The two switches are:
1.) The backup light switch (T5 location - driver's side of the transmission, has two brass screws)
2.) Netural Safety Swtich - it has nothing to do with the starter. T5 location is on the top front cover of the transmission. It's purpose is to tell the trans is is netural and it is safe to run the Engine or the clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.
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. 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 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
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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.
Starter solenoid wiring for 86-91 Mustang
Starter solenoid wiring 92-93 Mustang or upgraded high torque mini starter.
