Many machines burn up from electrical fires. Fires frequently caused by overheated wires. Wires overheat when additional electrical load added to system. Wires can also short because worn insulation. Another leading cause wires overheating is that amp fuse installed in circuit larger than specified. Instead of fuse burning out, wire burns out. Electrical solenoid wires on carburetors also potentially dangerous. Be sure have power disconnected before taking apart any wire connections.


Bypass starting usually described as using screwdriver or remote button switch to short “S” terminal on starter solenoid to connector on battery cable. Bypass starting maims and kills many people each year. Is unsafe habit should avoid.

An operator, instead of starting equipment from operator’s seat, may try to start it while standing alongside machinery. Operator may think transmission in neutral when actually in gear. Machine can start and run over operator before he can move to safety. Avoid temptation to take shortcuts that may cause injuries. Have a trained assistant in operator’s seat hold in clutch when you’re testing voltage in starter system. If you must work alone, always start engine from operator’s seat.


Generally a 12 volt system shouldn’t shock a person. Severe burns from short circuits are main danger. However, high voltages (20,000 volts) from spark ignitions can cause severe injuries. A secondary injury can occur when person jumps back from the wire and jerks an arm or an elbow into a wall or piece of machinery. Remember that any electrical spark can set off gas fumes from battery or leaky gas line.


Electrical systems on snow throwers vary model by model.

120 volt AC motor (instead of engine)—when replacing motor, careful to follow wiring color codes. Be sure motor is properly grounded according to manufacturer’s electrical diagram. Never replace a 3 wire cord with 2 wire cord.

12 volt DC electric starter—winter temperatures slow speed of chemical reactions in battery and produce less electrical current. Put battery on trickle charger for few hours or prior to starting. Trickle charger applies low current to battery, brings battery back to fully charge state over long period time. Also try switching to synthetic motor oil to make starting easier.

120 volt AC electric starter—because battery powered starting units less efficient in cold, the 120 volt AC system good alternative. Snow throwers with DC electric starters no longer sold, been replaced by 120 volt AC electric starters. Household line voltage unaffected by outside temperature and provides quick starts in all weather. Installation kits for 120 volt AC starter motors available. The starter motor wired to the switchbox. A push button on top of switch box activates motor, which starts engine.


Electronic devices are components used to control flow of electrons in a circuit. Those used in ignition systems are: the diode, the silicon controlled rectifier (SCR), and the transistor.

Conductor, remember, is material that allows electric current to flow easily through it. Insulator is material that resists flow of electricity. Semiconductor is element or compound (a combination of elements) that conducts electricity “part time”. Semiconductor sometimes acts like conductor, sometimes like insulator. Silicon, germanium, selenium are common semiconductor materials used to make electronic components.

Semiconductor materials specially processed, combined to form electronic devices such as diodes and transistors. Diodes and transistors capable of controlling flow of electrons.

A diode is simple electronic device that has two terminals called anode and cathode. Body of diode shaped like a small cylinder, and the terminals are thin wires that protrude from the ends of the cylinder. The electrical symbol for a diode is:

When voltage applied to cathode end of diode, current will move through the diode and come out anode end. In this situation, diode acts like conductor. If current applied to anode end of diode, diode will completely resist flow of current. In this situation, diode acts like an insulator. Diodes will allow current to flow through them only in one direction. If a diode is connected into a circuit, the diode will keep current flowing in just one direction. For this reason, the diode is sometimes called a “one way street” in a circuit.

A transistor is a semiconductor device that has three wire terminals. These terminals are called the base, the collector, and the emitter. Transistors are used to control the flow of current in a circuit. The electrical symbol for a transistor is:

A silicon controlled rectifier (SCR) is another type of semiconductor component. The SCR has three terminals called the anode, the cathode, and the gate. Note that the construction of an SCR is similar to that of a diode, except that the SCR has an additional terminal called a gate. The electrical symbol for an SCR is:

SCRs are used as switching devices in electronic circuits. If a small amount of voltage is applied to the gate of the SCR, a current will flow through the SCR between the cathode and the anode. The current will continue to flow until the voltage is removed from the anode. Thus, the SCR can be switched on by applying a voltage to the gate and off by removing voltage from the anode.

A bit later in the study unit, we’ll look at how these electronic components function in electronic ignition system circuits.


Are several testing tools technicians use to measure circuit quantities. Most common is multimeter or volt-ohm-milliammeter (VOM). This instrument enables you to measure voltage, current, and resistance. Multimeter has two wire test leads connected to it. Ends of leads hold probes used to make circuit tests.

To operate a multimeter:

1)Select quantity you want to measure by turning dial.
2)Take two test leads and touch probes to two points in circuit.
3)Read resulting information on meter’s display.

You can destroy a multimeter if you use it improperly; could also receive a serious electric shock. These instruments used to test electrical systems in certain types outdoor power equipment. Won’t usually need a VOM to perform tests on small engine ignition system.

