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CAN BUS system

2.1 CAN BUS system
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Quote:A serial BUS that was developed in 1983 by Robert Bosch GmbH and was officially
released in 1986 at the Society of Automobile Engineers (SAE) congress in Detroit
Michigan for in-vehicle network in cars. CAN BUS employ twisted wires to eliminate
radio frequency interference (RFI) and electromagnetic interference (EMI) from entering
the system. The twisted wire is terminated at each end with 120 Ohm resistor called a Termination Resistor. The total resistance of the two terminating resistors connected in a
twisted pair BUS sums up to 60 Ohms since the 2-resistors are connected in parallel. If
one end of a twisted pair wire is open a 120 Ohm will be measured across pin 6 and 14 of
the data link connector (DLC) with the ignition switch turned off and negative battery
terminal being disconnected. If both wires are open an OL will be indicated on a DMM.
Such CAN BUS is not reliable for data transmission and in many cases it might fail to
CAN is the fastest network and its twisted wire can transmit at speeds up to one million
bits per second.
CAN network allow different modules to share common sensor data like vehicle speed,
outside air temperature, coolant temperature and density of air. Such information is
essential for fuel trim and transmission shifting. Examples of modules that were
programmed to use same vehicle speed signal are powertrain control module (PCM),
cruise control module (CCM), anti-lock brake control module (ABS)and driver ’ s door
module (DDM)
CAN system was also designed to function in the harsh automotive / truck environment.
The CAN BUS is one of the 5-protocols used in the on-board diagnostics or OBD2
diagnostic standard. The OBD2 has been maintained for all cars and light truck sold in the
USA since 1986.

2.1.1 Why 120 Ohms terminating resistor?
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Quote:The 120Ω came up as a result of considering the Transmission Line Theory of Physics
This theory takes into consideration the length of BUS wires
The length is determined in terms of wavelengths
The terminating resistors prevent signal reflections causing interference
All devices in the network have to conform to the BUS impedance
When CAN BUS is at 60Ω (meaning two 120 Ω in parallel as shown in Fig 2.1.1) the
BUS can absorb all energy for maximum efficiency of the system

2.2.1 Classes of CAN Network
Quote:Class A = one wire low speed data, less than 10 Kbs, generally used for trip computers
and entertainment.
Class B = two wire mid speed data, 10-125Kbs, generally used for information transfer
among modules such as temperature sensor data.
Class C = twisted 2-wire high speed for PCM, ECM, Airbags, Antilock brakes. Class is
basically 100 times faster than Class B. Class 6 & 14 of the DLC

Class D = is at speeds of up to 1.0 Mbit / second and appears on some late model cars.

When CAN communication is possible or communicates with the scan tool it implies no
wiring problems. The technician’s job dealing with CAN BUS network is to ensure that:
The CAN BUS network is in good condition. The battery has proper B+ voltage 12.6V
No loose connection exists on a wire harness
All the control units are operating properly
Good ground on B- (GND) pin 4 & 5 of DLC
Proper B+ voltage on pin 16 of DLC
If all the above conditions are met then the CAN BUS system can be relied on such that
trouble shooting and repair won’t be a big deal. I will explain that in the proceeding
chapters. Problems that normally occur in the BUS communication are opens, shorts, and
unwanted resistance. Diagnosing and troubleshooting these problems requires a DMM
and an analogue meter. I will explain the diagnosis and troubleshooting procedures in the
accompanying chapters

2.2.2 Summary of CAN features

Quote:Faster than other BUS communication protocols, with maximum speed of 1000 000 bps.
Cost effective because it eliminates redundant wiring and it is an easier system to use as
compared to others
Less affected by EMI and RF due to twisted wires
Message based rather than address based which makes it easier to expand
No wake up needed because it is a 2- wire system
Supports up to 15 modules including a scan tool
Uses a 120 Ω terminating resistor at the ends of each pair to reduce electrical noise
Applies 2.5V on both wires:
A CAN_H (High) signal exists from 2.5V to a high approximately 3.5V when active.

A CAN_L (Low) signal exists from 2.5 V to a low approximately 1.5V when active
Possibility of assigning priority to messages and guaranteed maximum latency times[1]
Detection of possible permanent failures of nodes and automatic switching off of defective

2.3 Voltage levels of CAN BUS Digital Signals

Quote:CAN uses a differential type of module communication where the voltage of one wire is
equal but opposite to voltage of another wire. When no communication occurs both wires
have a voltage of 2.5V applied. So 2.5V is a reference level. If there is no data being sent
the voltage at CAN_H (High) and CAN_L (Low) will be 2.5V if the DMM is connected to
the chassis GND which happens to be pin 4.
The voltage difference between the peak of CAN_H (+) and the peak of CAN_L (-) ≈
2Volts in amplitude.
Peak CAN_H (+) - Peak CAN_L(-) ≈ 2Volts
When communication is occurring, CAN_H (High) goes by 1Volt up to 3.5 Volts and
CAN_L (Low) goes down 1 Volt to 1.5 Volt.

