Multiplex System

This particular multiplex system was used for body controlled functions on the 1996 to 2002 honda accords and 1996 onward honda preludes. It utilises three control units. The drivers door node, drivers side node and passengers side node.

This system works similar to CAN systems where the nodes communicate between each other in order to actuate the specified component.




Multiplex system: note the two white nodes are passengers side node and drivers side node. The drivers door node is at the bottom containing all the input switches


The testing procedure is divided in to three sections. Communication lines and nodes (mode 1), input (mode 2) and if the system passes these tests but is still faulty, the problem is most likely to be an output.

Using the wiring diagram that was provided I have identified the communication lines as; MCU(door) terminal A15 to MCU(drivers side) terminal A2 is a brown wire, MCU(drivers side) terminal B1 to MCU(passengers side) terminal B9 is a pink wire.

I have identified the earth and voltage supply lines as follows;

From the under-hood fuse/relay box (No. 54) up the yellow wireto the passengers under-dash fuse relay box (No.13). Pink wires connect this to the MCU(door) at terminal A1, the MCU(drivers side) terminal A12 and MCU(passengers side) terminal A24. They are then each earthed out black wires through MCU(door) terminals A12 and A19, MCU(drivers side) terminals B11 and A14, MCU(passengers side) terminals B22 and A8.

After a fault had been created via simulated fault switches, diagnosis went ahead;

All the windows work as normal. Drivers side windows operate normally, yet the drivers side passenger interior door light switch is not working/illuminating. The connection from the MCU(drivers side) terminal A22 through the drivers side passenger door light switch to earth is posibly shorting before the switch. The switch is normally open (when door is closed), yet it is behaving as though the switch is closed (thinks the door is open).

When testing further the system is put into mode 1. No codes are given. So communication lines and nodes are fine. They are communicating between each other, this is because the short/fault is occuring after the module. I was expecting this.

Voltage is tested to ensure proper communication is happening. MCU(passengers) at terminal B9 = 7.59v and MCU(drivers side) at terminal B1 = 7.59v. MCU(drivers side) at terminal A2 = 7.32v and MCU(door) at terminal A15 = 7.32v. This concludes that all communication is good.

Further testing brings mode 2 into play. And as presumed the fault is isolated as 'drivers rear door light switch'. So the final conclusion is that the fault is preventing signal from the input door switch to the drivers side module. Because the drivers door light switch is operating normally, this means the output (light) is not the problem.

The MCU(drivers side) terminal A22 green/yellow wire shall be tested and replaced accordingly.

Sleep mode on this system allows it to stay live for a few moments after the car has been turned off completely.

Anti-lock Brake System

Construction

The anti-lock braking system utilises exsisting components of a vehicle. Such as the brake calipers and wheel cylinders, brake rotors, brake lines, brake booster and master cylinder. It also however incorporates a few components of its own. Such as an electronic control module (ECM), wheel speed sensors, intergraded teeth on the rotors and/or axles and hydraulic control unit and motor pump (HCU).

ABS works very quickly in order to prevent the wheels from slipping. Yet still having enough pressure at the brakes to ensure stopping of the car when intended by the driver. So within milliseconds the system will release brake pressure when a brake is locked up, to prevent slipping of the tyre on the road. Then without delay, increase the pressure right back up to ensure stopping power. It will do this continuously until no wheels are locked up.

Under normal braking however, the ABS does not interfer at all. The only time it takes over is under heavy braking and any wheel becomes locked up and the driver is no longer in full control. It ensures that the vehicle maintains steerability (you can still steer) and stability (you won't spin out). It will also ensure that your vehicle has enough control through the wheels to manouvre around obstacles.

The three common ABS setups are single channel which the rear wheels are on the same channel (strictly stops the rear wheels only). 3 channel where again the rear wheels are on the same channel but the front wheels get their own individual channels. And 4 channel where each wheel has its own individual channel.

