One of the most important parts of a lawn tractor’s electrical system is the lawn mower’s solenoid. A solenoid is a relay that energizes the engines starter motor.
Common solenoids have three (3) electrical lugs or four (4) lugs. Three lug solenoids use the mounting bracket as an electrical ground. Four lug solenoids have a dedicated ground lug.
Mower Solenoid Wiring
The common wiring method uses the large screw type lugs on top of the solenoid as follows:
The positive battery cable attaches to one of the large lugs, in addition a smaller (usually red) wire comes from the ignition switch to the same lug. This small wire carries 12 volts DC from the charging system of the engine back to the lug and in turn to the battery insuring battery re-charging.
On the other large lug a battery cable runs to the energizing lug on the starter motor. When the solenoid is energized, 12 volts DC comes from the battery to the starter.
On a 3 wire solenoid there is a small lug that energizes the main relay allowing the battery voltage to flow through the solenoid to the starter. It delivers 12 volts to the lug and energizes the solenoid allowing the starter to engage.
The previously mentioned small red wire from the solenoids battery terminal to the ignition switches battery “B” terminal has two functions. One is to carry 12 volts from the battery to the switch and conversely through the internal wiring of the switch, it carries voltage from the charging system of the engine back to the battery.
Testing Your Mower’s Solenoid
In order to check a solenoid, first you need to be sure your battery is fully charged and the solenoid is grounded. If you are not getting power to the starter motor, try this test. Take a jumper wire from the battery cable to the energizer lug on the solenoid. If the solenoid acts and the starter motor turns over then the solenoid is good.
Your no-start problem lies in the ignition switch or related wiring. If however, the solenoid doesn’t energize, then it is time to take the test one step further.
Before you replace the solenoid, again check that the solenoid is grounded. It takes just a second to run an extra ground wire from the solenoid’s metal frame (3 lug) or the ground lug (4 lug) to the negative pole of the battery. Run the test again and try to energize the starter. If it turns over, then it was a ground issue.
Time to Replace the Solenoid
If neither test energizes the starter, then replace the solenoid with the proper replacement solenoid.