BMW Coupes and Sedans 1970-1988 Repair Guide

Motronic Emission Control System

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See Figures 1, 2, 3 and 4

The Motronic engine control system is an extremely complex, electrical control system. Most testing and repair on the system requires the use of very expensive, factory only, test equipment. The test procedures and explanations explained here, are to be used ONLY as a guide to basic system operation and testing. For any major problems, you should take the vehicle to an authorized factory shop for repair.

GENERAL INFORMATION



The Motronic Emission Control system is an electronically controlled, computerized engine system which controls the fuel injection and ignition timing as well as air/fuel ratio.

The system uses this information to determine engine operating conditions, and adjusts timing and fuel ratio accordingly. The Motronic control unit is located behind the speaker in the right kick panel of 635CSi and 735i models and in the glove compartment of the 3 and 5 series vehicles.

The Motronic control unit is the brain of the system. Various engine sensors supply the unit with operating information air flow, air temperature, throttle position, coolant temperature, engine speed, piston position and oxygen content of exhaust gases.



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Fig. Fig. 1: Motronic wiring diagram

The system receives electronic input signals from several engine sensors. Information supplied by these sensors is used to determine optimum ignition and fuel injection timing under various engine operating conditions.



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Fig. Fig. 2: Example of a Motronic emission control system



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Fig. Fig. 3: Motronic emission system with Digital Motor Electronics (DME) controls-528e models



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Fig. Fig. 4: Motronic emission system with Digital Motor Electronics (DME) controls-535i models

An ideal air/fuel ratio of 14:1 is maintained under most driving conditions. This is the ratio at which the catalytic converter operates most efficiently to reduce exhaust emissions.

The main components that make up the Motronic control system are: oxygen sensor, air flow sensor, 3 coolant temperature sensors, reference point pickup, engine speed sensor and the throttle position sensor.

SYSTEM SENSORS



Coolant Temperature

There are 3 components that supply temperature information to the Motronic control unit. They are the coolant temperature sensor, coolant temperature switch and the thermo timer. All 3 devices are located in the water jacket of the engine block. They supply the temperature information to the control unit in the form of electrical signals.

The system interprets these signals as cold or normal operating temperatures. During cold operating conditions, the air/fuel mixture is enriched by the cold start valve. This valve is located in the intake manifold, downstream from the butterfly valve. It supplies additional fuel to the inlet charge when signaled by the control unit. Extra rich conditions are maintenance until the normal operating temperature is reached.

Throttle Position

The throttle position sensor is located in the throttle linkage at the intake butterfly valve, where it detects the position of the throttle valve. This data is converted into an electrical impulse and sent to the control unit. The control unit interprets the signal as either full throttle, idle or normal operating condition and adjusts accordingly.

Engine Speed

The engine speed sensor is located on the bell housing, next to the starter ring gear. A steel ball, embedded in the ring gear, causes an electronic pulse in the speed sensor, with each engine revolution. These pulses are transmitted to the control unit to be used as the rpm reading.

Reference-Point Pickup

This sensor is located in the bell housing next to the engine speed sensor. It supplies the control unit with piston position information. When the control unit has determined optimum ignition timing data, the reference-point pickup is used to signal ignition firing.

Oxygen

Oxygen content of the exhaust gas is measured by the oxygen sensor, which is located in the exhaust manifold. This sensor measures the amount of oxygen present in the exhaust and sends the data to the control unit as an electrical impulse. The control unit uses this input to keep the air/fuel mixture at the optimum ratio for optimum engine performance.

Air Flow

Intake air flow is detected by the air flow sensor. It is located in the intake passage between the air filter and the intake manifold and informs the control unit of the rate of air intake. Incorporated into the air flow sensor is the air temperature sensor. This sensor informs the control unit of the ambient temperature of incoming air.