When multimeter set to read resistance, sometimes called an ohmmeter. When set to measure voltage, called a voltmeter. When set to measure current, called an ammeter.


Here are some electrical systems and accessory components found on typical riding mowers and garden tractors, and some of the controls used to operate them.


No one electrical system common to all types garden tractors. Variations possible, depending on how unit started. Schematic diagrams in service manual will show components. Most modern garden tractors use electric start that includes following components:

–Battery (almost always 12 V)
–Starter motor
–Battery charging system
–A solenoid
–A rectifier regulator
–Ignition switch

A typical garden tractor electrical system with electric start will work like this: when key turned to START, current flows from battery to starter solenoid. Solenoid is mechanical switch that contains small electromagnetic plunger. Plunger moves in and out to open, close switch. When current from battery reaches solenoid, plunger closes, which in turn closes circuit and allows current to flow to starter motor.

Two basic types starter systems used on garden tractors: the separate starter motor and solenoid system, and the combined starter motor and solenoid system.

In separate starter motor and solenoid, solenoid energized with small amount current when key switch turned to start. Solenoid closes circuit between starter motor and battery, and starter spins and engages flywheel.

In combined system, Solenoid mounted on starter motor. When key turned to START, solenoid energized and plunger pulls in. When plunger pulls in, causes solenoid’s shift arm to move out, which causes starter pinion to engage flywheel. As lever moves all way out, also closes circuit between battery and starter. When key switch moved back to RUN, magnetic field removed and large spring disengages starter pinion.

When starter rotates flywheel, this causes activation engine’s ignition system and battery charging system. The spark plug or plugs fire, igniting fuel and air mixture, and engine begins to run.

Tractor’s charging system contains alternator and voltage regulator. A battery can only supply current for certain amount time before discharges completely, so charging system needed to keep up charge. Charging system produces separate source power that recharges battery as long as engine running.

Two types charging devices seen in garden tractors: flywheel alternators and belt driven alternators. In flywheel alternator charging system, stator mounted underneath flywheel. Alternating current produced when flywheel magnets rotate over electric coils of stator. In belt driven alternator, alternating current produced when magnet is rotated inside stator. Belt driven alternator is separate device driven by belt off engine’s crankshaft.

Current produced by alternator is alternating current. Battery can only be charged by direct current. So electronic device called voltage regulator or rectifier placed in charging circuit to convert alternating current produced by alternator to direct current used by battery. Voltage regulator or rectifier contains diodes that convert alternating current to direct current.

Another important part electrical system in riding mower or tractor is safety interlock switch. Most riding mowers and garden tractors equipped with two safety interlock systems. One is prevent start system, other is operator presence system. Prevent start system won’t allow engine to start unless clutch disengaged, blade or PTO drive disengaged, transmission in neutral, and on some models, brake is engaged. Prevent start systems vary depending on type ignition system and starting system mower has. Operator presence system stops engine if operator gets off seat when clutch or PTO engaged. Seat switch works in conjunction with PTO switch and drive switch.

Prevent start circuit typically seen on riding mowers. System works with electronic control module. To start mower, blade must be disengaged and transmission switch closed or transmission in neutral. If blade engaged or transmission switch open, control module will ground ignition system if try to start. Once engine running, current from magneto opens circuit to ground in control module, and transmission and mower blade may be engaged.

Another type safety interlock system for recoil start mower requires drive clutch and blade mount be disengaged for engine to start. When recoil start handle unlocked, safety switch closes, makes connection to ground. If either drive clutch or blade engaged, magneto will be grounded, no spark will occur. If clutch and blade disengaged, mower can be started. After mower running, recoil starter handle locks back into place. This opens safety switch, breaks connection to ground, so drive clutch and blade can be engaged.

Another prevent start system found on certain riding mower designed so when transmission in neutral, transmission switch closed. When mower blade disengaged, mower blade switch closed. Both these switches must be closed for engine to start. If either switch open, current prevented from flowing to starter solenoid, engine can’t start.

Another type safety interlock system for electric start riding mower includes operator presence system with seat switch and prevent start system with switches at clutch and mower blade. All these switches must be in correct positions (be it open or closed) for engine to start. The seat switch, clutch switch, blade switches must be open, ignition switch must be closed. Seat switch will open when operator seated, clutch and mower switches will open when clutch and blade disengaged.

One safety interlock system for garden tractor contains prevent start system, and operator presence system. In this circuit, a neutral start relay used to allow circuit to perform combination safety functions. For engine to start, transmission must be in neutral and PTO disengaged. When transmission in neutral and PTO disengaged, electrical path flows through key switch to starter solenoid. Transmission switch energizes neutral start relay when in neutral. This enables operator to leave seat while engine running and not have it shut down.

After engine running, operator must be on seat if either PTO or transmission engaged. If PTO engaged and operator leaves seat, electrical path will form through energized relay, PTO switch, the seat switch, and ignition system will be grounded out. Another feature of system is special seat switch connector. If safety seat switch disconnected, pathway to ground will be completed and system will react as if no-one sitting on seat.