4 Fuses- devices used to protect the electrical circuit from too much current flowA fuse is designed to open the circuit when thecurrent reaches a predetermined level.Fuses are rated in AmpsDo not install a fuse larger than is designed forthe circuit
5 Why Use Relays? To direct electricity to a high current device like a starter motor, with the use of a light duty switch(key switch)To control devices-A relay is a switch that changes states when voltageis applied to its input. The input connects to an electriccoil. When voltage is replaced across the input, the coilmagnetizes and moves a switch contact.
6 Why Use Relays? Relay we use has a single switch that is normally open (N.O.)- refers to the effect of the switch in its normal(non-energized state)Electric symbol we use for theserelays - the symbol shows the coiland switch-When the coil is not energized a spring keeps the switchheld open or OFF. When the coil is energized the magneticfield pulls the switch against the normally open contactthereby closing the switch.
7 Starter motor solenoid or magnetic switch Consists of a contact plate, plunger, plunger shaft,pull-in coil, hold in coil and return spring-When power is applied to coil terminal (key turned to start), currentflows through pull in and hold in coils. Magnetic field pulls in plunger(the end of plunger has a contact disc). Disc makes contact with thetwo main contacts and an R terminal contact. Battery is connected toone main contact and the starter motor is connected to the other.Current flows between the battery and starter motor. Current alsobypasses the pull in coil. The hold in coil continues to keep the solenoidenergized until the key switch is opened. At that instant current no longerflows through the hold in coil and the contact disc is spring returned andstarter motor quits turning.
8 Solenoid- a coil of wire When voltage is placed across ends of coil, coilmagnetizes (magnetism moves a plunger)Cut away view of coil commonly found on hydrauliccontrol valvesSolenoids often take the place of what use to be done by mechanicallinkages, levers and switches.-Common uses for solenoids are to control mechanical, hydraulic andelectric circuits.
9 Sensors (referred to as senders) - devices that respond to a physical “stimulus”Variable Resistive Sensors - as physical propertychanges, resistance of sensor changesVariable Capacitance Sensors - (engine oil pressure andhydraulic oil charge pressure)three wires to these sensors: ground lead, supply voltage leadand sensor signalVariable Capacitance sensors- have a circuit built into them.The circuit converts the capacitance to an output voltage proportionalto the pressure. As the pressure increases the voltage increases.
10 SensorsSwitch sensors (air filter and hydraulic filter)close or open when the physical property reachespredetermined level
11 Harnesses - Suspected defect in a wire TestingHarnesses - Suspected defect in a wiretest for continuity with an ohm meterafter checking for continuity, check for a short to nearbypins, battery ground and battery positiveSwitches - Test leads on correct pinsWith wires disconnected from the switch, connect ohmmeter across the switchSwitch can often be tested with a voltmeter in a live circuit-With the switch open you should read infinite ohms. With the switchclosed you should read zero ohms. Anything other than thesereadings indicates a faulty switch.
12 Relays - Test activation signal TestingRelays - Test activation signalMay be able to test for activation signal by listening tothe relayTest switch outputsame test for any other switchactivate the relay and check that the switch functionsproperly-Activation Signal- When activated and or deactivated, many relays make aclicking sound. If the clicking sound is present, you can generally assume thatthe activation signal is present. A more positive method is to check for groundand the correct activation signal at the relay using a volt meter. If you do nothave the activation signal, the relay coil was shorted and should be replaced.If you do not have the activation signal, the cause for this must be found.
13 TestingRelays – Quick Testing TipsReplace relay with a known good relayquick and effectiveRemove relay and put a jumper across correct pins tosimulate a closed switchif output circuit functions, the circuit is ok
14 First, check for correct ground and power to module TestingModules / ControllersFirst, check for correct ground and power to moduleIf a known good module is available, replacement isthe best testObserve symptomsOften controllers are expensivecasual controller replacement is not recommended-Because modules often are complex internally and perform manyfunctions, it can be difficult to determine if a problem exists.
15 Voltage drop- the difference in voltage potential when Voltage Drop TestingVoltage drop- the difference in voltage potential whenmeasured across a circuit or component creating resistance-Voltage Drop- “Resistance decreases the amount of voltage available.”Resistance- an opposing force, created by a circuit orcomponent, to the flow of electrical current-Resistance- “There is a small amount of natural resistance when voltageflow through wires, switches, grounds or connections. The resistanceincreases beyond acceptable limits if corrosion develops, fittings becomeloose or wires fray. Resistance increases each time something, such as awire, a switch, connections, or the ground are added to the circuit.”
16 What Else Should be Tested Each wire, ground, connector, switch, solenoid and thecomplete circuit should be testedEach connection point is a potential source of increasedresistance