Idle Speed Control System

This system uses an electrically governed idle rpm control valve to keep the idle speed stable under the various engine operating conditions. Measured intake air from the air flow sensor bypasses the throttle plate through the idle rpm control valve and subsequently calls for additional fuel injection. The amount of bypassed air is determined by the variable orifice of the control valve.

An additional electronic control unit, the Idle Speed Control Unit, controls the orifice opening according to the engine speed and the engine operating conditions as related to the engine coolant temperature, transmission, air conditioning and heater intake air temperature.

TROUBLESHOOTING THE MOTRONIC SYSTEM



The following is a list of conditions and causes that can be used as a helpful guide in determining any problems with the Motronic system. This guide should be referred to before testing the Motronic system.

Cold engine will not start:

The possible causes are as follows:



Air flow sensor
 
Cold Start valve
 
Fuel injector
 
Fuel pressure
 
Ignition coil
 
Motronic control unit
 
Reference mark transmitter
 
Speed transmitter
 
Wire connections and plugs
 

Cold engine will start, but stalls immediately:

The possible causes are as follows:



Fuel injector
 
Motronic control unit
 
Wire connections and plugs
 

Cold engine is hard to start:

The possible causes are as follows:



Cold start injector
 
Fuel pressure
 
High voltage distributor
 
Motronic control unit
 
Spark plugs
 
Temperature time switch
 

Warm engine will not start:

The possible causes are as follows:



Fuel injector
 
Fuel pressure
 
Ignition coil
 
Motronic control unit
 
Reference mark transmitter
 
Secondary air of the engine
 
Speed transmitter
 
Wire connections and plugs
 

Erratic idle during the warm-up stage:

The possible causes are as follows:



Air temperature sensor
 
Coolant
 
Coolant temperature sensor
 
High voltage distributor
 
Idle control unit
 
Idle valve
 
Ignition circuit
 
Ignition system
 
Motronic control unit
 
Spark plugs
 
Temperature switch (112°F/44°C)
 
Throttle switch
 
Wire connections and plugs
 

The engine is backfiring:

The possible causes are as follows:



Exhaust system
 
Fuel injector
 
High voltage distributor
 
Ignition circuit
 
Motronic control unit
 
Spark plugs
 

The engine idle speed is incorrect:

The possible causes are as follows:



Cold start injector
 
Coolant temperature switch
 
Fuel injector
 
High voltage distributor
 
Intake system
 
Motronic control unit
 
Temperature switch
 
Throttle switch
 

The engine has a hesitation during acceleration:

The possible causes are as follows:



Fuel injector
 
High voltage distributor
 
Ignition circuit
 
Motronic control unit
 
Spark plugs
 
Wire connections and plugs
 

The engine is knocking during acceleration:

The possible causes are as follows:



Ignition circuit
 
Motronic control unit
 
Spark plugs
 
Wire connections and plugs
 

The engine has a coasting hesitation:

The possible causes are as follows:



Fuel injector
 
Motronic control unit
 
Wire connections and plugs
 

The engine is misfiring under all conditions:

The possible causes are as follows:



Air flow sensor
 
Air temperature sensor
 
Coolant
 
Coolant temperature switch
 
Exhaust system
 
Fuel injector
 
Fuel pressure
 
Ignition circuit
 
Intake system
 
Insufficient engine power
 
Motronic control unit
 
Secondary air of the engine
 
Throttle switch
 
Wire connections and plugs
 

The engine has high fuel consumption:

The possible causes are as follows:



Air temperature sensor
 
Cold start injector
 
Coolant temperature sensor
 
Fuel injector
 
Motronic control unit
 
Temperature time switch
 
Throttle switch
 

The engine CO level is incorrect:

The possible causes are as follows:



Cold start injector
 
Intake system
 
Oxygen sensor
 
Secondary air of the engine
 
Wire connections and plugs
 

The HC and NOx levels are excessive:

The possible causes are as follows:



Cold start injector
 
Fuel injector
 
Intake system
 
Motronic control unit
 
Oxygen sensor
 
Secondary air of the engine
 
Wire connections and plugs
 

TESTING



The Motronic engine control system is an extremely complex, electrical control system. Most testing and repair on the system requires the use of very expensive, factory only, test equipment. The test procedures and explanations explained here, are to be used ONLY as a guide to basic system operation and testing. For any major problems, you should take the vehicle to an authorized factory shop for repair.

