How to Test a Riding Mower Starter

Understanding the Basics of a Riding Mower Starting System

To effectively test a riding mower’s starter, you’ll need to know the basics of the starting system.

Riding Mower Starting System Components

The riding mower or lawn tractor starting system consists of these basic components:

• Battery: Supplies electrical power to power the starter motor.
• Ignition switch: Activates the solenoid coil to send electrical current to the starter motor.
• Starter solenoid: The starter solenoid is a high-current relay that sends current to the starter motor to spin the engine.
• Starter motor: Spins the flywheel to rotate the crankshaft and start the engine.

Once the engine rotates, the spark plugs fire and the pistons take over to run the engine.

Symptoms of a Failed Starter Motor

Many different problems can prevent the starter motor from working – including a failed starter motor. When you turn the ignition key and hear the click of the solenoid but the engine doesn’t crank, you can suspect that the starter motor is bad. When the starter motor gets electrical current through the solenoid but the starter motor doesn’t spin, the starter motor very well could be defective. Other failures can also cause this symptom.

To know for sure if the starter motor is bad, you’ll need to properly diagnose the starting failure. Read on to find out how to determine whether you have a failed starter motor.

Diagnosing the Cause of a Riding Mower Starting Failure

Here’s a detailed video and written troubleshooting procedure for determining whether the starter motor is bad:

Check for a Weak Battery

First, check for a weak battery. The click you hear when you turn the key means that the starter solenoid coil is getting power.

The starter solenoid only needs a little current to make the solenoid coil click, but the starter motor needs a lot of current to spin the motor. So a weak battery could have enough current to trip the solenoid coil but not enough current to crank the engine.

To check the battery, use a multimeter to measure the DC voltage across the battery terminals.

1. Turn off the ignition switch.
2. Access the battery—in this type of riding lawn mower, you lift the seat to get to the battery.
3. With the multimeter set to measure DC voltage, touch the red multimeter probe to the positive or red battery terminal and the black meter probe to the negative or black battery terminal.

A fully charged battery measures more than 12 volts DC. To provide enough current to spin the starter motor, the battery typically needs to be at least 75 percent charged. Charge or replace the battery if the meter measures less than 12 volts.

The starter motor should spin the engine after you recharge the battery. If not, the next step is to check the battery cables.

Check the Battery Cables

If the battery is okay, then corroded or broken wire cables could prevent the starter motor from getting power.

Removing Battery Cable Corrosion

If you find corrosion on the battery terminals and cable ends, here’s how to safely remove it:

1. Put on work gloves and safety goggles.
2. Disconnect the negative cable and then the positive cable from the battery.
3. Use a wire brush to clean the cable ends and battery terminals. If brushing doesn’t remove it all, sprinkle baking soda over the terminals and cable ends and then moisten the baking soda. The baking soda and water will bubble vigorously and dissolve the corrosion. When bubbling stops, use a toothbrush to remove any remaining corrosion.
4. Wipe off the terminals and cable ends with a shop rag.

Using a shop rag, thinly coat the battery terminals with petroleum jelly to help prevent corrosion.

Testing the Battery Cables

If you found no corrosion, or if the motor still won’t start after you remove corrosion, check if the red battery cable is delivering power to the solenoid post by measuring voltage on the red terminal post.

With the multimeter set to measure DC voltage, touch the red meter probe to the red post on the starter solenoid and the black meter probe to the negative terminal on the battery. It should measure more than 12 volts. If it doesn’t, replace the red battery cable.

A word of warning here: Don’t let the meter lead touch both the solenoid posts at the same time or you’ll see a severe spark.

If the motor still doesn’t turn over with a good red cable, the solenoid is next on the list of likely suspects.

Test the starter solenoid

Check the starter solenoid by measuring voltage on the black post at the same time as the solenoid clicks. Here’s how. (You can also see details in the above video.)

You'll need some clip-on meter probes to hold the probes on the wires as you turn the ignition key, unless you have a helper to turn the key while you hold the probes on the wires.

1. Disconnect the black starter cable from the solenoid post and attach the red meter probe to the post.
2. Clip the black meter probe to the negative battery terminal.
3. Hold the ignition key in the start position—you should hear the solenoid coil click.
4. Check the voltage reading and then turn the key to the off position.

When the solenoid coil clicked, the multimeter measured more than 12 volts if the black post gets power. If it measured no voltage when the solenoid coil clicked, replace the starter solenoid.

Test the Starter Motor Cable

If the multimeter measured more than 12 volts on the black solenoid post after the solenoid clicked, you know that the starter solenoid works. Now, check the heavy cable that connects the solenoid to the starter. That cable needs to carry enough current to turn the starter motor.

To check the starter motor cable, use a voltage drop test to measure resistance through that cable. Using a voltage drop test is more accurate than a simple resistance test for a large cable with many copper strands, because a simple resistance check simply detects any current; it can’t detect whether the cable can carry enough current to spin the starter motor.

Measure voltage—also called voltage drop—from the solenoid post to the starter motor stud. Ideally, you should measure less than 1 volt during this test, indicating voltage is nearly the same at the solenoid post and the starter motor stud and that heavy current is flowing freely through the cable. If it’s more than 1 volt, it means wire strands inside the cable are broken.

1. To start the test, reconnect the black starter cable to the solenoid post and clip the red meter probe to the black post.
2. Lift the mower hood and disconnect the spark plug wire so the engine has no chance of starting.
3. Clip the black meter lead to the starter motor stud that connects the black cable to the starter.
4. Hold the ignition key in the start position and check the voltage reading after the click. Turn the key to the off position after checking the voltage.

If you measure a drop of more than 1 volt through the starter cable, replace the starter cable.

Check for a seized engine

If the starter cable is okay, you know that the starter motor is getting power.

Since the motor isn't turning, we’ve narrowed the failure down to a seized engine or a failed starter motor.

Check for a seized engine and if it’s not locked up, you’ll know that the starter motor failed.

To see if the engine spins, remove the plug from the screen above the flywheel to access the flywheel bolt. Use a socket wrench to rotate the flywheel bolt clockwise and try to spin the engine. If the engine won’t spin because it’s locked up, have a service technician diagnose and repair the riding mower problem.

If you can spin the engine, replace the starter motor because it’s not spinning the engine when activated.

If you follow these troubleshooting steps, you should be able to get that riding lawn mower engine started so you can get back to mowing.

If you encounter any difficulties or are unsure about any step, stop and schedule lawn repair near you. We’ll send a local Sears Home Services technician to your home to fix the starting problem and get your riding mower back in top shape.