For additional information about the location and diagnostics of the information sensors described in this Section, refer to Chapters Supply system and Ignition system.
Coolant temperature sensor
General description
coolant temperature sensor (shown by arrow) usually located next to the temperature gauge, near the fuel pressure regulator and is a thermistor (resistor whose resistance changes according to temperature). The change in resistance determines the voltage drop across the sensor. At low temperatures, the resistance of the sensor is high. As the temperature rises, the resistance drops. Failures in the sensor circuit in most cases occur due to a loose wire contact or a short circuit; if there is no problem with the wires, check the sensor as described below.
Examination
1. When checking the sensor, first of all measure its cold resistance (typically 2100 to 2900 ohms).
2. Next, start the engine and warm it up to normal operating temperature. The resistance should be lower (typically 270 to 400 ohms).
If limited access to the temperature sensor makes it difficult to connect electrical probes to its contacts, remove the sensor as described below and test in a container of heated water simulating operating conditions.
Wait until the engine has completely cooled down before starting this procedure.
Replacement
1. To remove the sensor, press the spring clip, disconnect the electrical connector, then carefully unscrew the sensor. Be prepared for a small amount of coolant to escape; to reduce leakage, prepare a new sensor and install it as soon as possible.
Handle the coolant sensor very carefully. Damage to this sensor will affect the operation of the entire fuel injection system.
It may be necessary to drain a small amount of coolant from the radiator before removing the sensor.
2. Before installing the sensor, make sure its threads are clean and apply a small amount of compound to them.
3. Installation is reverse to removal.
Oxygen concentration sensor
General description
Usually oxygen sensors are installed on vehicles with a catalytic converter. Most oxygen sensors are located in the exhaust pipe behind the exhaust manifold. On the 535, the oxygen sensor is installed in the catalytic converter. Sensor electrical connector facing front for easy access (left-hand side).
oxygen sensor (shown by arrow) usually located in the exhaust pipe behind the exhaust manifold, monitors the oxygen concentration in the exhaust gases.
Oxygen in the exhaust gases interacts with the oxygen sensor and creates a voltage that varies from 0.1 V (high oxygen concentration, lean mixture) up to 0.9 V (low oxygen concentration, rich mixture). The ECU constantly monitors this change in voltage to determine the ratio of oxygen to fuel in the mixture. ECU, controlling the duration of the fuel injectors opening pulse (opening time) changes the air/fuel mixture ratio. A fuel mixture of 14.7 parts air and 1 part fuel is the ideal mixture for minimizing exhaust emissions, allowing the catalytic converter to operate at its most efficient. It is this ratio of 14.7 to 1 that the ECU and the oxygen sensor are trying to maintain all the time.
The oxygen sensor does not produce any voltage if its temperature is below its normal operating temperature of about 320°C. During this initial warm-up period, the ECU operates in "open circuit" (that is, without information from the sensor).
When the engine warms up to normal operating temperature and/or has been running for two minutes or more, and if the oxygen sensor produces a constant voltage below 0.45 V at 1500 rpm or higher, the ECU fault code memory is activated.
If there is a problem with the oxygen sensor or its circuit, the ECU will operate in "open loop", that is, it controls the fuel supply in accordance with the programmed value of the failure codes instead of information from the oxygen sensor.
Proper operation of the oxygen sensor depends on four conditions:
1. Electrical - The low voltage generated by the sensor depends on good, clean contacts, which should be checked whenever a sensor malfunction is suspected or detected.
2. Outside Air Intakes - The sensor is designed to allow air to enter the inside of the sensor. Whenever removing the sensor, make sure that the air passages are not clogged.
3. Correct operating temperature - The ECU does not respond to temperature sensor signals until it has warmed up to approximately 320°C. This factor must be taken into account when evaluating the performance of the sensor.
4. Unleaded Fuel - The use of unleaded fuel is essential to proper sensor operation. Make sure you are using the correct fuel.
In addition to the above conditions, special attention should be paid to sensor maintenance.