Before suspecting the Motronic control system to be at fault, be sure that all other systems are in proper working order. Any engine system that would normally be checked in a vehicle not equipped with the Motronic control system, should be checked first.

If the Motronic control unit has been found to be causing the problem, determined which component or area is the most probable source of performance difficulty and begin testing there. Many component failures may be traced to faults in the wiring circuit. Before beginning other diagnostic procedures, check the appropriate circuit for breaks or shorts and be sure that all electrical connections are clean and tight.

In order to properly diagnose and repair any defects in the Motronic control system the following test equipment will be necessary. The BMW service test kit, Bosch L-Jetronic fuel injection test kit and service procedures, BMW test meter 22 13 100 and a standard volt/ohm meter. Failure to use the proper test equipment may result in unnecessary replacement of good components or damage to the system. These special tools are expensive and designed for factory authorized and trained technicians. They are listed here only to give you a reference to the tools used.

Cold Start Valve
VALVE DOES NOT OPEN
  1. Remove valve, leaving fuel lines connected. Supply battery voltage to valve with jumper wire and be sure valve is properly grounded. Pull off relay 1. Apply battery voltage to connector 87 in the relay plug and check that the fuel pump runs. The cold start valve should deliver fuel. If not, replace the valve.
  2.  
  3. If the valve functions properly, check power supply to the valve: Pull plug off the valve and connect a voltmeter between the plug wires. Start the engine. The meter should read battery voltage while cranking the engine. If not, trace the circuit and repair the wiring.
  4.  
  5. Check the thermo timer and replace if resistance values are not correct.
  6.  

VALVE LEAKS

Check valve operation as in first cold start valve test. If valve operates properly (fuel is delivered), remove jumper wire to battery voltage and check that fuel delivery stops. If fuel is still delivered, or leaks, or seeps out, replace valve.

Idle Control Valve

See Figure 5

  1. Valve should be open when vehicle is at rest (no voltage to valve). When voltage is applied to valve (engine on), valve should close. Remove 2 valve hoses and observe valve operation. If valve does not operate as described, replace valve.
  2.  



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Fig. Fig. 5: Idle control valve control unit wiring diagram

  1. If valve operates properly, pull of connector plug and connect voltmeter between the 2 wires in plug. Start the engine and turn the A/C on. Voltmeter should read battery voltage. If it does not, see idle control diagnosis test.
  2.  

Coolant Temperature Sensor

Check that sensor is properly installed and firmly seated. Check that cooling system is full. Bleed system. Check resistance between switch connections. If resistance is incorrect, replace sensor. If resistance is correct, trace sensor circuit and repair wiring.

Coolant Temperature Switch

Switch must be tightly installed. Check that cooling system is full. Bleed system. Check resistance between switch contacts. Resistance below 106°F (41°C) should be zero. At higher temperatures, resistance should be infinite. If values are correct, trace circuit and repair wiring. If values are incorrect, replace switch.

Thermo Timer

See Figure 6



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Fig. Fig. 6: Cut-away view of thermo timer and schematic of the switch

  1. Check that the timer is properly installed and firmly seated. Check the radiator for correct coolant level. Bleed the cooling system.
  2.  
  3. Disconnect the timer and check resistance values between plug terminals G and W, G and ground, and W and ground. If values are correct, trace timer circuit and repair the wiring. If values are incorrect, replace the timer. Specifications are as follows:
    1. G to ground is 40-70 ohms.
    2.  
    3. G to W & W to ground is infinite when the temperature is above 60°F (15°C).
    4.  
    5. G to W & W to ground is zero when the temperature is below 60°F (15°C).
    6.  

  4.  

 
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