Inside the hydraulic control unit there are solenoids which are the muscle of the system. They work in pairs for each channel. They have three positions; normal braking position and pressure increasing position where the inlet valve from the master cylinder is open and the outlet valve to the reservoir is closed. Pressure holding position where both the inlet and the outlet valve are closed, so pressure will not increase nor will it decrease. And pressure releasing where the inlet valve is closed so pressure from the master cylinder will not increase and the outlet valve to the reservoir is open in order to release pressure to somewhere remote.

http://www.youtube.com/watch?v=Vy0p3OnF_18

This link shows the ABS operating with a scan tool displaying the variety of signals that are being processed. As you can see the wheel speed sensors take in all the information rather quickly and communicate it to the ABS ECM. Also as the brakes are applied the brake light swtich is active (on). It shows that it understands the engine is running as well as reading the throttle position. The ABS actuator will obveasly be on too as the ABS warning light would be alluminated if any so called problems were to occur.

Above is the ABS relay being signalled against the wheel speed sensor signalling that it is 'locked up'.

Actuators

Injectors


The injectors are one of, if not the most important part of an internal combustion engine. It is basically a switching electromagnet that controls a tapered valve seat. Opening and closing in milliseconds according to a duty cycle signal sent by the ECU, allowing high pressurised amounts of fuel to be precisely injected into the manifold or combustion chamber.


As the throttle is opened, the ECU tells the injectors to stay open for longer and close for shorter periods at a time. It is usually explained as a percentage, where at wide open throttle the injector may be open around 80 to 90% and closed 10 to 20% of each cycle. At idle the injectors may only open 10 to 20% and closed the other 80 to 90% of the time in each cycle.


The ECU collects alot of data feedback from the engines sensors and the injection cycle is determined by this various, ever changing information. That is why it is important to have sensors that are operating effectively.




An Injector Pattern



Here is an example of an injector operating correctly. And as you can see the voltage spikes high, quickly (about 60v) and then slowly returns to a normal voltage. This happens within about 4 milliseconds.


Injectors are most accurately tested off the vehicle. They are benched tested in order to visually examine the spray pattern, delivery volume or leakage (dribbling). Sometimes even possible seal damage.


Ignition System


Ignition systems are another key component in an internal combustion engine. They create a spark at the desired time the ECU signals and intends combustion of the air/fuel mixture.


Components often include a power source, an ignition switch, coil or coil pack, an igniter module, a trigger generating component/distributor, distributor cap, rotor, HT leads and spark plugs. A newer, more commonly used setup will contain a direct ignition coil. This does away with most of the components previously used such as HT leads, distributor/distributor cap, rotor and in some cases the igniter module as well.

A Basic Ignition System

Testing Ignition Coils


Older ignition coils were very basic where they had two sets of windings.The secondary winding is constructed of thin wire wound tightly around the iron core, located in the center of the coil. The primary winding is constructed of thick wire and is wound around the outside of the secondary windings.

A Single Coil Cross-section



Low voltage is coming through the positive terminal attached to the primary windings and is earthed out the negative terminal on the coil housing. When switching occurs in the primary triggering circuit and over the primary winding, the voltage built up in secondary windings collapses and creates a giant energy surge over the spark plug gap.

Testing coils using an ohmmeter off the vehicle is a reliable way of retrieving knowledge of their condition.

Testing the Primary Winding

When testing primary windings the ohmmeter must be connected between the positive and negative terminals on the coil housing.

Testing the Secondary Winding

When testing the secondary windings, the ohmmeter must be connected to both the positive terminal on the coil housing and the negative side of the secondary windings. Being where the HT lead conducts the power to the distributor cap.

