BLOG 3 TTEC 4848

WS6 OXYGEN SENSORS ON VEHICLE

what an oxygen sensor is?

http://i01.i.aliimg.com/img/pb/837/867/265/1283765874043_hz-myalibaba-web5_8478.JPG

An oxygen sensor is an electro mechanical device that is fitted in the exhaust stream to sense the amount of oxygen content in the exhaust gases left over after combustion. The oxygen sensor measures the amount of free oxygen in the exhaust gases and compares the difference to the amount of oxygen that is in the atmosphere. By this, a voltage is generated within the sensor which is sent as a voltage signal to the ECU which is then judged to be rich or lean.

Practical

In practical we had to find a vehicle fitted with an oxygen sensor to test with an oscilloscope.

The vehicle I made test on is a FORD KA. The oxygen sensor is located under the hood in the engine bay up front by the radiator, which is easy to get to. The sensor is a 4 wire Zirconium Narrow range type. The colour of the wires and what they stand are below:

http://www.obd2tech.com/img/p/196-328-thickbox.jpg

White = heater +

White = heater –

Black = signal +

Grey = sensor ground

To test the sensor I had to back probe the signal positive wire connection using a pin.

Above is a photo I took of the oxygen sensor in closed loop mode cycling from rich to lean. The engine at this stage is running at 2500 rpm. The voltage goes as high as 0.9 volts and low as 0.2 volts. The average voltage is about 0.45 volts.

This sensor is functioning good as it has a “cross count” of about one per second. This means it is sending its signal back to the ECU at the correct speed to keep the ECU up to date with the air fuel ratio in order to make adjustments to the air/fuel. If the signal was not signalling properly then the amount of cross counts will be reduced. This will indicate that the sensor is becoming sluggish. If the sensor is sluggish you could remove it and clean it with a solvent spray suitable for it. Then replace it and see if it made any changes.

http://www.volvoclub.org.uk/faq/ImagesProcedures/OxygenSensor1.gif

In the picture above it shows an oxygen sensor like the one I am testing. As the exhaust gases pass by the sensor a voltage is generated based on the amount of “free” oxygen in the exhaust compared to the oxygen in the atmosphere. The sensor tip can become contaminated with engine oil, engine coolant and/or wrong additives in the fuel. This causes faulty voltage signals back to ECU. For example if the tip is contaminated and blocked then the sensors going to get false impression that the air fuel ratio is leaner than it actually is. The ECU will then try to make the air fuel ratio richer to try and maintain a perfect ratio of 14.7:1.

Consequences of this will damage the catalytic converter. A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a huge drop in highway performance or stalling because of a buildup of backpressure in the exhaust stream. It is also very expensive to replace.

http://davisconverters.com/images/cracked-broken-catalytic-converter.jpg

Below I have copied and pasted parts out the WS6 practical workbook and added to my blog to help show my work.

5.0 Make this Oxygen Sensor go rich by accelerating once or twice. (The fuel system should normally make the system go rich when you do a sudden acceleration.) Push on the accelerator quickly but don’t let the rpm go high enough to hurt the engine. (If you act like you will hurt the engine you will be asked to leave lab.) The signal should go over 0.85V.

5.1 Freeze your pattern as it goes rich and draw or photograph it onto the graph below: Note the voltage and time per division or scale next to the graph.

In the frame above the voltage goes as high as 0.85 volts. The response time is good as you can see how accelerating the throttle causing a rich mixture has affected the waveform.

6.0 Make this Oxygen Sensor go lean by doing a sudden deceleration. Gently run the rpm up to about 3,000, and let the RPM drop suddenly. The fuel system should make the system go lean on deceleration. The signal should go below 0.2V.

6.1 Freeze your pattern as it goes rich and draw or photograph it onto the graph below: Note the voltage and time per division or scale next to the graph.

