Electric mirrors
9.9.2 Electric mirrors
Many vehicles have electrical adjustment of mir- rors, particularly on the passenger side. The system used is much the same as has been discussed above in relation to seat movement. Two small motors are used to move the mirror vertically or horizontally. Many mirrors also contain a small heating element on the rear of the glass. This is operated for a few minutes when the ignition is first switched on and can also be linked to the heated rear window cir- cuit. Figure 9.34 shows an electrically operated mirror circuit, which includes feedback resistors for positional memory.
9.9.3 Electric sunroof operation
Figure 9.32 Motor reverse circuit
The operation of an electric sunroof is similar to the motor reverse circuit discussed earlier in this
Figure 9.34 Feedback resistors for positional memory and Figure 9.33 Position memory for electric seats
circuit
Electrical systems 223
Figure 9.36 Door lock circuit
alarm systems often lock all the doors as the
alarm is set.
Figure 9.35 Sunroof circuit
Figure 9.36 shows a door locking circuit. The main control unit contains two change-over relays, which are actuated by either the door lock
chapter. However, further components and cir- switch or, if fitted, the remote infrared key. The cuitry are needed to allow the roof to slide, tilt
motors for each door lock are simply wired in and stop in the closed position. The extra compon-
parallel and all operate at the same time. ents used are a micro switch and a latching relay.
Most door actuators are now small motors
A latching relay works in much the same way as which, via suitable gear reduction, operate a linear
a normal relay except that it locks into position rod in either direction to lock or unlock the doors. each time it is energised. The mechanism used to
A simple motor reverse circuit is used to achieve achieve this is much like that used in ball point
the required action.
pens that use a button on top. Infrared central door locking is controlled by The micro switch is mechanically positioned
a small hand-held transmitter and an infrared such as to operate when the roof is in its closed
sensor receiver unit as well as a decoder in the position. A rocker switch allows the driver to
main control unit. This layout will vary slightly adjust the roof. The circuit for an electrically
between different manufacturers. When the operated sunroof is shown in Figure 9.35. The
infrared key is operated by pressing a small switch switch provides the supply to the motor to run it
a complex code is transmitted. The number of in the chosen direction. The roof will be caused
codes used is well in excess of 50 000. The to open or tilt. When the switch is operated to
infrared sensor picks up this code and sends it in close the roof the motor is run in the appropriate
an electrical form to the main control unit. If the direction until the micro switch closes when the
received code is correct the relays are triggered roof is in its closed position. This causes the latch-
and the door locks are either locked or unlocked. ing relay to change over which stops the motor.
If an incorrect code is received on three consecu- The control switch has now to be released. If the
tive occasions when attempting to unlock the switch is pressed again the latching relay will once
doors, then the infrared system will switch itself more change over and the motor will be allowed
off until the door is opened by the key. This will to run.
also reset the system and allow the correct code to again operate the locks. This technique prevents a scanning type transmitter unit from being used to
9.9.4 Door locking circuit
open the doors.
When the key is turned in the driver’s door lock all the other doors on the vehicle should also lock. Motors or solenoids in each door achieve
9.9.5 Electric window operation
this. If the system can only be operated from the The basic form of electric window operation is driver’s door key then an actuator is not required
similar to many of the systems discussed so far in in this door. If the system can be operated from
this chapter, that is a motor reversing system either front door or by remote control then all the
either by relays or directly by a switch. More doors need an actuator. Vehicles with sophisticated
sophisticated systems are now becoming more
224 Advanced automotive fault diagnosis popular for reasons of safety as well as improved
This is important, as the window must not reverse comfort. The following features are now avail-
when it stalls in the closed position. In order for able from many manufacturers:
the ECU to know the window position it must
be initialised. This is often done simply by oper- ● one-shot up or down;
ating the motor to drive the window first fully ● inch up or down;
open, and then fully closed. If this is not done ● lazy lock;
then the one-shot close will not operate. On some ● back-off.
systems Hall effect sensors are used to detect motor speed. Other systems sense the current
When a window is operated in one-shot or being drawn by the motor and use this as an indi- one-touch mode the window is driven in the
cation of speed.
chosen direction until either the switch position Lazy lock feature allows the car to be fully is reversed, the motor stalls or the ECU receives
secured by one operation of a remote infrared
a signal from the door lock circuit. The problem key. This is done by the link between the with one-shot operation is that if a child,
door lock ECU and the window and sunroof for example, should become trapped in the win-
ECUs. A signal is supplied which causes all dow there is a serious risk of injury. To prevent
the windows to close in turn, then the sunroof this, the back-off feature is used. An extra com-
and finally locks the doors. The alarm will also mutator is fitted to the motor armature and pro-
be set if required. The windows close in turn to duces a signal via two brushes, proportional to
prevent the excessive current demand which the motor speed. If the rate of change of speed of
would occur if they all tried to operate at the the motor is detected as being below a certain
same time.
threshold when closing, then the ECU will reverse
A circuit for electric windows is shown in the motor until the window is fully open.