The readings against the specifications as follows (two separate coils are being tested):

Spec: Coil #1  No.CIT-118  Voltage - 12v 

 Primary - 1.0-1.3ohm Secondary - 8.5-9.5k/ohm

Spec: Coil #2  No.CIC-31  Voltage - 12v 

 Primary - 3-4ohm  Secondary - 7-8k/ohm

Results are:

Coil #1  Primary - 2ohm  Secondary - 9.38k/ohm

Coil #2  Primary - 3.9ohm  Secondary - 8.46k/ohm

Resistance through the primary windings is a little high, but other than that these coils will perform as normal.

Wasted spark type coil packs can be tested using a multimeter also as follows (one individual wasted spark type coil pack is being tested)(four cylinder type):

Coil #1  Primary - 1.1ohm  Secondary - 6.95k/ohm

Coil #2  Primary - 1.1ohm  Secondary - 7.13k/ohm

Ballast resistors (used more commonly with 9v coils) can also be tested, as follows:

Spec: Ballast #1  No.BR1  0.9 - 1.1ohm

Spec: Ballast #2  No.BR3  1.5 - 1.7ohm

Results are:

Ballast #1  No.BR1  2.2ohm

Ballast #2  No.BR3  2.4ohm

Results are a bit high, this wont cause damage to any other components. Although it may affect the performance of the ignition system.

Testing the Current and Voltage Drops

Current can be tested in a circuit. Here we test a coil and a ballast resistor in series. As you can see the multimeter needs to be in the circuit. So the multimeter is connected between the positive terminal at the power source and the entry terminal of the ballast resistor.

We can now collect the current value of the circuit. Then disconnect the multimeter and collect voltage drop values over the coils primary windings and the ballast resistor. As follows:

Current in the circuit: 1.9A

Coil (CIC-31) calculated Voltage drop: A x R = V

 = 1.9A x 3.9ohm

 = 7.41v

Coil measured Voltage drop: 7.59v

Ballast (BR3) calculated Voltage drop: A x R = V

 = 1.9A x 2.4ohm

 = 4.56

Ballast measured Voltage drop: 3.55v

Types of Ignition Systems

Basic Single Coil Schematic

Here we have the original, more basic type of ignition system. It consists of the typical components found in a basic ignition system. A 12v power supply, a 12v coil (no ballast resistor in necessary), an igniter module, an HT lead and a spark plug. A distributor is not used, as a function generator which takes its place here can produce alot more variety of signals. It can also change signal speed alot more effectively. This makes it alot more accurate in this demonstration than any distributor could be.

This link shows the above single coil circuit in action. It also has an oscilloscope hooked up showing the triggering pattern in the primary circuit:

http://www.youtube.com/watch?v=XTsG_1EjmtU&feature=related

Wasted Spark System Schematic





Wasted Spark wired up



In this picture above, we have a wasted spark type ignition system. This is a good upgrade from the single coil ignition system. The coils in this system don't have to work as hard as a single coil on its own. This also allows the secondary windings dwell time to stay at an effective time right throughout the increasing engine rpm.

Links involving the wasted spark show the system in action:

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

http://www.youtube.com/watch?v=eVHW-XKapdM

Each coil is only igniting two spark plugs. They are in fact ignited simultaneously. One spark plug is ignited on the combustion stroke as normal. But the opposing spark plug is ignited at the opposing stroke which is effectively, the exhaust stroke. In a way this is good because it will ignite any unburnt fuel leaving the combustion chamber and in turn slightly reduce emissions.

The Smart Direct Ignition Individual Coil System





Direct Ignition System wired up

 And of course the most recent and commonly used now is this system. Its the individual coil direct ignition type system. This inteligent design compacts all the traditional components up into one small attachment that sits on top of each spark plug. It reduces production costs by removing the need for a distributor/distributor cap, HT leads and in some cases the igniter module. It also, like the wasted spark system enables dwell time to stay at the most effective time throughout because each coil is only operating one spark plug.

http://www.youtube.com/watch?v=w_O_t5Wow0I

Sensors

Throttle Position Sensor (TPS)


