91 GT
New Alternator (140AMP)
New Battery
New Battery Cables
New Starter
Stock Engine
No sound system

Car starts fine and I when I start it, I can diconnect the battery cable and the car will stay on. When I'm driving the battery charge goes up to about half way, but as soon as I come to a stop, the charging goes down to about 8 and if driving at night, all the lights (dash and external) goes dim when car is idle. And brightens when accelerator is press. Why does it do that? Why would reving the engine cause the charging to go up?

 

sounds like a voltage regulator. I dont know where they are on a mustang. sometimes they're built into the alternator. Or, do you have the right size pulley on the alternator. If not, it may be spinning to slow when you're at idle.

 

Most interesting. Alternator spinning too slowly. That could be it. I could change the pulley? May be put some Underdrive Pulleys on.

 

Wait ... don't underdrive pulleys slow these things down already?

 

Yes, have the alternator output checked. Just b/c it's new, doesn't mean it's working correctly.

 

I had it tested and its fine.

 

my car does the exact same thing. so i know how you feel. i'm about to change the wires running from the alt to the solenoid in a couple of days. how did you wire it? did you buy the kit or splice the wires? ill let you you know if it fixes the problem.

 

if you have underdrive pullies I'd remove them.. they don't help with charging.. quite the opposite

 

No underdrive pulleys. Just not charging when idle. If I give it some gas ... the charge goes up. I'm going to check the size of the pulley on the new alt to see if it's the same size as the old one. I don't remember if I had this problem with the old alt I had ...

Yeah let me know how that re-wiring goes cruizin92red5.0

 

i would have to say it is the pulley, it can't be the same size