Figure 9.37. Note the connections to other sys- By counting the number of pulses received the
tems such as door locking and the rear window ECU can also determine the window position.
isolation switch. This is commonly fitted to allow
Figure 9.37 Electric window circuit
Electrical systems 225
the driver to prevent rear window operation for
9.10.2 Body electrical systems
child safety for example.
fault diagnosis
Symptom
Possible fault
9.10 Diagnosing body
Electric units
If all units not operating:
electrical system faults open circuit in main supply
not operating
main fuse blown relay coil or contacts open circuit
9.10.1 Testing procedure
or high resistance
(unit ⫽ window,
If one unit is not operating:
The following procedure is very generic but with
door lock,
fuse blown
a little adaptation can be applied to any electrical
mirror etc.)
control switch open circuit
system. Refer to manufacturer’s recommenda-
motor seized or open circuit
tions if in any doubt. The process of checking back-off safety circuit signal any system circuit is broadly as follows. incorrect (windows)
Auxiliary systems diagnostic chart Start
Hand and eye checks (loose wires, loose switches and other obvious faults) –
all connections clean and tight. Check battery
Check motor (including linkages) or actuator or
bulb(s) – visual check
Fuse continuity – (do not trust your eyes) check voltage at both sides with a meter or a test lamp
No
Voltage supplies at the
Check item with separate
device/motor/actuator/
Yes
fused supply if possible
bulb(s)/heater are correct?
before condemning
Supply to switch – battery
No
If used does the relay
Supplies to relay (terminal volts
click (this means the
Yes
relay has operated, it
30 for example) – battery
is not necessarily
volts
making contact)?
Supply out of the switch
Feed out of the relay
and to the relay – battery (terminal 87 for example) – volts
battery volts
Relay earth connection – note also that the relay may
Voltage supply to the light – have a supply and that the
within 0.5 V of the battery control switch may have the
earth connection
Earth circuit (continuity or
voltage) – 0 ⍀ or 0 V
End
226 Advanced automotive fault diagnosis
Figure 9.38 Power hood circuit
9.10.3 Circuit systematic testing
7. Check continuity from hood up and down relays to earth on B wire.
The circuit shown in Figure 9.38 is for a power
8. Check switch operation. hood on a vehicle. The following faultfinding
9. Check pump motor operation. guide is an example of how to approach a
problem with a system such as this in a logical If the power hood will operate in one direction manner.
only proceed as follows.
1. Check for 12 V on N wire at hood up or down ignition switch at the correct position and the
If the power hood will not operate with the
relay as appropriate.
handbrake applied proceed as follows.
2. Check continuity from hood up or down relay to earth on B wire.
1. Check fuses 6 and 13.
3. Check relay.
2. Check 12 V supply on N wire from fuse 6.
3. Check for 12 V on GS wire at power hood relay.
4. Check continuity from power hood relay to
9.11 Instrumentation
earth on BW wire.
5. Check power hood relay.
9.11.1 Gauges
6. Check for 12 V on NW wire at hood switch.
Check for 12 V on N wire at hood up and down Thermal gauges, which are ideal for fuel and relays.
engine temperature indication, have been in use
Electrical systems 227
Figure 9.39 Thermal gauge circuit
for many years. This will continue because of their simple design and inherent ‘thermal’ damp- ing. The gauge works by utilizing the heating effect of electricity and the widely adopted benefit of the bimetal strip. As a current flows through a simple heating coil wound on a bimetal strip, heat causes the strip to bend. The bimetal strip is con- nected to a pointer on a suitable scale. The amount of bend is proportional to the heat, which in turn is proportional to the current flowing. Providing the sensor can vary its resistance in proportion to the measurement and (e.g. fuel level), the gauge will indicate a suitable representation as long as it has
Figure 9.40 Principle of the air cored gauge together with the
been calibrated for the particular task. Figure 9.39 circuit when used as a fuel lever or temperature indicator and shows a representation of a typical thermal gauge the resultant magnetic fields
circuit. Thermal type gauges are used with a variable
use as a temperature indicator. The ballast resis- resistor and float in a fuel tank or with a thermis-
tor on the left is used to limit maximum current tor in the engine water jacket. The resistance of
and the calibration resistor is used for calibration! the fuel tank sender can be made non-linear to
The thermistor is the temperature sender. As the counteract any non-linear response of the gauge.
thermistor resistance is increased the current in The sender resistance is at a maximum when the
all three coils will change. Current through C tank is empty.
will be increased but the current in coils A and B
A constant voltage supply is required to
will decrease.
prevent changes in the vehicle system voltage The air cored gauge has a number of advan- affecting the reading. This is because if system
tages. It has almost instant response and as the voltage increased the current flowing would
needle is held in a magnetic field it will not move increase and hence the gauges would read higher.
as the vehicle changes position. The gauge can Most voltage stabilisers are simple zener diode
be arranged to continue to register the last pos- circuits.
ition even when switched off or, if a small ‘pull Air cored gauges work on the same principle
off’ magnet is used, it will return to its zero pos- as a compass needle lining up with a magnetic
ition. As a system voltage change would affect field. The needle of the display is attached to
the current flowing in all three coils variations
a very small permanent magnet. Three coils of are cancelled out negating the need for voltage wire are used and each produces a magnetic field.
stabilisation. Note that the operation is similar to The magnet will line up with the resultant of the
the moving iron gauge.
three fields. The current flowing and the number of turns (ampere-turns) determine the strength of