- The oxygen sensor is equipped with a cable with an electrical connector that must not be disconnected from the sensor. A damaged or disconnected cable or connector will adversely affect sensor performance.
- Grease, dirt or other foreign matter is not allowed on the electrical connector and on the perforated end of the sensor.
- Do not use any type of cleaning solvent on the sensor.
- Do not drop the sensor and handle it carefully.
- The silicone case must be installed correctly so that it does not melt and the sensor works properly.
Examination
1. Warm up the engine and leave it to idle. Disconnect the electrical connector from the oxygen sensor and connect the positive probe of the voltmeter to the output terminal of the sensor (refer to the following table), and the negative probe to the body. Oxygen sensor being warmed up (up to 320°C), produces a very small voltage signal. Typically, the signal voltage is between 0.1 and 1.0 V).
Options for oxygen sensor connectors on the cable side. For test procedures, use the appropriate sensor pins (there are three different 4-wire oxygen sensor connectors - do not mix them up).
The electrical connectors for most oxygen sensors are located at the rear of the engine near the front panel. Find a large rubber boot with a thick cable. On early 535i models, the oxygen sensor heater circuit connector is located under the vehicle. Find a small protective cover. These models must have their own oxygen sensor in order to access it in the same way as other models. For more information, consult your dealer.
2. While monitoring the voltage, increase and then decrease the engine speed.
3. With an increase in speed, the voltage should increase to 0.5 - 1.0 V. With a decrease in speed, the voltage should decrease to 0 - 0.4 V.
4. If applicable, inspect the oxygen sensor heater (models with multi-wire sensors). With the ignition on, disconnect the electrical connector of the oxygen sensor and connect to the contacts indicated in the table (see below), voltmeter. There must be battery voltage between the contacts (approximately 12 V).
5. If the reading is incorrect, check the oxygen sensor heater relay (see chapter Onboard electrical system). If relay information is not available, refer to the vehicle owner's manual for the exact location of the oxygen sensor heater relay. The relay should be receiving battery voltage.
6. If the oxygen sensor fails at least one of these checks, replace the sensor.
Replacement
Because the sensor is installed in the exhaust manifold, converter, or pipe, which shrinks when cooled, the oxygen sensor can be very difficult to remove when the engine is cold. Rather than risk damaging the sensor (if you plan to use it again in another manifold or pipe), start the engine and let it run for a minute or two, then shut it off. Be careful not to burn yourself when performing the following procedure.
1. Disconnect the negative battery cable.
If your vehicle's radio is equipped with an anti-theft system, make sure you know the correct activation code before disconnecting the battery. Before disconnecting the wire, refer to Section Anti-theft audio system and instrument panel language.
If a message in a different language appears on the instrument panel display after connecting the battery, refer to Section Anti-theft audio system and instrument panel language.
2. Raise the vehicle and place it on stands.
3. Disconnect the electrical connector from the sensor.
4. Carefully unscrew the sensor.
Excessive force may damage the thread.
5. To facilitate subsequent removal, a high temperature anti-corrosion compound should be applied to the threads of the sensor. The threads of the new sensor are already covered with this compound, but if the old sensor is removed and installed, apply the compound to the threads.
6. Install the sensor and tighten it tightly.
7. Connect the encoder cable electrical connector to the main motor cable.
8. Lower the car and connect the battery.
Type of oxygen sensor | Sensor output | Heater Power (12 V) |
not heated (single wire) | black wire () | absent |
heated (three-wire) | contact 1 () | contacts 3 () and 2 (-) |
heated (four-wire) | contact 2 () | contacts 4 () and 3 (-) |
Throttle position sensor (TPS)
General description
Throttle position sensor (TPS) located at the end of the throttle shaft in the throttle body. By monitoring the TPS output voltage, the ECU can determine the fuel delivery based on knowledge of the throttle valve angle (set by the driver). In this system, the TPS works more like a switch than a potentiometer. One group of throttle valve switch contacts closed (contact connection) only at idle. The second group of contacts closes when the throttle is fully opened. Between these positions, both groups of contacts are open (no connection). A bad or weak TPS can cause erratic fuel injection and erratic idle because the ECU thinks the throttle is moving.
