Well after thinking I had my problem figured out I am in a rut again. My alt is good, I had it tested at the store, and also swapped it temporaily with a brand new one and it made no difference. My battery voltage is the same as the alt post as it is at the battery, little to no voltage drops. My plug that goes into the alt is getting battery voltage on the B side, and 1.9 volts on the a side. I'm guessin that I should be having battery voltage on the a side too? Also my battery is good, I replaced it with a new one as my first attempt at fixing this problem. Anything else I can check or ideas you may have? thanks!

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Original Blue 87GT 5spd.
125,000 Miles, Stock 2.8L

Centerforce Dual Friction, H4 Blue headlights, Cleared Taillights, Mr. Mikes Leather, MoMo Carbon Fiber Steering Wheel, Lowered 1.5", Konig Verdict 17" Rims, Power everything, Speed Hut Gauges, Short Shifter.

Believe it or not, Im having the exact same problem! Any clues guys?

For SI alternators....

Terminal A is the Sense line and should have battery voltage at all times.

Terminal B is the Lamp line and should only have voltage when the key is on.

The SI Alternators will not work right unless these wires both function correctly.

People with later CS alternators (86-88 L4 and 88 V6) can read in my cave. Link at top of page.

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Nobody loves me. Everybody hates me. Guess I'll just go eat worms.

If the alt is good and the battery is good, about all thats left is a slipping belt or a bad/dirty connection. Check battery posts, starter connections, and that all the wires a good and clean at the alt. You could have a bad battery cable and look perfectly fine too...corroded or partially broken inside the insulation.