Electrical circuits consist of switches, wires, and sometimes unfamiliar components that may cause confusion when trying to troubleshoot system. Can usually save time by referring to wiring schematic before beginning job.

When examine a schematic, trace through all the windings and electrical contacts. This will help you become familiar with the electrical system. The glow plugs are the components on the lower left side of the schematic. The battery is next to the glow plugs. The negative battery cable goes to ground on the chassis. From the battery, you can trace the positive battery cable up through the fusible link to the main switch (starter switch) that’s in the upper left corner of this particular drawing of schematic for a Kubota GF1800 turf mower.

When the key turned in the ignition, electricity from the battery passes through the main switch, goes directly to all the glow plugs to preheat them, and then goes to ground. Other engines may have relays to control the higher current needed to preheat larger or more numerous glow plugs.

To the right of battery is starter motor containing solenoid switch. To right of starter are engine stop solenoid and safety start switches. Both safety start switches, PTO and seat, connected to starter motor controller. Only if switches closed will controller send current to starter solenoid to energize 12 volt motor. Wire to right of seat switch is ground wire that grounds controller to chassis.

To right of starter safety switches is switch that controls oil lamp. Lamp used as pressure indicator light. Comes on before engine started to show that bulb working and only comes on while engine running if oil pressure gets dangerously low.

Fuel unit to right of oil switch is variable resistor sending unit. Sends voltage signal to fuel gauge on instrument panel. Fuel unit functions using a float in fuel tank. As float goes up and down, resistance in sending unit changes. As result, voltage signal that unit sends will be in proportion to level fuel in tank. The fuel gauge is voltmeter that’s calibrated to display fuel level.

Next to fuel unit is coolant temperature sensor. Both fuel unit and sensor essentially resistors. Difference in symbols on diagram indicates small difference in construction of each resistor. Fuel unit resistor responds to changes in position of float in fuel tank. The special type of resistor in coolant sensor is known as thermistor. A thermistor changes resistance in response to changes in temperature. A change in resistance causes change in voltage sent to temperature gauge on instrument panel. Some systems also have sensor coming from fuel tank used to activate low fuel warning light.

On far right of schematic are three lights turned on by light switch. These wires, like all in schematic, are marked with letters indicating colors. Wire color system allows technician to identify and trace wires that belong to particular components. The alternator, located above gauges, has built-in IC voltage regulator. Connected to alternator is large red wire that delivers charging current to battery through fusible link. This wire also main power bus that provides power to all other electrical devices like switches and starter motor controller. In addition to fusible link, other fuses are identified for each circuit in schematic. The current rating for each fuse is listed within its symbol.

Complexity of schematic may seem intimidating at first, but it will become much clearer as you trace through diagram component by component. If need to work on electrical system of a machine, make sure to have service manual and electrical schematic standing by for consultation.


Need to understand some basic terminology. Electric circuit is complete electrical path. When you’ve connected electric components to allow electricity to flow from power supply, through components, back to power supply, created an electric circuit. Electrons leave negative terminal of power supply, flow through wires and components, return to positive terminal. This is electron theory of current flow. In conventional theory of current flow, current leaves positive terminal of power supply, returns to negative terminal.

Most outdoor power equipment, automotive applications, easier to understand if use conventional theory. Positive terminal of battery in garden tractor called “hot” wire or terminal (terminal from which power starts to flow). Negative terminal “ground” or ending point for flow of electricity. Ground point of circuit usually thought of as 0 volts. So, in conventional theory, current flows from high voltage point (such as positive terminal on 12 volt battery) to low voltage point (negative or zero voltage terminal).

Negative or ground terminal battery connected to metal frame of equipment. Frame conductive like piece of wire, functions like negative battery terminal. Instead of having two wires running to every component in electric system (a positive wire from + battery terminal and negative wire back to – battery terminal), negative connections can be made to almost any point on metal frame of equipment.

A schematic drawing is map that uses symbols for electric components in a circuit to show how system should be connected. Four types circuits should be familiar with: (1) series circuits, (2) parallel circuits, (3) open circuits, (4) short circuits.

–Series circuits: components connected end to end, current follows one path through circuit. If one of components in series circuit fails, current stops.

–Parallel circuits: current has two or more electrical paths it can follow. Flows from power source, branches into two or more paths, returns to power source or ground. If garden tractor has two headlights, probably wired in parallel. If one burns out, remaining one still lights.