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2.3.1 CAN Electron Current
Quote:Current carrying wire has magnetic field around it, such fields induces voltage but in
opposite direction according to Lenz’s Law of electromagnetism. This induced voltage
termed voltage spike can destroy electronic components that are fundamental to an
automotive system and therefore twisted pair cable is a good idea to curb such problems.
CAN_H and CAN_L wires are twisted pair and therefore the electron signal current will
flow in different directions and that signal is in a state of balanced. This makes the CAN
BUS immune to electrical noise since any electrical noise and induced voltage present in
one wire exists in the other but in opposite directions and therefore cancellation effect will
occur. This will minimise signal loss and increase signal conductivity of the lower level
of the CAN signal.

2.3.3 Measuring the Resistance of CAN BUS network

Quote:The rule of thumb says measure the resistance when no current is flowing. Measuring the
resistance of the CAN network is done on pin 6 & 14 of the DLC. Extreme caution should
be exercised whenever dealing with the DLC since the pins on DLC are very delicate and
vulnerable. The following steps to be taken when measuring the resistance of the CAN
Before measuring the CAN network resistance disconnect B- of the battery first.
This is because some voltage will still be present on pin 6 & 14 even at Key Off.
The B- is disconnected first because electric current flows from negative to
Wait for maximum of 10 minutes before probing pin 6 & 14 with a DMM to ensure
that voltage is negligible.
Measure the resistance on pin 6 & 14 after you are sure that voltage does not exist
on those pins
The DMM test leads should be on pin 6 & 14, red test lead on CAN_H (High) and
black test lead on CAN_L (Low)

2.3.4 CAN BUS network resistance values interpretation

Quote:If both wires are open an “ OL ” will be indicated on a DMM
If one end of a twisted pair wire is open a 120Ω will be measured across pin 6 and
14 of the data link connector (DLC), this shows that the two terminating resistors are no
o longer connected in parallel.
A 0.0 Ω or very low resistance far much less than 60 Ω implies that the twisted
wires are shorted together
The data bus will remain operational when one of the two modules containing a
terminating resistor is not connected to the network. However the data bus will fail
when both terminating resistors are not connected to the network.
Do a thorough visual inspection and look for network wire insulation damage, repair or
even replace the wire if possible, look for loose connections as well. Repair the damaged
resistors if possible. A reading of 60 Ω implies a perfect connection between different
control units and the CAN BUS.

2.3.5 Measuring Voltage of CAN BUS network

Quote:The rule of thumb says measure voltage when current is flowing in the circuit. This is
done to monitor communications and to check the CAN BUS for proper operation. The
following steps are to be taken when measuring voltage of the CAN network;
Check the battery voltage, the battery voltage should be at least 75% state of charge
before Measuring Voltage of CAN BUS network
Use a DMM set to DC volts
Pin 4/5, 6 & 14 to be used for voltage measurements
0.0 Volts indicates short-to-ground, check for the short by disconnecting one
module at a time until a module causing problems is found.
When no communication occurs both wires have a voltage of 2.5V applied and this
phenomenon is called recessive state.
Signal that is 12 Volts all the time indicates short-to-voltage, the BUS circuit could
be shorted to 12 Volts. Check the repair history of the car with the customer before
doing anything, and then start unplugging one module at a time until a problem is
Variable voltage indicates a normal operation of the CAN BUS; this usually
indicates that messages are being transmit sent (Tx) and received (Rx) by a
transceiver circuit embedded in different modules

Battery Voltage State of charge

12.66 100%
12.45 75%

12.24 50%

12.06 25%
11.89 0%

Summary of network communication diagnosis
Check the battery voltage, the battery voltage should be at least 75% state of charge before
doing any electrical diagnosis.
Do a thorough visual inspection to check accessories that do not function properly as this
will help to identify a module or a BUS circuit at fault. A power window that does not
work properly can be due to a BCM problem. A miscommunication between an engine
and transmission can be attributed to a TCM in some cars. I remember fixing a Nissan
extera 2001 model that had such problem, it was cranking with no start condition.
Use a scan tool usually a factory scan tool to perform module status test. Check if the
components/ systems/ actuators can be controlled by the scan tool
Check CAN BUS network for voltages
Disconnect the PCM terminals and check for corrosion, spray anti- corrosion solvent to
remove rust on PCM pins, W40 is a good solvent to achieve that. Corrosion causes PCM
intermittent malfunction.
Check for loose terminals in a connector
Check for loose ground connections
Check owner’s manual for procedures and steps to fix BUS network problems

