Chapter 10

Chassis Electrical Circuits

Objectives (1 of 2)

• Describe how a light bulb functions.• Explain the operating principles of halogen and

high-intensity discharge (HID) lamps. • Describe the function of the reflector and lens in a

headlamp assembly. • Aim truck headlights. • Troubleshoot lighting circuit malfunctions.• Describe the operation of typical truck auxiliary

equipment.• Explain how a trailer electrical plug and connector

are connected.

Objectives (2 of 2)

• Outline the operating principles of truck instrument cluster components.

• Diagnose and repair some typical truck instrument cluster failures.

• Explain the function and operation of warning and shutdown systems.

• Identify the types of circuit protection used in truck electrical systems including fuses and cycling and non-cycling circuit breakers.

• Describe the procedure and material required to solder a pair of copper wires.

• Outline the procedure required to quickly check out a truck electrical system.

Lighting Systems (1 of 2)

Lighting Systems (2 of 2)

• Light bulb operation

• Fluorescent lights

• Halogen lamps

• Halogen infrared (HIR) headlamps

• Xenon headlamps

• HID optics

• Managed headlamp voltage

HID and Halogen Luminosity and Efficiency

• See Table 10-1 on page 252 of the textbook.

Light-emitting Diodes (LEDS)

Caution

• When replacing halogen bulbs, take care not to contact the bulb with fingers because this can cause a rapid failure. When installing a bulb, handle it by the base only.

Shop Talk

• Some manufacturers recommend coating the prongs and base of the new sealed beam with terminal grease for corrosion protection.

• Use an electrical terminal protective approved by the manufacturer.

Night Vision Systems

• Although a typical truck operates only 30 percent of its operational life at night, more than half of truck accidents occur when it is dark.

• This has led to the emergence of infrared thermal imaging systems such as the Bendix Xvision system. – XVision consists of a front-mounted infrared camera that

senses heat from the environment and processes the signal electronically to produce a virtual display as an output on the vehicle dash.

– The driver display consists of a black-and-white image. Warmer objects such as people, animals, and headlights appear in shades of white, while cooler objects such as guardrails, trees, and abutments appear in shades of gray and black.

Shop Talk

• Lights should be turned off when cranking the engine to avoid transient voltage spikes.

• As the engine is cranked, 650 to 1,200 amps are drawn through the system.

• At the precise moment when the starter is disengaged, electricity may surge into any closed electrical circuits. – This random surge can shorten the life of the

lights, causing them to burn out prematurely.

Headlight Dimmer Switch

• The headlight dimmer switch, or courtesy switch, can either be mounted on the floor or it can be a part of the turn signal assembly.

Stalk Switch

• Trucks today may use either electromechanical switches or smart switches.

• The procedures you use for diagnosing and repairing switches differ vastly so make sure you know what you are working on before attempting any repairs.

• The stalk switch or turn light switch may be either “smart” or electromechanical.

• A smart stalk switch is designed to send nothing more than signals to a module.– The module then converts the signals into outputs.

Turn Signal Switch Diagnosis

• Before replacing the turn signal switch, ensure that the trouble is in the switch and not elsewhere in the circuit.

• Check that the circuit breaker and fuse are functional, and inspect the signal light bulbs for broken filaments.

• Also, check the flasher relay, and replace it if necessary.

• If the turn signal switch must be replaced, make sure the key is off and the battery negative cable is disconnected.

Auxiliary Electrical Equipment

• Trailer circuit connector

• Instrument panels and gauges– Dash design

• Dash components

Thermistor Resistance Check

• To check thermistor-type temperature sensors, the approximate temperature of the component being checked must be known to determine the resistance that should be read at the cluster harness connector. Consult a chart such as this:

• See Table 10-3 on page 263 of the textbook.

Caution

• Static electricity can cause permanent damage to the cluster.

• Before working on the cluster, be sure to remove all static electricity from your body by touching grounded metal.

• Do not touch pin connectors during removal and installation of gauges.

Fuel Gauge

• The fuel level sensor used is a float/rheostat type.

• When a sending unit sensor is shorted to ground, the gauge should read empty.