–Open circuits: there’s a break in electric connection that won’t allow current to flow through circuit. Can be intentional break like a switch that’s turned off, or defect like a broken wire.

–Short circuits: current flowing through circuit follows path to ground different than one it was designed to follow. Wires that connect headlight to battery or generator in garden tractor insulated to prevent current from flowing to any destination other than headlight. If insulation damaged, wire contacts metal frame of tractor, current flows to ground instead of headlight.



Accessory circuits like lights and horn simple. Include battery supply, ignition switch, and accessory switch. End of headlight and end of battery both connected to ground. Ground is point zero of voltage, such as negative terminal of battery. Since headlight, battery negative terminal both grounded, same as if electrically connected at same place; like wire running from end of headlight back to negative terminal.

Consider circuit with battery a series or end to end connection six cells. Output voltage battery 12 volts DC. Ignition switch only one contact, in reality ignition switch contains many additional contacts. In this case, labeled as headlight switch. When ignition switch, accessory switch closed, light turns on because is in complete circuit.

Consider typical symbol for ignition switch. Six connection points on this type switch. Coded with letters such as M for magneto, B for battery, S for solenoid, etc. With switch in rest or off position, G or ground terminal, M or magneto terminal connected together. Creates electrical path from kill terminal to ground which short circuits ignition system, shutting off engine.

When engine started, keyswitch turned clockwise. This causes S or solenoid terminal to connect to B or battery terminal. With these two terminals connected, battery voltage and current flow to solenoid. This causes solenoid contacts to close, connecting starter motor to battery which powers starter. Opposite side starter connected to ground to complete circuit. After engine starts, switch changes position.

Movement of internal assembly ignition switch placed in run position by spring. Starter solenoid, starter motor deenergized. B or battery terminal connected to R or rectifier terminal and A or accessory terminal. Lights, horns, electric clutches, other accessories all connected to this terminal of switch. Connection normally prevents lights, other accessories from being left on when engine isn’t running and battery isn’t being charged.

Accessory circuits normally just simple series circuits. Consider typical connection for a light. The A terminal of ignition switch is source power for lighting circuit. A dashboard mounted switch used to turn headlight on or off, depending on whether switch open or closed. If are other lamps such as taillight or second headlight, usually connected in parallel. In this type system, headlights connected across or parallel with each other. If power supplied to either headlight, both headlights illuminate. If one burns out, other can remain on.

Consider simple horn circuit with single headlight circuit. This is simple series circuit. Path for headlight also a series circuit. If look at two circuits together, however, horn circuit and headlight circuit connected in parallel. If horn or headlight fail, other component still operates.


Other accessories may also be connected in this series arrangement. Consider oil sentry light connected to pressure switch. Electric clutch would also be wired in series configuration. An electric clutch allows operator to engage or disengage blade drive or other accessory from switch on control panel.

Smaller equipment may have headlight driven directly off magneto. The 1.25 A and 3 A unregulated charging system stator had optional set coils and poles called lighting stator. This stator connects directly to headlight with no provision for turning the light on or off. Once engine begins to operate, headlight illuminates. When engine at 3,000 rpm, headlight at full intensity. With battery powered lighting and accessory system, lights and accessories always operate on 12 volts DC independent of engine rpm.


A circuit is complete electrical path. Circuit includes power source, conductors, a load, and switch. A power source is source of electrical power. In household circuit, typically a wall outlet. Source in cordless appliance is battery. Conductors are wires that carry electricity. Load is device you want to run. Switch used to turn circuit on and off.

Circuits may be open or closed. In closed circuit, when switch turned on, electrical power from power source flows through unbroken path to load, flows through load, returns to power source. In open circuit, switch is turned off. With switch off, path of circuit is broken, and power can’t reach load.

Electricity is natural force produced by movement of electrons. Electrons are tiny atomic particles with negative charge. Note that battery has two different ends. End of battery labeled with negative or minus sign (-) called negative terminal. End of battery labeled with positive or plus sign (+) called positive terminal. Negative terminal has negative charge; it contains abundance of electrons. Positive terminal has positive charge; it contains too few electrons.

Negative and positive charges in battery produced by chemical reaction. Battery contains solution called electrolyte. Terminals or electrodes are two strips of metal. Each electrode made from different type metal. When these metal strips placed into electrolyte, chemical reaction occurs. Negative charge forms on one electrode, positive charge on other electrode.