2.4 No network communication diagnosis

Quote:Check the battery voltage, the battery voltage should be at least 75% state of charge
before doing any electrical diagnosis

Check pin 16 of the DLC for the presence of B+ voltage. At least 10Volts should be
available at pin 16 during cranking when the battery is good. If no voltage is found
on pin 16 check the (1) B+ cable to the PCM if properly connected, if properly
connected check for (2) a damaged PCM fuse or circuit breaker, PCM fuses are
normally damaged when the B+ polarity is reversed. The B+ pin which is fused
maintains the PCM’s volatile memories such as KAM and DTC’s memories when
the ignition is turned off. If (1) & (2) is valid check for any loose connections in the
wire harness that might cause a no communication. If the above mentioned steps
are valid the following procedures listed below may be done:
Be sure that your scan tool is not a problem; you may as well test it on a different
Test for 5 volt reference signal from any accessible sensors like MAF, TPS and
MAP. The 5 volt reference originates from the PCM. If the 5 volt reference signal
is present then the PCM is partially operational, and if no 5 volt reference exists
then one of the sensors is shorted to ground, implying that it pulls all the 5 volt
reference to the ground. The best procedure to deal with that problem will be to
disconnect one sensor after the other until the 5volt reference appears on line. This
could be very monotonous but we have to do it.
In some cases check the PCM relay
Check the PCM for any external damages, PCM can be damaged by overcharging
alternator, alternator with shorted rotor excessive moisture or any catastrophic
Use an oscilloscope to check for voltage measurements on testing points, in this
exercise you have to open the PCM before the testing and then afterwards run the
testing whilst the ignition is running.
Consult a PCM connector diagram to determine the configuration of the connector

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2.6 OBD2 Data Link Connector (DLC)

Quote:DLC or Data Link Connector is an OBDII standardized and has got 16 pin configurations.
It has been used in all automobiles manufactured since 1996. DLC design and location
depends on the manufacturer however most vehicles have DLC located under the dash.
Some pins function is standard like pin (4,5, 6,14 & 16 ) and required by all
manufacturers, while others like (1,8 & 12) are left to the individual manufacturer’s
discretion. DLC is very delicate and vulnerable and extra care and caution is required
whenever dealing with it.
DLC has pins 1-8 running on the upper segment of DLC and 9-16 running on the lower
segment of the DLC as shown below:

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2.6.1 The DLC allows you to:

Quote:Turn ON / OFF actuators using a bidirectional scan tool
Test Signal & Chassis ground
Testing of computer network system, protocol communication & signalling
Provides power to the Scan tool
In brief it is a diagnostic tool
2 BUS +ve line (square wave signal)
3 OEM reserved
4 Chassis ground
5 Signal / Sensor ground
6 CAN_ H
7 K Line ISO 9141 Protocol
8 OEM reserved
10 BUS –ve
11, 12 & 13 OEM reserved
14 CAN_L
15 L Line ISO 9141 Protocols
16 B+ voltage

2.6.2 Accessing OBD2 Data Link Connector (DLC) pins

Quote:Breakoutbox (BOB) offers easy access to DLC pins for diagnostic measurements. The

BOB is a true reflection of the DLC. The BOB protects the possible damage of DLC if

probed with DMM test leads. BOB has 16 pins that match the 16 pins on the DLC. Pin

2,4,5,6,7,10 & 16 have LEDs that light up. LEDs are used to indicate status of power,

grounds and communication activity on each of the pins listed above of the DLC.

Pins 2, 6, 7, 10 light up to identify vehicle data OBD-II protocol and activity.

A CAN_ H & CAN_L voltage appears on pin 6 & 14 of DLC respectively. Pin 16 has B+

voltage at Key ON and charging voltage at Key ON Engine ON. The intensity of

brightness of LED on pin 16 increases during charging as compared to Key ON. The

diagram below illustrates the BOB with the 16 pins configurations and two connectors.
One connector will be connected to the DLC and the other to the scan tool.

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#voltage #bus #dlc #network #pin #check #pcm #wire #signal #quote

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