汽车电器设备（英文版）7仪表及指示系统

Chapter 7 Instrument and indicator system Chapter 7 Instrument and Indicator System 1. Instrument system Instrument system is used to indicate the operating condition of engine and vehicle. It includes various meters or gauges on the instrument panel, such as: · speedometer · odometer · trip odometer · tachometer · coolant temperature gauge · amperemeter(or voltmeter) · fuel gauge · oil pressure gauge · air pressure gauge(for air pressure braking system),etc. 2. Indicator system Indicator system is usually used to monitor the abnormal or limited operating condition of engine and vehicle. It includes various warning or indicator lights and chimes ,such as: · Oil pressure warning light · Low air pressure warning light (for air pressure braking system) · Low vacuum warning light (for vacuum brake booster) · Low brake fluid warning light · Braking signal warning light · Air cleaner jam warning light · Low fuel warning light · Exhaust brake indicator light (for exhaust brake system) · Turn signal/hazard indicator light · High beam indicator light · Charging indicator light · Seat belt reminder light · Glow plug indicator light (for diesel engine) · Low washer fluid indicator light · Water in fuel filter warning light(for diesel engine) · Rear window defogger indicator light · 4-wheel drive indicator light (for 4-wheel drive vehicle) · A/T oil temperature warning light (for automatic transmission) · Engine oil temperature warning light, etc. 3. Speedometer and odometer 3.1 Function of speedometer and odometer Speedometer—It indicates vehicle speed. Odometer—It records the total distance the vehicle has been driven. 3.2 Construction of speedometer and odometer ( magnetic induction type for example) 1-permanent magnet 2-metal(aluminium)ring 3-magnetic screen 4- coil spring 5-speedometer face 6-pointer 7-number rings(decimal odometer) 8,9-worm gears 10-driving shaft 3.3 Operating principle The magnet is driven by a flexible cable from the transmission. The faster the car goes ,the faster the magnet spins. This action produces a rotating magnetic field that drags on the metal ring surrounding the magnet. The faster the spinning, the more drag on the ring. The spinning causes the ring to swing around against the tension of a spring. This, in turn, moves a pointer attached to the ring, which indicates car speed. The odometer is operated by a pair of gears from the same rotating flexible cable that drives the speedometer. The motion is carried through the gears to the number rings on the odometer indicator. These rings turn to show how many kilometers the car has been driven. 4.Tachometer 4.1 Function of tachometer The tachometer indicates the engine speed in order to · Check and adjust engine. · Monitor the engine operating condition. · Control the optimum shift timing. · Utilize the economical vehicle speed( for low fuel consumption). 4.2 Types of tachometer There are two types of tachometer respectively for gasoline and diesel engine. The circuit varies with the different signal source. 4.3 Tachometer circuit and operation The following is a typical tachometer circuit for gasoline engine. Ignition switch R3 C2 D1 1KΩ 0.7μF B ignition coil R1 R2 D2 T DW Contact 101KΩ 1 KΩ (8v) points C1 Fig. Typical tachometer circuit for gasoline engine Operation: This circuit utilizes the capacitor action of charging and discharging. (1) When contact points closed, the base electrode of triode T (NPN) doesn’t have offset voltage and the triode blocks. The capacitor C2 is being charged. The charging circuit is B+→ ignition switch→ R3 →C2 →D2 →B─. (2) When contact points opened, the base electrode of triode obtains a positive potential as a signal voltage and the triode passes. The capacitor C2 is discharging now. The discharging circuit is C2 positive→ T→ amperemeter →D1 →C2 negative. This action will drive the amperemeter. (3) When contact points repeat the process (open and close), C2 charges and discharges through amperemeter. The average current is in direct proportion to engine speed. Then the scale of amperemeter can be calibrated to indicate the engine speed. Here the Zener diode DW stabilizes the voltage to charge C2 for more accuracy. 5. Fuel gauge 5.1 Function and component parts of fuel gauge The fuel gauge indicates the fuel level of the fuel tank. It consists of a fuel indicator gauge on the instrument panel, and a sensor in the fuel tank. The sensor is a variable resistor. 