The type of TPS sensor we use here is called a Linear throttle position sensor or Pontentiometer type sensor. Potentiometer type sensors hold a variable resistor with a slide contact inside of them. As you push in the accelerator and the throttle butterfly moves, the contact changes its position on the variable resistor. As the throttle position changes the output voltage from the sensor changes. As the throttle opens (throttle position angle increases) the resistance is decreased, thus increasing the output voltage from the sensor:


15` - voltage is at 1.4v
30` - 2.1v
45` - 2.6v
60` - 3.1v
75` - 3.6v
90` - 4.7v


Idle puts out a low voltage (the least voltage) and this is how it is recognised. Usually about 0.4 - 100mV.




TPS Readings



Throttle Position Switch (TPS)

Throttle Position Switch type sensors are very basic and are rarely used on the modern engine. They detect when the throttle is at full throttle or at idle. This particular throttle switch is a two position sensor and has only three pin outs. Another type is a three position sensor making it a four pin out switch, this gives the driver that extra gradual throttle response for better economy. These are both also adjustable, to ensure optimum operation and also good economy.




TPS Readings



From zero to fifteen degrees, the ohm reading is at 0.5ohm.
 At fifteen degrees, the resistance shoots to an overload reading (O.L.).
 As the angle increases, up to seventyfive degrees resistance stays in overload (O.L.). From seventyfive degrees the resistance is back in range, at 0.5ohm.


AIR FLOW SENSORS (MAF, MAP, THA, AFM, VAF)


The air flow sensor is located at the point of entry of air into the fuel injection system. The air flow sensor sends air volume, pressure and temperature information to the ECU. As intake manifold pressure is directly related to engine load, the ECU needs to know intake manifold pressure in order to calculate how much fuel to inject and when to ignite the air/fuel charge.

 The air flow sensor is an expensive and complex device. Any air leaks between this sensor and the engine will cause problems with your fuel injection system. Failure of this sensor can cause a variety of performance problems including erratic idle, loss of power under load, bad air/fuel mixture, and even complete lack of vehicle operation.


The air filter resides in the housing below the air flow sensor and must be routinely changed for continued quality of operation.


Vane Type Air Flow Meter (VAF)


A Vane type air flow meter measures air pressure/volume and air temperature seperately. Similar to a throttle position sensor in that the Vane type air flow meter uses a slide contact running along a variable resistor to get a variation in voltage outputs as load is applied.


 There are two main types of VAF sensors. The first design, uses battery supply voltage. As the measuring plate opens, voltage is increased. The second design uses a regulated 5v input.


 During Engine operation, intake air flow reacts against the measuring plate and its return spring, and deflects it in proportion to the volume of air flow passing the plate. A compensating plate, found in the dampening chamber acts as a shock absorber to prevent rapid movement or vibration of the slide contact attached.


VAF Sensor Wiring Schematic


Testing this sensor can be done with an ohm meter, testing individual cricuits for variable resistance in response to air vane movement.
 However it is more accurate to test the the voltage output signal on the vehicle. By back probing the multi-plug with the ignition turned on.




VAF Sensor Readings

 

This photo shows the readings when tested off-car but hooked up to a power supply of only 5v and not battery voltage (12v). The graph shows the desired voltage output pattern of:

0' - 0.83v

20' - 2.7v

45' - 3.9v

60' - 4.3v

75' - 4.75v

90' - 5v

Manifold Absolute Pressure (MAP)

The MAP sensor measures the voltage difference between a reference resisted output and a varing output resisted as to the pressure in the intake manifold.

In the Map sensor there is a silicon chip mounted inside a reference chamber. On one side of the chip is a reference pressure, this reference pressure is at a calibrated pressure. On the other side is the pressure to be measured. The silicon chip changers its resistance depending on the changers of pressure, this change in resistance alters the voltage signal. The ECU interprets the voltage signal as pressure and any change in the voltage signal means there is a change in pressure.