The voltage goes low as 0.131 volts. This is a good a voltage reading as the signal should go below 0.2 volts. A lean mixture is caused when there is a lot of oxygen left over in the exhaust gases. A lean mixture tends to produce NOX pollutants and a rich mixture produces HC pollutants. If a vehicle was running lean it doesn’t mean that the oxygen sensor is the main problem. Parts like a faulty EGR valve can cause the air fuel ratio to be lean.

WS5 SCAN TOOL DIAGNOSTICS

The vehicle I worked on is a 1987 Toyota sprinter. It has an OBD 1 connection under the hood. The scan tool turns on when you plug it in to the diagnostic connector with the key ON. This provides power to the scan tool. With the scan tool on I then started the engine to read the data.

 Above is a picture of an 0BD 1 connection under the steering coloumn of a nissan pulsar 1996.

Here is a list of information the vehicle is showing on the screen:

Type of information (PID = Parameter Identification)	Letters to describe it E.g. TPS	Value of data	Units for data E.g. volts
Engine Load (how much air comes in)	Intake	29	kpa
Engine RPM	RPM	800	RPM
Throttle angle	TPS	0.8	v
Engine coolant temperature	ECT	85	degree
Intake air temperature	N/A	N/A	N/A
Fuel Injection opening pulse	FIOP	2	ms
Transmission select position	N/A	N/A	N/A
Vehicle Speed	N/A	0	km/h
Oxygen sensor(s)	N/A	LEAN RICH	N/A
Fuel Trim	N/A	N/A	N/A
Idle control	ISC	39	%
Power steering condition	N/A	N/A	N/A
Air conditioning condition	A/C Signal	off	switch on/off
Exhaust Gas Recirculation (EGR)	N/A	N/A	N/A
Fuel Evap or Purge condition	N/A	N/A	N/A
Malfunction Indicator Light (MIL)	N/A	N/A	N/A
Barometric Pressure	N/A	N/A	N/A

I then checked to see if the system holds any fault codes. It has no codes detected.

The tutor then created a fault without me looking so that I could go through the scan tool and try to diagnose the fault my self.

The fault codes that are now in the system are code numbers 7 and 22. Systems affected are throttle position sensor (7) and water temperature circuit (22).

Before doing anything to fix these faults I had a look at the PID’S (Parameter identification of system voltages) to see how the faults affected the readings that I got at the beginning. The readings that don’t make sense are:

Type of information (PID = Parameter Identification)	Letters to describe it	Value of data	Units for data
Throttle position sensor	tps	0	volts

I then located the fault to be a disconnected tps and ect sensor. While I was accelerating the throttle the tps voltage didn’t change. At closed throttle it remained zero and at WOT it remained zero.

To repair the fault I re connected the plugs to their sensors. I then played around with the throttle to see if the voltage would change, and it did. At idle the voltage was 0.8 volts and as I started to open the throttle up more the voltage started increasing.

I then cleared the codes using the scan tool function, and then checked if the codes were still in the system. The codes are now cleared.



Using live data when fault finding, allows you to look at all the values of the sensors on one screen which makes it easier and faster for diagnosing faults. This is because I can compare readings of one component to the other. I don’t have to go around with my multi meter trying to look for sensors that are hard to reach and impossible to get to.

BLOG 4 TTEC 4848

Anti Lock Braking Systems (ABS)

ABS is fitted to vehicles to prevent the wheels from locking during heavy braking conditions. This will generally reduce the stopping distance and provide safer driving.

Overview of ABS

ABS controls the hydraulic pressure of all 4 wheels during sudden braking and when braking on slippery or uneven road surfaces.

Without ABS, when the brakes are suddenly applied, the wheels can lock and the tyre will skid. This causes the vehicle to veer to the side where the tyre has the greatest friction with the road surface.

