Starter, alternator, battery, or wire path?


New Member
Feb 23, 2008
Having problems, want to get an opinion. I recently did a whole bunch of work to my 91 lx. The car starts up fine when the engine is cold, but when the engine is warm it will barely even crank. Here is what I have done so far:

had battery tested and it passed the autoparts store test
used jumper cables to act as extra path on the battery to starter (hooked up in parallel to the regular battery wires).
hooked up another battery in parallel (jumpered)

nothing seems to help out starting when the car is warm. Based on all those tests I am suspecting the starter is bad and/or not grounding out properly. Someone else suggested the alternator is pushing voltage but not current, when driving the voltage reads 14v.

What else could it be?
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SN Certified Technician
Mar 10, 2000
Dublin GA
Pay special attention to the voltage drop tests. If all the connections and wireing check out good, the starter is the problem...

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

AutoZone wiring diagrams

For 79-88 model Mustangs | Vehicle Selection - Year

For 89-93 model Mustangs | Vehicle Selection - Year

For 94-98 model Mustangs | Vehicle Selection - Year

For 1999-2005 model Mustangs | Vehicle Selection - Year

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.

Starter solenoid wiring for 86-91 Mustang

Starter solenoid wiring 92-93 Mustang or earlier Mustang with upgraded high torque mini starter.
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