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Transcript
Chapter 9 Cranking Systems Objectives (1 of 2) • Identify the components in a truck cranking system. • Explain the operating principles of magnetic switches, solenoids, and starter motors. • Describe the operating principles of lightweight, planetary gear reduction starter motors. • Test and troubleshoot a cranking circuit using voltage drop testing. Objectives (2 of 2) • Disassemble a heavy-duty truck starter motor. • Test an armature for shorts using a growler. • Test an armature for grounds and opens. • Use a test light to check out field coils. • Outline the procedure required to rebuild a Delco-Remy 42MT starter motor. Cranking Systems (1 of 3) Cranking Systems (2 of 3) • The cranking system in any vehicle is designed to turn the engine over until it can operate under its own power. • A cranking system can be divided into two sub-circuits: – Control circuit – Starter circuit Cranking Systems (3 of 3) • In a typical cranking system, the components are: – Battery – Key switch (or starter button) – Battery cables – Magnetic switch – Starter motor Battery Terminals • Cleaning terminals – If any evidence of corrosion is observed on visual inspection, a battery terminal should be removed from the post and cleaned. • Testing terminals – The only way to determine whether a battery terminal is doing its job is to test it by measuring voltage drop. Cables • Cables conduct the heavy current load from the battery to the starter and from the starter back to the battery. • All cables must be in good condition. • Cables must be heavy enough to carry the required current load. • Cables can be corroded by battery acid. • Corrosion will cause a voltage drop and decrease circuit amperage, reducing power available to the starter. Cranking Circuit Components • See Table 9-1 on page 230 of the textbook. Starter Motor Operation (1 of 2) Starter Motor Operation (2 of 2) Starter Solenoids Override Clutches Reduction-gearing Starter Motors (1 of 4) • Reduction gearing starters are lightweight starters capable of cranking engines up to 16 liters. • Lightweight starters are able to crank large bore, high-compression engines because they use a planetary gearset. • This arrangement provides for a reduction ratio of about 3.5:1. Reduction-gearing Starter Motors (2 of 4) Reduction-gearing Starter Motors (3 of 4) Reduction-gearing Starter Motors (4 of 4) Cranking Circuit Testing • The cranking circuit requires testing when: – The engine will not crank – The engine cranks slowly – The starter motor will not turn Preliminary Checks • Cranking output obtained from the starter motor is affected: – By the condition and charge of the battery – The wiring circuit – The engine cranking requirements Troubleshooting • See Table 9-2 on pages 240-241 in the textbook. Shop Talk • Starter motors can only operate for short periods without rest. • The high current needed to operate the starter creates considerable heat, and continuous operation will cause overheating. • A starter motor should never operate for more than 30 seconds at a time and should rest for 2 minutes between cranking. This permits the heat to dissipate without damage to the unit. Starter Relay Testing • The starter relay bypass test is a simple method of determining if the relay is operational. • Connect a jumper cable around the starter relay. This bypasses the relay. • Then, crank the engine. • If the engine cranks with the jumper installed, the starter relay is defective and should be replaced. • If the motor still will not crank, check the control circuit. Shop Talk • In some starters, the solenoid also performs the function of a relay. • The control circuit is wired to the windings of the solenoid. • Battery current is routed directly to the starter motor through the drive solenoid. Cranking Current Testing (1 of 2) • The cranking current test measures the amount of current, in amperes, that the starter circuit draws to crank the engine. – 1. Connect the leads of a volt-amp tester (Figure 9-20). – 2. Set the carbon pile to its maximum resistance (open). – 3.Crank the engine and observe the voltmeter reading. – 4. Stop cranking and adjust the carbon pile until the voltmeter reading matches the reading taken in step 3. – 5. Note the ammeter reading. Shop Talk • If the analyzer uses an inductive pickup, ensure that the arrow on the inductive pickup is pointing in the right direction as specified on the ammeter. • Then, crank the engine for 15 seconds and observe the ammeter reading. Cranking Current Testing (2 of 2) • See Table 9-3 on page 242 of the textbook. Shop Talk • The starter should not be operated if the voltage at the battery is below 9.6 volts. • Some leasing companies now use a voltagesensing module to prevent starter operation if voltage is below 9.6 volts to prevent drivers from destroying starters. Starter Circuit Testing Warning • When performing this test, do not operate the solenoid for extended periods of time as severe overheating will occur. Adjusting Pinion Clearance Shop Talk • When testing starter circuits, use the OEMrecommended method of preventing the engine from starting. • Whenever possible, use autoranging DMMs to perform voltage-drop tests. Summary (1 of 3) • The vehicle cranking circuit functions to crank the engine until it can operate under its own power. • A cranking circuit is managed by a control circuit that uses low current to switch and energize a highcurrent starter motor circuit. • A starter motor converts the electrical energy of the vehicle batteries into mechanical energy for cranking the engine. • A powerful electro-magnetic field in the starter motor field coils is used to rotate the armature assembly in a typical starter motor. Summary (2 of 3) • Most starter motors are series wound, so there is only one path for current flow through the assembly. – This means that all of the current flowed through the field coils also flows through the armature producing peak torque at close to stall speeds. • A new generation of lightweight, high-torque starter motors uses planetary gear reduction to multiply cranking torque and greatly reduce motor weight. • Voltage-drop testing should be used to troubleshoot a cranking circuit malfunction because it does so dynamically. Summary (3 of 3) • A heavy-duty truck starter motor found to be defective should be removed from the engine and either rebuilt or replaced. • A growler is used to test an armature for shorts: A steel strip such as a hacksaw blade is placed on top of the armature during testing and if it vibrates, a short is indicated. • A test light is used to test an armature for grounds and opens. • Field coils should be checked using a test light. • After reassembling a starter motor, the pinion clearance should be checked with thickness gauges and adjusted if necessary.