1987 5.0 starting problem?

to start off i fully charge the battery and i have checked all the wiring with voltmeter and test light, as soon as i attempt to crank the car the battery drops into the red and cranks a couple times slowly then its like the battery is dead???? I can take the battery out and start my other 5.0 just fine i am lost? I have replaced the ignition coil, the starter, the distributor, cap, rotor, wires, plugs, alternator, relays, everything i can think of? If anyone has had the same problem or any ideas some feed back would be nice thanks!:SNSign:
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Check the battery cables and motor grounds. Also consider that the starter itself has an excessive draw. With your new starter, chances are you ruled that out.

The coil, cap, rotor, wire, plugs, alt and other stuff outside the primary electrical system (battery, battery cables, solenoid, starter interlock, grounds, etc) should not affect cranking. They can keep it from catching, but it should crank even if all that crap is missing.

I'd be replacing cables next if you have doubts about them.

Good luck.
No Crank checklist for 5.0 Mustangs

Since some of the tests will bypass the safety interlocks, make sure that the car is in neutral and the parking brake is set. Becoming a pancake isn’t part of the repair process…

Check battery, terminal connections, ground, starter relay switch (also known as solenoid) and starter in that order. The clamp on with 2 bolts battery terminal ends are a know problem causer.

A voltmeter is handy if you are familiar with how to use it to find bad connections. Measure the voltage drop across a connection while trying to start the car: more than .5 volts across a connection indicates a problem.
See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103 for help for help troubleshooting voltage drops across grounds.

1.) Will the car start if it is jumped? Then clean battery terminals and check battery.

2.) Check the battery to engine block ground, and the ground behind the engine to the firewall.

3.) Jump the big terminals on the starter relay next to the battery with a screwdriver - watch out for the sparks! If the engine cranks, the starter and power wiring is good. The starter relay is also known as a starter solenoid.

4.) Then pull the small push on connector (small red/blue wire) off the starter solenoid (Looks like it is stuck on a screw). Then jump between the screw and the terminal that is connected to the battery. If it starts, the relay is good and your problem is in the rest of the circuit.

5.) Remember to check the ignition switch, neutral safety switch on auto trans and the clutch safety switch on manual trans cars. If they are good, then you have wiring problems.
Typical start circuit...
Diagram courtesy of Tmoss & Stang&2birds

See http://www.autozone.com/images/cds/gif/large/0900823d80195963.gif for 88-90 year cars
See http://www.autozone.com/images/cds/gif/large/0900823d80195964.gif for 91-93 year cars.
See http://www.autozone.com/servlet/UiB..._us/0900823d/80/1d/db/3c/0900823d801ddb3c.jsp for 94-95 model cars.

6.) The starter may be hung, loosen up the bolts that hold it on, and give it a good whack with a big hammer. Tighten up the bolts and try again.

7.) Pull the starter and take it to Autozone or Pep Boys and have them test it. Starter fails test, then replace it. If you got this far, the starter is probably bad.