All models (except early 535i models with automatic transmission) have a combined idle and full throttle switch; a separate idle switch indicates closed throttle, while TPS is used to indicate full throttle. On 535i models with automatic transmission, the TPS is connected directly to the automatic transmission control unit. At full throttle, the transmission control unit sends an open throttle signal to the Motronic control unit.
All models except early 535i with automatic transmission
Examination
The L-Jetronic TPS is located under the intake manifold (contacts shown).
1. Disconnect the electrical connector from the TPS and connect an ohmmeter to pins 2 and 18. Check continuity between pins 2 and 18 with throttle closed (late Motronic system shown).
2. Slightly open the throttle by hand. Slowly release the throttle so that 0.2-0.6 mm remains before the stopper. The ohmmeter should indicate a short circuit.
3. Check the resistance between pins 3 and 18 with the throttle open. There should be a short circuit within 8-12 degrees from the fully open position. If readings are incorrect, adjust TPS.
4. If all resistance readings are correct and TPS is properly adjusted, check sensor power (5 V) and if necessary look at the wires between the sensor and the ECU (see chapter Onboard electrical system).
Adjustment
1. If the adjustment does not meet the requirements (p. 1-2), loosen the TPS screws and rotate the sensor to the correct position. Follow the TPS check procedure above, and when you're done adjusting, tighten the screws.
2. Check TPS again; if the reading is correct, connect the TPS cable connector.
Early 535i models with automatic transmission
Examination
1. Check TPS for continuity first. To check for continuity, follow the procedures in steps 1-2.
2. Next, check the idle position switch. Disconnect the idle position switch cable connector and connect an ohmmeter to pins 1 and 2. There must be a closure. Slightly open the throttle and measure the resistance. There should be no closure.
Idle position switch and TPS on early 535i models with automatic transmission.
3. Check for correct TPS output voltage signals with throttle closed and ignition on. Connect the voltmeter probe to pin 3 (black wire) on the back of the TPS connector and check for voltage against the chassis. Should be 5V. Also, check the voltage between pin 3 (black wire) and pin 1 (brown wire). It should also be 5 V.
4. Check the voltage between pin 2 (yellow wire) and pin 1 (brown wire) and slowly open the throttle. The voltage should constantly rise from 0.7 V (throttle closed) up to 4.8 V (throttle fully open).
Adjustment
1. First, measure the stabilized voltage. With the ignition on and throttle fully closed, measure the voltage between pin 3 (black wire) and pin 1 (brown wire). It should be around 5V.
2. Next, loosen the sensor mounting screws and connect between pin 2 (yellow wire) and pin 3 (black wire) voltmeter. With the throttle fully open, rotate the switch until the voltage is 0.20 to 0.24 V less than the stabilized voltage.
To measure such small voltage changes, you will need a digital voltmeter.
3. Check TPS again; if the reading is correct, connect the electrical connector to the TPS. It is advisable to use paint or thread locking compound to lock the TPS screws.
Air flow meter
General description
The air flow meter is located in the intake duct. The air flow meter measures the amount of air entering the engine. The ECU uses this information to control fuel delivery. A large amount of air means acceleration, while a small amount of air means deceleration or idling. For all diagnostic checks and replacement procedures for the airflow meter, refer to Chapter Supply system.
Ignition timing sensors
The ignition timing in Motronic systems is electronically controlled and cannot be adjusted. When starting, the crankshaft position sensor sends a signal to the ECU and this determines the starting point of the ignition moment. When the engine is running, the ignition timing changes continuously depending on various inputs to the ECU. The engine speed is determined by the speed sensor. In early Motronic systems, the reference sensor and speed sensor are mounted above the flywheel on the clutch housing. Late Motronic systems have a single sensor (impulse sensor), mounted above the crankshaft pulley. This sensor works as a speed sensor and as a position sensor. For more information, see the Head Ignition system.
Some models are equipped with a TDC sensor at the front of the engine. This sensor belongs to the BMW Service Test Unit and is not part of the Motronic ignition system.