Ok, I have a 1987 350 TPI IROCZ. Alternator failed and drained a old battery. So I picked up a replacement alternator, and a new battery because the mentioned drained battery was old. (why not kill two birds with one stone?)

Replaced both, all together fine, hooked up a Sun Vat machine and the alternator only puts out 12.33-+ a few .00 volts... Its peak voltage seems to be around 12.40 at idle, with the VAT machine regestering 11.98 at 2000 RPM. All wires and connections were cleaned of corrosion, and everything is hooked up right.

Any ideas?

 

I'd first take the alternator back to the store and have them bench test it. No sense beating yourself to death looking for a problem until you're sure the alt. is up to par.

 

how old is that sun vat machine?
hook up a voltmeter to the battery yourself
measure the voltage with the engine off and again with the engine running
12.6 with engine off
around 14 with engine running

 

Thanks the MicroVat is not exactly new, but it works. I tested it out on my daily driver, charging voltage fine etc. The IROC is still fubar I think Im going to pull the alternator today and take it to work tomorrow and test it again.

 

Took the new alternator off, and took it too two parts stores. Tested fine. I watched it output the correct voltage on the test machine. Put it back on the car... 12.30 volts peak output. WTF.

 

sounds like you have some voltage drop along the positive cable going from the alternator to the battery might try replacing that assuming of course that this is the case. take a multimeter and see how much voltage the alternator is putting out at the post on the alternator where the batery connects if that is higher than the voltage getting to the battery id say you found your problem.

 

i second that assessment
if the alternator is putting out 14 volts and ur getting 12.3 at the battery theres a huge voltage drop accross the positive wire from the alternator to the battery
if you measure the voltage across alternator output and battery negative
and compare it to voltage across battery positivbe battery negative
and it says 14, and 12.3
then the wire is bad

 

Yea I kinda feared that, but the wire splices off at some point. I'll have to get a multimeter and stuff tomorrow at work. (and print out a wiring diagram)

 

did you figure it out?

 

I know what's wrong. This will seem unbelieveable but it's true. Check your 20-amp "FP Fan" fuse. The alternator receives voltage to its "F" terminal which causes it to charge. This terminal is fed through the 20-amp FP Fan fuse circuit. This is true for 1987 model year F-bodies. I don't know if it holds for other years though.

I had this problem years ago and went nuts trying to figure it out when someone told me this. My fuse was blown!! It fixed the problem!

 

that fuse was the same on my 91 as well. but i agree it could be possible.

 

Where is this fuse, in the fuse block in the car? I see a 20a fuse container bolted to the side of the fender near the battery, but it is ok. I've been busy this week, I might get a few minutes to work on it tonight but probably not till the weekend.

 

Yea thank you! That was it, a friggin blown 20amp fuse. Replaced it and boom! 13.50 volts!

In my 3 years as a Air Force mechanic, Ive never run into anything like that. Strange, huh?

 

If the fan fuse is blown, would you not notice it when your fans don't kick on?

I've always had a low guage reading on my instrument panel volt meter and could never figure it out. I replaced the battery and had the alternator rebuilt with 150 amps. It still shows less than 12 volts on the guage when lights are on and turn signal is blinking.

I checked my fuse but could not tell for sure if it was broken. I'll find a new fuse tomorrow and try it.

 

I didnt drive it when the alternator wasnt working, so it would really not get hot enough. I have the same issue on the cluster. Sometimes it shows accurately, other times, its low. Still the same 13.5 reading from the alternator though, I just assumed the gauge is going out.