5.2 Types of fuel gauge There are two basic types: Electromagnetic fuel gauge Bimetal fuel gauge 5.3 Fuel gauge circuit and operation of electromagnetic type (1) Electric circuit Fig. Fuel gauge circuit of electromagnetic type 1-left coil 2 –right coil 3-iron armature(or rotor) 4-pointer 5-variable resistor 6-sliding blade 7-float 8,9,10-terminals 11-ignition switch (2) Operation When the float is low, left coil(called holding coil) has great current and produces strong magnetic field. This attracts the rotor and the pointer moves leftwards. The fuel gauge indicates a low level. As the level changes to higher position, the magnetic field of right coil (deflecting coil) increases because of parallel connection with variable resistor, causing the movement of pointer rightwards to indicate a high level. By using two coils, the accuracy of the gauge is unchanged by any alteration in battery voltage, as both are equally affected. 5.4 Fuel gauge circuit and operation of bimetal type (1) Electric circuit Fig. Fuel gauge circuit of bimetal type 1-contact point 2-bimetallic strip 3-heating coil 4,11,12-terminal 5,9-adjusting geared sector 6-bimetallic strip 7-heating coil 8-pointer 10-spring strip 13-variable resistor 14-sliding blade 15-float (2) Operation When the float is low, all resistance of sensor (or variable resistor ) is connected into circuit. As a small current flows, the coil heats the bimetallic strip in the gauge a little. The strip bends a little because of the unequal expansion of the two metals. The pointer moves a little to indicate a low level When the float is high, the resistance of sensor becomes smaller. The higher the current, the more the heating and bending of bimetallic strip in the gauge. The pointer moves across the scale to indicate a high level. Note: The voltage stabilizer equipped is to reduce the influence resulting from the voltage fluctuation of power source. 6. Coolant temperature gauge 6.1 Function and component parts of coolant temperature gauge The coolant temperature gauge indicates the temperature of engine coolant. It consists of a temperature indicator gauge on the instrument panel, and a temperature sensor at the water jacket of engine head. 6.2 Types of coolant temperature gauge There are two basic types: Bimetal coolant temperature gauge Electromagnetic coolant temperature gauge 6.3 Coolant temperature gauge circuit and operation of bimetal type (1) Electric circuit Fig. Coolant temperature gauge circuit of bimetal type 1-stationary contact-point 2- bimetallic strip 3-contact tongue 4,5,11-terminal 6,9-adjusting geared sector 7- bimetallic strip 8-pointer 10-spring piece (2) Operation The temperature sensor is a sealed tube with a bimetallic strip ,and a heating wire wound around it. The contact points(moveable and stationary points) has a pre-pressure applied by the bimetallic strip. When the coolant temperature becomes high, it bends upwards, the pressure on contact points decreases. At a low temperature: The bimetallic strip is heated by heating coil and bends up. The contact points open. As the surrounding temperature is low, the bimetallic strip cools down easily and the closed time of contact points is long. The average current through the heating coil in gauge is high. The bimetallic strip bends much, causing the pointer to move rightwards and indicate a low temperature. At a high temperature: The bimetallic strip is heated by heating coil and bends up. The contact points open. On the contrary, the surrounding temperature is high, the bimetallic strip cools down hardly and the closed time of contact points is short. The average current through the heating coil in gauge is low. The bimetallic strip bends little, causing the pointer to move leftwards and indicate a high temperature. 6.4 Coolant temperature gauge circuit and operation of electromagnetic type (1) Electric circuit Fig. Coolant temperature gauge circuit of electromagnetic type 1-left coil 2-right coil 3-iron armature(or rotor) 4-pointer 5-sensor(thermistor) (2) Operation The circuit sensor is a NTC (negative temperature coefficient) thermistor . When the temperature is high, the resistance is low. When the temperature is low, the resistance is high. As the coolant temperature changes, the resistance of thermistor changes. This alters the current through right coil and results in a variation of magnetic field. The resultant magnetic fields of left and right coil cause the pointer to move and indicate the coolant temperature, high or low. 7. Oil pressure gauge 7.1 Function and component parts of oil pressure gauge The oil pressure gauge indicates the oil pressure of engine lubricating system. It consists of a oil pressure indicator gauge on the instrument panel, and oil pressure sensor at the main oil gallery or on the coarse filter of the engine. 