Did it do this before the 3G upgrade or after? If it was before, it could be a whole other animal. If it was right after, you know where the problem lies and thats where to troubleshoot. It could be you used the wrong AWG wire and it is getting hot therefore mandating a cool-down period to restart. It could be grounds. It could poor connections. Etc, etc, etc...
Crank quickly. Get it to crank slowly (when hot). Then add a jumper ground wire and see if it now cranks fast.
It should not damage anything to try and start the car.
No Crank checklist for 5.0 Mustangs
Revised 30-Jan-2009 to reorder checklist and add cleaning procedure for connections.
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…
1.) Will the car start if it is jumped? Then clean battery terminals and check battery.
The battery posts and inside of the battery post terminals should be scraped clean with a knife or cleaner brush tool.
The clamp on with 2 bolts battery terminal ends are a know problem causer. Anyplace you see green on a copper wire is corrosion. Corrosion gets in the clamped joint and works its way up the wire under the insulation. Avoid them like the plague...
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 Automotive Test Tools for help for help troubleshooting voltage drops across connections and components. .
2.) Check the battery to engine block ground down near the oil filter, and the ground behind the engine to the firewall. All grounds should be clean and shiny. Use some sandpaper to clean then up.
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 cranks, 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
6.) 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.
AutoZone wiring diagrams
For 79-88 model Mustangs
AutoZone.com | Get In The Zone!
For 89-93 model Mustangs
AutoZone.com | Get In The Zone!
For 94-98 model Mustangs
AutoZone.com | Get In The Zone!
For 1999-2005 model Mustangs
AutoZone.com | Get In The Zone!
Starter solenoid wiring for 86-91 Mustang
Starter solenoid wiring 92-93 Mustang or earlier Mustang with upgraded high torque mini starter.
Revised 30-Jan-2009 to reorder checklist and add cleaning procedure for connections.
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…
1.) Will the car start if it is jumped? Then clean battery terminals and check battery.
The battery posts and inside of the battery post terminals should be scraped clean with a knife or cleaner brush tool.
The clamp on with 2 bolts battery terminal ends are a know problem causer. Anyplace you see green on a copper wire is corrosion. Corrosion gets in the clamped joint and works its way up the wire under the insulation. Avoid them like the plague...
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 Automotive Test Tools for help for help troubleshooting voltage drops across connections and components. .
2.) Check the battery to engine block ground down near the oil filter, and the ground behind the engine to the firewall. All grounds should be clean and shiny. Use some sandpaper to clean then up.
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 cranks, 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
6.) 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.
AutoZone wiring diagrams
For 79-88 model Mustangs
AutoZone.com | Get In The Zone!
For 89-93 model Mustangs
AutoZone.com | Get In The Zone!
For 94-98 model Mustangs
AutoZone.com | Get In The Zone!
For 1999-2005 model Mustangs
AutoZone.com | Get In The Zone!
Starter solenoid wiring for 86-91 Mustang
Starter solenoid wiring 92-93 Mustang or earlier Mustang with upgraded high torque mini starter.