Opposite electrical charges (positive and negative) attract, and try to balance each other out. When lots electrons concentrated in one place, will try to move to place where there are fewer electrons. This is basic operating principle of battery. Negative terminal has high number electrons, positive terminal few electrons. Electrons at negative terminal strongly drawn to positive terminal. To actually move between terminals, electrons need path. Can create path by connecting wire between terminals.

In order to use electrons to perform work, can connect lightbulb or other load to circuit. Can also connect switch to turn circuit on and off. When turn on switch, circuit closed, electrons from negative terminal move to positive terminal. As electrons flow through bulb, cause filament to heat up and glow. Flow of electrons through circuit called electric current.

Flow of electricity produced by battery will continue as long as chemical reaction in battery keeps up. When chemical reaction in battery stops, battery will stop functioning, will need to be recharged or replaced.


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.


Schematic is drawing that uses symbols to illustrate circuit’s construction. Good idea to understand simple schematics. Most schematics used in small engine applications label symbols. Since not all manufacturers use same types symbols for various components, this labeling helpful.

Consider circuit that uses battery to power electric starter. An alternator powered by engine charges battery. Are two headlights controlled by a switch, several safety switches, an ammeter that monitors charging current to battery, and fuse that protects battery from high current surges. A single diode used as half wave rectifier. Keyswitch completes electric connections that start engine, also grounds ignition system when engine shut off.

Currently engine isn’t running. Ignition module that supplies voltage to spark plug connected to ground. With switch in this position, any current generated by ignition system goes to ground instead of spark plug. If trace wire from battery to starter and ignition switch, should see several reasons why engine can’t start. Since solenoid contacts aren’t connected, no complete path from battery to starter and ground. Since are open switches between solenoid and start switch, battery current can’t flow to solenoid and close contacts. Remember that open switch is a break in electrical path, so current shouldn’t flow through open switch unless switch faulty. Symbols in figure include some found in schematic described above. Each manufacturer small engine powered equipment has own way illustrating electric systems.

For engine to start, first need complete path from battery to start switch. This will energize solenoid, closing contacts and supplying current from battery to starter. To do this, ignition switch needs to be turned to “S” or start position. This results in two things:

1)Ignition system no longer connected to ground, so high voltage generated by ignition system can flow to spark plug.
2)Battery is now connected to solenoid, which closes contacts, allowing current to flow from battery to starter. This allows starter motor to turn which should start engine.

Note that two other switches have to change for engine to start. Safety switch between start switch and solenoid has to close. Also safety switch between ignition system and ground has to open to disconnect ignition from ground so spark plug can fire.

Safety switches common in outdoor power equipment. A manufacturer will often place one under seat of garden tractor to make sure someone sitting in seat before engine starts. Switch can also be connected to blade of lawn mower to be sure blade disengaged before engine can start. Let’s assume both switches connected to seat of garden tractor.

Consider condition of electrical system with operator sitting on tractor seat and key in start position. First, current flows from battery, through keyswitch, down to solenoid. This closes contacts of solenoid. Current flows from battery, through solenoid, to starter motor then on to ground. Note that only one connection to starter visible. Second connection is metal case of starter that’s bolted to engine. Since engine grounded, and starter electrically connected to engine, starter case also connected to ground.

Once engine started, want starter motor to stop turning. Once operator stops turning key, spring in keyswitch moves contact in switch to run position. Ignition system remains isolated from or not connected to ground, and battery charged by rectified voltage from stator or alternator. As follow path current from stator to battery, notice it passes through diode. This allows only positive pulse electricity to pass to battery, providing DC charge. Light switch has also been closed. Current can flow through lights to ground, allowing lights to illuminate. Lights connected directly to stator so can only operate when engine running. No rectifier between lights and stator, so lights being powered by AC voltage.

If want to stop engine, can turn keyswitch to M position, which would ground ignition system. Would prevent spark from being generated at plug and thus stop engine. If operator gets off tractor while engine running, though keyswitch still in run position, safety switch between ground and ignition system would close, shorting out ignition system and shutting off engine.

The schematic for a complex piece outdoor power equipment would include more components and circuit paths. May be taillights and brake lights, horns, many additional safety switches, warning lights on dashboard, any number other possibilities. When reading schematic, recognize what positions switches should be in, and follow current path from battery or alternator.

Notice that wires connected to components labeled as to color. This indicates actual color of wire in equipment, helps technician follow proper wire when troubleshooting electrical problems.