7.2 Oil pressure gauge circuit and operation (1) Electric circuit 1-oil chamber 2-diaphragm 3-spring tongue 4- bimetallic strip 5-adjusting gear 6-connecting strip 7,9,15-terminal 8-calibrated resistance 10,13- adjusting geared sector 11- bimetallic strip 12-pointer 14-spring piece 16-ignition switch (2) Operation The operating principle is similar to that of coolant temperature gauge of bimetal type. The main difference is that the contact-point pressure changes with the sensor temperature for coolant temperature gauge, whereas the contact-point pressure changes with the sensor oil pressure for oil pressure gauge. 8. Oil pressure warning light 8.1 Function of oil pressure warning light Oil pressure warning light warns of low engine oil pressure. If the light flickers or comes on during normal driving, pull off the road in a safe area, stop the engine immediately and call a mechanic to check. Running the engine with the oil pressure warning light on could cause serious damage to the engine. 8.2 Sensor types of oil pressure warning light There are two types of sensor for oil pressure warning light: Spring tube type and diaphragm type. They operate similarly. 8.3 Construction and operation of diaphragm-type sensor (1) sensor construction Fig. Oil pressure warning sensor of diaphragm-type 1- adjusting screw 2-diaphragm 3-moveable point 4-stationary point (3) Operation When the oil pressure drops to some extent, the moveable point touches the stationary point. Then the circuit is put through and the warning light comes on. Some cars are equipped with two warning switch, one switch is for low pressure warning, the other is for high pressure warning. 9. Low brake fluid warning light 9.1 Function of low brake fluid warning light The low brake fluid warning light warns of a low brake fluid level. If the light comes on while the engine is running, stop the vehicle and perform the following: · Check the brake fluid level. Add brake fluid as necessary. · If the brake fluid level is correct, check the warning system. 9.2 Construction and operation of brake fluid level sensor (1) sensor construction Fig. Brake fluid level sensor 1-case 2- terminal 3-reed switch 4-permanent magnet 5-float 6-fluid level (2) operation When the float drops below the specified value along with the fluid, the permanent magnet closes the reed switch. Then the circuit is put through and the warning light comes on. When the fluid level and the float go up above the specified value, the magnetic force becomes weak. The reed switch will open and the warning light goes out. 10. Low fuel warning light 10.1 Function of low fuel warning light The low fuel warning light warns the driver that the fuel tank is almost empty. Refuel at the nearest gas station. 10.2 Construction and operation of low fuel warning light sensor (1) Sensor construction and circuit Fig. Sensor construction and circuit of low fuel warning light 1-case 2-explosion-proof metal screen 3-thermistor 4-fuel tank 5-terminal 6-warning light (2) Operation The sensor is actually a NTC thermistor. When the thermistor is immersed in fuel, it dissipates heat fast and the temperature is low. Therefore the resistance of thermistor is high,a small current flows in the circuit. The warning light is “off”. When the fuel level drops below the specified value, the thermistor surfaces out of the fuel. Therefore the temrerature is high the resistance is low, a high current flows in the circuit. The warning light will be “on”. 11. Charging indicator light 11.1 Function of charging indicator light If the charging indicator light comes on while the engine is running, it may indicate that there is something wrong with the charging system. Turn the engine off and check the alternator belt. If the light remains on, see a mechanic immediately. 11.2 Charging indicator light circuit and operation (1) Electric circuit K R2 Neutral T2 T1 R1 D Fig. Charging indicator light circuit R1=1KΩ R2=680Ω D-2CZ85D T1=3DK1A T2-3DD15A (2) Operation After closing the ignition switch K, with the alternator not working, T1 is blocked, T2 passes, the indicator light is “on”. As the alternator is working, the neutral point produces a DC voltage about 6V. This voltage makes T1 pass, and thenT2 blocked. The indicator light is “off” now. This denotes the alternator is operating normally. See Fig.7-11 on page 212 mA See Fig.7-5 on page 208 See Fig.7-3 on page 207 See Fig.7-7 on page 209 See Fig.7-8 on page 210 See Fig.7-9 on page211 See Fig.7-24 on page 218 See Fig.7-20 on page 217 (Fig.7-19. on page 217) See Fig.7-27 on page 220 PAGE 6