With ABS working, the wheels are prevented from locking during braking. The vehicle stays straight, and the stopping distance is reduced. It prevents the brakes from locking and the wheels from skidding by modulating the hydraulic pressure in the brake system. It can hold the brake pressure, decrease the brake pressure or restore it to provide the most effective braking for the road conditions.

http://www.youtube.com/watch?v=ngKSirE7zJA

Main system components

antilock-brake-diagram.jpg

The location of the main parts of an anti lock braking system are shown in the picture above. The master cylinder assembly and the disc brake assemblies at the wheels are normal brake-system components. The ABS parts of the system are the:

1.     Hydraulic control unit

2.     Electronic control unit (ABS computer)

3.     Speed sensors at the wheels

4.     Electrical circuit

Operation

When the brakes are applied the sensors supply wheel speed information to the ABS computer, the computer then operates the hydraulic unit and the hydraulic unit adjusts the pressure at the brakes to prevent the wheels from skidding.

Hydraulic control unit:

The hydraulic unit is used to control brake pressure. It has eight solenoid valves – two for each hydraulic circuit of 4-channel system.

For all normal braking, when the ABS is not operating, pressure from the master cylinder reaches the hydraulic control unit and passes on through the brake lines to the wheels cylinders.

When the ABS comes into operation, fluid pressure from the master cylinder is controlled by closing and opening the solenoid valves at the appropriate times, so that it provides maximum braking, but at the same time prevents skidding.

ABS Channels

Channels refer to the lines, or circuits, to the wheels.

Three-channels:

Vehicles with a rigid rear axle have a three-channel system while those with independent rear suspension have a four-channel system.

A three-channel system has a hydraulic line and a sensor at each wheel. The hydraulic control unit has a pair of solenoid valves for each front wheel, but only one pair of solenoid valves for both rear wheels.

With a rigid rear axle-axle, each rear wheel has its own sensor, but there is only one channel to serve both wheels. So, if the pressure at one wheel has to be altered, the pressure at both wheels will be altered.

Four-channel:

With independent suspension at both the front and the rear of the vehicle, a four-channel system can be used. Each wheel has its own sensor and its hydraulic line. The hydraulic control unit has separate solenoids and valves for each channel; therefore each wheel can be controlled independently. There are two solenoid-operated valves for each channel.

Where a vehicle has independent suspension at both front and rear, the pressure at each of the wheels is adjusted separately.

http://www.drivingfast.net/images/technology/abs/overview.png

The above picture shows a four-channel system.

Speed sensors at the wheels

There are three main types of wheels sensors on modern vehicles. One sends an analogue signal using an inductive pick up; the others send a digital signal using either Hall Effect or magneto resistance encoder.

http://www.madehow.com/images/hpm_0000_0002_0_img0013.jpg

The picture above shows a magnetic pick up sensor and tooth rings that are used with an independent rear suspension. The pulse rings are mounted to the differential housing and the pulse rings are fitted to the drive shafts. Independent rear suspension enables a four-channel system to be used.

The wheel hub or drive shaft has a pulse ring (toothed rotor) that rotates with the wheel and a sensor that is mounted close to it. As each tooth of the pulse ring passes under the sensor, a small voltage pulse is induced in the sensor. The pulses are sent as input signals to the electronic control unit.

The frequency of the pulses is related to road speed, and the control unit uses these signals to determine the rate of deceleration which could produce skidding. If the brake is about to lock, the pressure is relieved for a moment and skidding is prevented.

Electronic system

The electronic control system has sensors at the wheels and an Electronic control unit in the engine compartment. The wheel sensors send speed signals to the control unit, which monitors them and decides when wheel lock is about to occur.

Before the wheel locks the electronic control unit operates a solenoid in the hydraulic control unit. The solenoid valve reduces the hydraulic pressure to the brake of that particular wheel and so prevents the brake from locking and the wheel from skidding. When the possibility of skidding has been overcome, the solenoid is again operated to restore normal brake pressure at the wheel.

Operating modes

The hydraulic system has three operating modes. In operation the, the pressure is quickly changed from one mode to the other, as required, to obtain the maximum braking effect without locking the wheels, the fluid pressure modes are:

1.     non- ABS braking pressure

2.     holding pressure

3.     reducing pressure

4.     Increasing (restoring) pressure.

The inlet and outlet valves control the fluid in the hydraulic control unit. They are solenoid valves that can be opened or closed by energizing their solenoids. This is done by an electric signal from the electronic control unit. The inlet valves are normally opened, but are closed when their solenoids are energized. The outlet valves are normally closed, but are open when their solenoids are energized. Each valve can be operated independently 

By signals from the electronic control unit

The inlet valves control pressure fluid to the brakes, and the outlet valves control pressure fluid from the brakes.

