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