• Locate the fuel sensor gauge input in the cluster harness. – If resistance for the fuel

tank level matches OEM spec and the gauge reading does not correspond, replace the fuel gauge.

• See Table 10-4 on page 264 of the textbook.

Shop Talk

• Before checking the fuel gauge, be sure the cab interior is warmed up (during cold weather), and that the vehicle has been sitting still long enough to allow the fuel to settle.

Circuit-protection Devices

• Fuses

• Circuit breakers– Cycling circuit breakers– Non-cycling circuit breakers

Relays

Terminal Assignments• Terminal Designation Old NumberNew

NumberCoil circuit

• coil circuit (Control Pos) 86 1• coil circuit (Control Neg) 85 2

Switched circuit• common (supply current or B+ ) 30 3• normally closed (NC) 87a 4• normally open (NO) 87 5

Standard SAE Relay Terminal Assignments

Circuit Analysis

Rapid Checking of a Truck Electrical Circuit

• General voltmeter test– Battery voltage– Cranking voltage– Charging voltage– Voltage-drop test

• These tests are used to determine the general condition of a vehicle electrical system.

• The idea is to produce a report card on the battery, the cranking circuit, and the charging circuit.

• In fact, the test is so fast and easy to perform, it should become part of routine service procedure.

Battery Voltage

• Battery voltage can only be accurately measured when there is no surface charge.

• The voltmeter readings produced at 70°F may be interpreted as follows:

• 12.6V or higher 100 percent charged

• 12.4V 75 percent charged

• 12.2V 50 percent charged

• 12.0V 25 percent charged

• 11.9V or lower fully discharged

Cranking Voltage

• Ensure the engine is not capable of starting by no-fueling the engine.

• Connect the voltmeter leads across the cranking motor terminals.

• Crank the engine for 15 continuous seconds. The voltmeter reading should read above 9.6V for the full 15 seconds.

• If the reading falls below 9.6V, a problem with one of the following is indicated:– Defective or corroded battery cables or terminals– Defective or discharged batteries– Defective cranking motor, solenoid, or relay

Charging Voltage

• Start the engine and run at 75 percent of rated speed with no load.

• Now turn on all the electrical accessories on the vehicle.

• Use the voltmeter to test battery voltage. It should read between 13.5V and 14.5V.

Summary (1 of 6)

• Truck and trailer lighting systems can be broadly divided into interior and exterior.

• Circuit wiring usually proceeds from a power source to a circuit breaker, to a switch, to a junction block, and then finishing at the load. – The circuit is completed by grounding it at or close to

the load.

• Headlamp assemblies consist of either a pair (a single lamp on either side of the chassis) or double units (two lamps on either side of the chassis).

Summary (2 of 6)

• Truck chassis lighting systems use incandescent, gaseous-discharge, or LED operating principles.

• Halogen headlamps use a tungsten filament within a gas envelope.

Summary (3 of 6)

• Xenon or HID headlamps are becoming popular due to longer service life and a brighter, whiter light characteristic.

• LED light units are increasingly being used on truck chassis marker and taillight assemblies because they last longer and illuminate faster. – Faster illumination makes them ideal as brake

lights.

Summary (4 of 6)

• The wiring output junction block for the trailer is usually located inside the tractor cab, often directly behind the driver seat.

• Most current trucks use computer-managed instrument clusters that are networked with the chassis data bus.– For this reason, the OEM service literature

should be consulted before attempting troubleshooting.

Summary (5 of 6)

• Relays are a means of using a low-current control circuit to switch a high-current circuit. Standard SAE relays use standard numeric coding to indicate the terminal assignments.

• Fuses and cycling and non-cycling circuit breakers are all circuit-protection devices.– Fuses fail when overloaded. Circuit breakers

trip when overloaded, opening the circuit; they reset either automatically or when the circuit is switched open.

Summary (6 of 6)

• SAE Type 1 circuit breakers are cycling and automatically reset.

• SAE Type 2 circuit breakers are non-cycling and the circuit must be switched open to allow them to cool after tripping.

• Voltage-drop testing is a key to diagnosing truck chassis electrical systems. – Note the procedures for performing a fast electrical

system assessment on a truck in this chapter, and study voltage-drop testing with a DMM in the next.