Non – ABS braking pressure

During normal braking the solenoids are not energized so the pressure holding valve remains open and the pressure reduction valve remains closed.

http://www.autoshop101.com/forms/brake09.pdf

When the brakes are being used normally and there is no ABS action required, the system acts as follows:

1.     The inlet valves are held open and the outlet valves are held closed.

2.     When the brakes are pressed, fluid flows through the open inlet valve to the wheel cylinders to apply the brakes in the normal way.

3.     When the brake pedal is released, fluid flows back through the inlet valve to release the brakes.

4.     Braking occurs in the usual way, with pressure in the system dependent on the force applied to brake pedal by driver.

Holding pressure


The pressure reduction valve closes, preventing hydraulic fluid from going to the reservoir

http://www.autoshop101.com/forms/brake09.pdf

When the electronic control unit detects the sudden deceleration of a wheel, indicating that the wheel is about to skid, it signals the hydraulic control unit to hold the pressure at the brake.

1.     The outlet valve is already closed and the signal from the electronic unit closes the inlet valve.

2.     Fluid under pressure is then held in the part of the circuit between the inlet valve, the wheel cylinder and the outlet valve.

3.     This pressure will remain constant regardless of any change in pressure at the master cylinder as long as the outlet valve remains closed.

Reducing pressure

http://www.autoshop101.com/forms/brake09.pdf

When the slip ratio of any wheel exceeds 30%, the ABS ECU energizes both the holding valve and the reduction valve.

If the electronic control unit detects that the pressure at a wheel needs to be reduced to prevent skidding:

1.     The outlet valve is opened by a signal from the electronic control unit.

2.     Fluid flows through the outlet valve to reduce pressure at the brake.

3.     The fluid flows to the accumulator and to the pump.

4.     The accumulator holds fluid under pressure and the pump sends fluid back to the master cylinder.

Increasing pressure

http://www.autoshop101.com/forms/brake09.pdf

The ECU turns OFF both the Pressure Reduction Valve and the Pressure Holding Valve.

After the pressure at the wheel has been reduced and the wheel is no longer about to skid, the pressure can be increased:

1.     The outlet valve is closed so that fluid cannot pass to the accumulator and pump.

2.     The inlet valve is opened so that pressure fluid from the master cylinder circuit can again reach the wheel cylinder.

3.     the braking effort is restored

The cycle of reducing and increasing the pressure at the brakes to prevent skidding will continue as long as the brake pedal is held depressed.

Pressure can be decreased and increased in the braking circuit by ABS action, but it cannot be increased above master cylinder pressure.

http://www.youtube.com/watch?v=d2tp3YXbb0c&feature=related

Accumulator and pump operation

The accumulator is basically a spring loaded piston in a cylinder. It comes into operation during the pressure reduction mode. Fluid from the outlet valve is temporarily stored in the accumulator. The pump starts to operate at this time and fluid is returned to the master cylinder.

By providing somewhere for the fluid to flow, the accumulator causes a quick reduction of pressure at the brakes. The fluid cannot flow directly back to the master cylinder because the master cylinder pressure is higher than the brake pressure at this time.

The master cylinder pressure is blocked from entering the pump by the pump outlet valve, but when the pump comes into operation, it causes a fluid pressure that is little higher than master cylinder pressure. This enables the pumped fluid to pass through the pump outlet valve and be returned to the master cylinder.

Fault diagnosis

Possible ABS faults can be separated into hydraulic faults, electrical faults and electronic faults. ABS hydraulic faults are the same as those likely to be encountered in a system without ABS. These include faults in the master cylinder, wheel cylinders, hoses, leaks and so on. Electrical faults could be related to cables and connections and might be found by a close inspection.