High vacuum is low pressure, or less air coming in. Low vacuum is high pressure, or more air coming in. The more the throttle is held open, the more air comes in, the higher the MAP reading.





Wiring Schematic of a MAP Sensor


MAP sensors are tested using a mity-vac which applies pressure to the sensor. Hooking up the MAP sensor to a 5v voltage supply and to a mity-vac, the following readings are displayed:

MAP Sensor Readings

0 cmHg - 9.84v

10 cmHg - 4.16v

20 cmHg - 3.44v

30 cmHg - 2.75v

40 cmHg - 2.01v

50 cmHg - 1.31v

60 cmHg - 0.65v

70 cmHg - 0.06v

Air Flow Meter Hot Wire Type (MAF)

MAF sensors are the newest and most commonly used type of air flow measuring sensor. They consist of a thermistor, a platinum hot wire and an electronic control circuit.

The thermistor measures the temperature of the incoming air. The platinum hot wire stays at a constant temperature in relation to the thermistor by the control circuit.

An increase in air flow will cause the platinum hot wire to lose heat faster and the control circuit will compenstate by sending more current through the hot wire. The circuit simaltaneously measures the current flow and puts out voltage signal accordingly.

MAF Sensor Diagram

TEMPERATURE SENSORS (ECT, IAT)

The ECU uses different temperature measurments of the engine to adjust various systems (fuel injection, ignition timing). It is important that proper operating temperature is reached and properly signalled to the ECU, in order for these systems to operate effectively.

Coolant Temperature Sensor (ECT, THW)

The ECT sensor responds to change in the engine coolant temperature and signals the ECU accordingly. This type of sensor is a NTC thermistor which change their resistance depending apon the temperature.

Testing this sensor with an ohmmeter gives us readings as follows:

Coolant Temp Sensor Readings

35'C - 1.53 k/ohm

40'C - 1.46 k/ohm

45'C - 1.02 k/ohm

50'C - 0.89 k/ohm

60'C - 0.70 k/ohm

70'C - 0.522 k/ohm

75'C - 0.415 k/ohm

80'C - 0.345 k/ohm

Intake Air Temperature (IAT, THA)

The IAT sensor responds to change in the intake air temperature and signals the ECU accordingly. Most older vehicles have a seperate IAT sensor in the intake manifold. The mordern, more common systems have the Hot Wire type air flow meter which incorporates the IAT sensor in its internal operations.

The air temperature sensor is much like the coolant temp sensor, in that it is a NTC thermistor. They change their circuits resistance according to the temperature. Also an ohmeter is used to test the operations of the sensor, as follows:



Intake Air Temp Sensor



This graph shows the resistance readings as temperature increases. When coolant temperature is below 40'C the resistance reads over load, meaning no voltage signal will be recieved from the sensor. The graph shows a slight spike in the resistance decrease (voltage increase) curve. Readings as follows:


20'C - O.L.
30'C - O.L.
40'C - 1.2k/ohm
50'C - 0.8k/ohm
60'C - 0.5k/ohm
70'C - 0.41k/ohm
80'C - 0.34k/ohm
90'C - 0.24k/ohm
100'C - 0.22k/ohm


Thermo Fan Switch


The thermo fan switch contains a bi-metal strip that has two different metal types fused together. They both have different expanding rates when the same amount of heat is applied. This means one side of the strip will expand sooner than the other side, causing it to bend in the direction of the side in which the metal does not expand as early.


In light of this, if the metal strip was touching the contact which switched it to ground, it would flex and the contact would disconnect from the strip. Or, if the strip was not touching the contact, the metal strip will flex and come into contact with the earth switching contact. Thermo switchs come as 'normally on' or 'normally off'.

Thermo Fan Switch Readings



As you can see the contact in this thermo switch is 'normally off'. It will read over load up until about 95'C (depending on specs) then it will switch to ground and have a reading of 0.3ohms.