Volvo850-Ignition.pdf

Chapter 5 Part B: Ignition system

Contents

Dstributor cap and rotor arm

removal and refitting

Ignition system power stage

removal and

refitting

5

Distnbutor cap, rotor arm and HT lead check

See Chapter

Ignition system sensors

removal and refitting

Electronic control unit (ECU)

removal and refitting

8

Ignition timing

checking

9

General Information

On-board diagnostic system

general information and operation

3

Igniticn HT coll

removal and refitting

Spark plug renewal

See Chapter 1

Ignition system

testing

2

Degrees of difficulty

Easy; suitable for novicewith littie expenence

Specifications

General

System type:

B5204 S, and 85234 S

85204 T, and B5234 T engines

85254 $ engines

Firíng order

Spark plugs

Type

Ignition timing'

85204 T and B5234 T engines

B5254 $ engines:

With EZ-129K ignition system

With Motronic 4.3 engine management system

'Velues given are for checking purposes only, no adjustment is possible

Ignition coil

Primary resistance

0.5 to 1.502

Secondary resistance

Pre-1994, 85252 $ engines:

Prmary resistance

Not specitied

Secondary resistance

Torque wrench settings

Nm

Krock sensors

20

Fairty difficult suitable for competent

DlY mechanic

Difficult suitable for experienced DlY mechanic difficult

suitable for expert DIY

Very

Or professional

EZ-129K ignition system with LH3.2 Jetronic fuel system Fenix 5.2 engine management system Motronic 4.3 engine management system EZ-129K ignition system with LH3.2 Jetronic fuel system; or Motronic 4.3 engine management system (according to market territory) 1-2-4-5-3 (No. 1 cylinder at timing belt end of engine)

See Chapter 1 Speclfications

10" + 2 BTDC @ 800 + 50 rpm

58 + 2 BTDC @ 850 + 50 rpm

General information

The ignition system Is responsible for igniting the compressed fuellair charge in each cylinder in turn at precisely the right moment for the prevailing engine speed and load. This is achieved by using sophisticated engine management system which utilises computer technology and electro-magnetic circuitry to achieve the required ignition characteristics Three systems are used on 850 models covered by this manual, dependant on engine size and type. On the Fenix 5.2 and Motronic systems, a single electronic control unit (ECU) is used to control the complete management system including both the fuel and ignition functions. On the LH3.2-Jetronic system, two ECU's are used , one for the fuel system and one for the EZ-129 K ignition system. On this system both ECUs work in conjunction with each other to form one Interrelated engine management package. The operation of all three systems in terms of ignition control is virtually identical; with only minor differences in component arrangement. engine

A series of holes drilled in the periphery of the engine flywheel, and an APM sensor whose inductive head runs just above the drllled flywheel periphery allow the ECU to compute engine speed and crankshaft position As the crankshaft rotates, the land (or "teeth") between the drilled holes In the flywheel , pass the RPM sensor , which transmits pulse to the ECU every tlme a tooth passes it. There is one missing hole in the Ilywheel periphery , which allows the land (or tooth) at that to be twice as wide as the others. The ECU recognises the absence of a pulse trom the RPM sensor at this point, and uses It to establish the TDC position for No 1 piston. The time interval between pulses, and the location of the missing pulse, allow the ECU to accurately determine the position of the crankshaft and its speed. The camshaft position sensor enhances this information by detecting whether a particular piston is on an inlet or an exhaust point cycle.

The main components of the ignition side of the system are the ECU, the ignition power stage, the ignition coil, the distributor , the spark plugs and HT leads and the various sensors that supply information to the ECU on engine operating conditions. The operation of the system is as follows

Information on load is supplied to the ECU via the mass air flow sensor (or via the manitold absolute pressure sensor and inlet air temperature sensor on the Fenix 5.2 system) . The load being determined by computation based on the quantity of air being drawn into the Further information is sent to the ECU from two knock sensors. These sensors are sensitive to vibration and detectthe knocking which engine engine occurs when the engine starts to "pink" (preignite) Sensors monitoring coolant temperature, throttle position, road speed, automatic transmission position (where applicable) and air conditioning system operation, provide additional Input signals to the ECU on vehicle operating conditions gear

With the firing point established, the ECU sends signal to the ignition power stage, which Is an electronic switch controlling the current to the ignition coil primary wIndings. On receipt of the signal from the ECU, the power stage interrupts the primary current t0 the ignition coil, which induces a high-tension voltage in the coil secondary windings. This HT voltage Is passed to the distributor cap, and then on to the spark plugs, via the distrbutor rotor arm and HT leads. The Is then repeated many times a second for each cylinder in turn. cycle

From this constantly-changing data the ECU selects, and if necessary modifies, a particular ignition advance setting from a map of ignition characteristics stored In its memory

In the event of a fault in the system due to loss of a signal from one of the sensors, the ECU reverts to an emergency ("limp-home") program This wlll allow the car to be driven, although engine operation and performance will be limited A warning light on the instrument panel will illuminate if the fault is likely t0 cause an increase in harmful exhaust emissions.

In addltion to the above operations, many of the ignition system components have a second function in the control and operation of the fuel injoction system. Further details will be found in Part A of Chapter 4.

To facilitate fault diagnosis, the ignition system is provided with an