM to measure voltages and not the gauge on the dash.

 

ok im gona check this out later today! thanks allot Jermdm

 

ok heres what i got.

key off
s wire 12v
f wire 0v
l wire 0v

key on
s wire 12v
f wire .04
l wire 0

So I am not getting 12 v with ignition on to the brown "f" wire.
i checked the fuse box and fuses are good. is there a in line fuse or something i am not aware of?

 

according to the diagram you should have 12v to the battery lug all the time (or battery voltage to be exact, on the back of the alternator)

the plugin should be:
(S wire) red --> battery voltage all the time (key off)
(F wire) should be hot in key run position
(L wire) should also be hot in key run position

you should also see a battery icon on your dash and it should be on with the key in the run position, based on what your test results your not getting power to the F and L wire on the plug when you should, what is the result of testing the FP FAN Fuse on both sides of the fuse with the key in the run position? and with the key in the run position do you get the battery light on your dash cluster? My 87 GTA is below the volt meter and should light up with the key in the run position...all of this is with the engine not running...

Let me know your results

RJMcEachern

 

-ok i will update tomorrow thanks for the help!

 

acording to the Chilton i only have a c/h-fan fuse and an FP-INJ2 fuse. I will test both of those tomorrow. i have to use the chilton is because the fuse box text is worn off.

 

here is a good step by step diagnostic for the alternator charging system

http://alternatorparts.com/cs130_sbpage3.htm

RJMcEachern

 

If the text on the fuse box is gone, here is a diagram of the fuse box and location of the fuses. Your main problem is not having 12v at the F terminal with the key turned on. Need to find out why. Could be the FP fan fuse blown. If fuse is ok and you have voltage there, you need to trace the wire and find the open. The L terminal, you dont need to worry about right now. Thats only for the charge light on the dash and doesnt affect alternator operation. Did you check your voltages with the connector plugged in or unplugged?

 

So i checked the CH-fan fuse and had a solid 12+ voltage.
checked the wire.... was good. tried everything without fixing the problem.

Then i decided the only thing i did not do to charging system was change that CH-Fan fuse.

Then i fired it up i got a almost 14v with all accessories on. which is strange because looking at the fuse it looks flawless. I put it back in to see if it was the problem and the car failed at charging. Bad fuse was the whole problem. I think it was just caroded enogh to break the conection but the apread to be no serious corrosion

i have never had the car run this kind of voltage before. at the most it would run @max 12 volts a little under with every thing going. She has never run better and i now i know how dependent these cars are on the charging system.

Thanks everyone for your help on this. This website is the only reason that car is not in the junk yard

 

Glad you got it fixed. Fuses can be funny sometimes. It might not been blown all the way but causing alot of resistance in the circuit and not able to carry the 12v load. Seen things like that and alot of other weird things.