How to Troubleshoot a Faulty Starter Motor Circuit on Your Car

Introduction

Having car trouble starting your engine can be frustrating. In many cases, a faulty starter motor circuit is the culprit. As the main component responsible for turning over the engine, a bad starter motor can leave you stranded. However, with some basic mechanical skills and testing equipment, you can often troubleshoot and fix a faulty starter motor circuit yourself.

In this article, I will walk through the complete process of diagnosing common starter motor issues, from testing the battery and cables to bench testing the starter solenoid and motor. With the right approach, you can get your car starting again and avoid an expensive tow or repair bill.

Safety Precautions

When working on any car system, safety should always come first. Follow these precautions when testing a starter motor circuit:

• Wear protective eyewear - Safety glasses will protect your eyes from debris, sparks, or acid.
• Disconnect the negative battery terminal - This prevents accidental short circuits while testing components.
• Allow the engine to cool - Hot engine parts can burn you. Let the car sit before starting any work.
• Use jack stands - Support the vehicle properly before getting underneath.
• Avoid jewelry or loose clothing - These can get caught in moving parts.

Following basic safety measures will help ensure you avoid injury while diagnosing starter issues.

Step 1 - Test the Battery

The first thing to check with starter trouble is the car battery. A weak or dead battery is the most common reason a starter motor will not turn over or crank slowly. Use a multimeter to test battery voltage and health.

Follow these steps:

• Set multimeter to DC volts - Select the 20V DC or similar range on your multimeter.
• Disable the ignition system - Remove the main relay or fuse for the ignition/fuel system. This prevents any drain during testing.
• Test voltage at rest - With the car off, probes should read 12.4-12.6V on a fully charged battery. Lower readings indicate a battery needing a charge.
• Test voltage during cranking - While simulating a start by turning the key or crossing starter wires, voltage should stay above 9.6V. Dropping below indicates a weak battery.
• Load test if needed - Use a battery load tester to verify battery health if voltage checks are inconclusive.

A battery providing insufficient cranking amps to the starter will cause slow cranking or failure to start. Recharge or replace the battery before further starter diagnosis.

Step 2 - Inspect Battery Cables and Connections

After verifying battery condition, next inspect the battery cables and connections to the starter. Corrosion or loose connections can cause high resistance and voltage drop.

Follow these guidelines:

• Check cable ends - Look for frayed wires, damaged insulation and excessive corrosion.
• Verify tightness - Battery cable ends should be tight on the terminals, free of dirt or oxidation.
• Clean surfaces - Use a wire brush to clean battery posts and cable clamps.
• Check resistance - Use a multimeter to check for abnormal resistance between cables.
• Inspect grounding - Ensure the battery negative cable has a clean, tight chassis ground.

Faulty cables and connections can mimic a bad starter. Cleaning or replacing damaged cables often fixes no-crank or slow-crank issues.

Step 3 - Check the Neutral Safety Switch

The neutral safety switch (or inhibitor switch) is a sensor that prevents the starter from engaging unless the transmission is in Park or Neutral. Issues here can prevent starter operation.

To test:

• Locate the switch - The neutral safety switch is located on the side of the transmission or brake pedal assembly. Consult a vehicle repair manual for the exact position.
• Test switch continuity - With the vehicle in Park/Neutral, check for continuity through the switch contacts with a multimeter. Continuity should only be present in P or N.
• Verify switch wiring - Check for loose or damaged wires leading to the switch sensor.
• Replace if needed - Faulty neutral safety switches are a common, inexpensive repair. Installing a new one is straightforward.

Proper functioning of the neutral safety switch ensures the starter only operates when safe. Rule this out before looking at other starter components.

Step 4 - Check the Starter Relay or Solenoid

The starter relay (also known as a starter solenoid) is the switch interface between the battery and starter motor. When engaged, it closes high current contacts to send electricity to the starter. Issues here can prevent starter actuation.

To test, locate the starter relay or solenoid on the fender well or along the firewall. Then:

• Check power terminals - Verify battery voltage is present on the power studs. No voltage indicates an open fusible link or bad wiring.
• Check trigger wire - When the key is turned, 12V should be present on the trigger/control wire. No voltage points to failure of the ignition switch or circuit.
• Listen for click - Turn the key and listen for an audible click from the relay/solenoid. No click indicates a bad solenoid.
• Test solenoid winding - Use a multimeter to check solenoid coil resistance. Specs vary, but values should fall between 2-5 ohms.
• Bench test if uncertain - Directly apply 12V to the trigger terminal. If the contacts do not close audibly, the solenoid is faulty.

Replacing a malfunctioning starter relay will generally resolve no-crank situations. Fortunately these parts are inexpensive and simple to change out.

Step 5 - Check Wiring Continuity to Starter

Wiring problems between the relay and starter can also prevent proper starter operation. After verifying the relay or solenoid is functioning, trace the heavy starter cables and check:

• Tight connections - Starter cables should be securely fastened with no looseness or corrosion.
• Insulation condition - Inspect sheathing for cracks, chafing, or other damage that may cause shorts.
• Voltage drop testing - Use a multimeter to check for excessive voltage drop along the starter cables during cranking. Some drop is normal, but more than 0.5V indicates high resistance in the circuit.
• Continuity check - With cables detached from the starter, check end-to-end continuity between the relay and starter terminals. An open circuit points to a damaged wire or bad connection.

Re-securing loose connections or repairing damaged wires can often resolve starter engagement problems without replacing any components.

Step 6 - Bench Test the Starter

If all previous electrical checks are good, the starter motor itself likely has a mechanical or winding failure. To isolate the starter, remove it and conduct bench testing:

• Remove starter - Disconnect all wiring from the starter. Then unbolt it from the engine block. Note shim locations.
• Mount in vise - Clamp the starter body securely in a vise equipped with soft jaws or rags to prevent damage.
• Power and ground - Attach cables from a 12V power source to the starter power and ground terminals.
• Engage and test - While wearing insulated gloves, briefly connect the trigger pin to 12V. The starter should spin strongly. Odd noises or sluggish turning indicate issues.
• Check draw - Use a multimeter to verify starter amp draw is within spec. Excessive draw points to shorted windings.
• Inspect internals - If still questionable, remove the starter end caps and inspect the brushes, armature, magnets and bushings for obvious wear or damage.

Bench testing the starter itself will definitively diagnose problems within the motor. From there you can rebuild or replace the faulty component.

Summary

Diagnosing starter motor issues requires methodically isolating each component in the starter circuit. Follow these general steps:

• Verify battery condition and connections
• Check neutral safety switch operation
• Test starter relay or solenoid
• Inspect wiring continuity
• Bench test starter motor

With patience and proper testing procedures, you can accurately pinpoint what is preventing starter engagement in your car. Repair or replacement of the faulty component will then get your engine starting reliably again.