[Jim's 89 GT]

I got a 89 gt 5.0 reading repair manual, and was wondering if it had one? for some reason when I turn my headlights on or hit the brakes the amps go down a bit on my car. I was wondering if this maybe have something to do with it? I checked out all wiring new alter. new battery. Where is this located at if it has one, and if so you think this could have something to do with it?

[Canyon_Red_86]

If the voltage only drops at idle or low RPM's, there's a chance someone has put a set of underdrive pulleys on the car. When you replaced the alternator, did it come with it's own pulley or did you swap the one that was on your old alternator? There are multiple manufacturers of underdrive pulleys and even a small difference in the pulley circumference can make a 200-300 RPM difference on the alternator -- which can make a huge difference in voltage output @ idle.

[Jim's 89 GT]

it had one on there got it from advance auto. like when i turn the lights on its drops the amps alot but still will run but amp needle will move alot when i use the brakes samething. I checked all the wires etc cant find anything. you think if i upgrade to a 3g alternator it will have enough power where it wont do that? like it wont stall or anything just dont think that stock alt. can handle the power. Would a 3g make evrything run ok? if so are they hard to install?

[Canyon_Red_86]

Don't think a 3G swap would help. You need to try and get the existing system to work if you can. Don't 'butcher' your wiring quessing at the problem. Do you have multiple amps in your stereo system? You say your amp guage drops -- how much does it drop? You may still have an underdrive crank pulley. A stock pulley is about the diameter of a quart paint can and steel. An underdrive pulley is about the diameter of a soup can and usually aluminum. If you run the RPM's up does the amperage come up?

[EagleAutosports]

To answer your other question, it doesn't have an external regulator, it's an internal one.

Something you could check is the two connections at the back of the alternator. They're plastic and tend to melt in between the posts, and interrupt the electrical voltage.

[Jim's 89 GT]

Originally Posted by Canyon_Red_86

Don't think a 3G swap would help. You need to try and get the existing system to work if you can. Don't 'butcher' your wiring quessing at the problem. Do you have multiple amps in your stereo system? You say your amp guage drops -- how much does it drop? You may still have an underdrive crank pulley. A stock pulley is about the diameter of a quart paint can and steel. An underdrive pulley is about the diameter of a soup can and usually aluminum. If you run the RPM's up does the amperage come up?

yea when i run the rpm's up it goes up, its stays 3/4 up on idle but if i use lights brakes it will drop past half but goes back up when the rpm's go up. no pullies either all stock. No amps, the radio is not installed yet.

[Canyon_Red_86]

Seems like the charging system is o.k. I don't think it would be charging that well if the plug @ the altenator was the culprit. Problably an unwanted ground on the lighting circuit. Check the harness where it comes through the metal @ the brake lights and any other area where the harness isn't in loom and could possibly be touching metal. Should be an easy fix just may take a while to track it down.