On older vehicles, the alternator and engine were never really connected. The alternator was internally or externally self-regulated to a voltage, but that was it. Under certain conditions, the idle would drop when loads increased.
Under some conditions, this wasn’t ideal for performance and emissions. Also, as electrical loads increased for systems like heated seats, electric power steering and infotainment systems, the need for better power management was required.
Honda’s solution was to use a dedicated local interconnect network (LIN) line that connects the alternator and the powertrain control module (PCM). The LIN line uses a master/slave protocol to send and receive data to the different components on the line.
The alternator is the slave. The alternator receives data from the PCM and provides feedback, which allows the PCM to operate the engine electrical system efficiently and with little change in engine performance. If there are problems with the LIN line, code P16E2 will be set.
The alternator has a continuous monitor that is running when the engine starts and reaches 500 rpm. Once the engine starts, the alternator has three seconds to communicate with the PCM. Also, the voltage can’t drop below 10.0-volts, or a code will be set.
The PCM sets a target generating voltage in the range of 12.5- to 14.5-volts using the LIN line. If the output voltage does not match the commanded voltage for the engine speed, it sets a code that the voltage is too high or too low. DTC P0562 is for the charging system voltage that is too low. DTC P1549 is for the charging system voltage being too high. If there is no voltage coming from the alternator, code P056A will be set for “No Charging Malfunction.” Both of these codes are cleared after one key cycle.
If the sensor detects the alternator is overheating and has exceeded 320 degrees F, it will set code P16E4.
Most late-model Honda alternators have a Overrunning Alternator Decoupler (OAD). This type of pulley also has a one-way overrunning clutch inside the hub, as well as an internal torsion spring to dampen vibrations in the belt drive system further. The spring acts as a shock absorber to cushion the hub. This reduces noise at idle and low engine speeds, and helps dampen harmonic vibrations at higher speeds. If the clutch or spring inside the pulley has failed, the pulley may fail to drive the alternator, or it may create vibrations and noise.
On most 2008 and newer Honda models, there is a battery sensor attached to the negative battery terminal. The sensor measures internal resistance, temperature and current. The sensor sends a signal to the PCM through the gauge control module.
If you don’t have much experience working with CAN bus communications systems, you’re not alone. Although CAN bus was introduced by Mercedes-Benz during the early 1990s and adopted by many European manufacturers, many domestic and Asian auto manufacturers waited until the 2004 model year to introduce CAN bus in their lower-end, bread-and-butter vehicles.
Since the fast transmission speeds of CAN bus systems are needed to operate the new air bag, active braking and body control accessories of today’s “connected” vehicles, CAN was required as standard equipment in 2008. While the 2004 and later model-year CAN vehicles have performed reliably during their first 100,000 miles of service, we’re going to see more bus communications problems occur during their second 100,000 miles of service. Since CAN is very application specific and complex, the sole objective of this column is to introduce you to basic CAN bus architecture, terminology and diagnostics.
THE OBDII DLC
Let’s begin at the 16-pin data link connector (DLC) found under the dash. The DLC pins are arranged in two rows of eight, numbered left to right, with pin #1 located at the top left corner and pin #16 at the bottom right corner. Pin #16 provides a constant B+ power source for your scan tool. Pin #4 normally grounds the scan tool to the chassis, while pin #5 is a common sensor or signal ground. All other pins are application specific, so avoid any testing that might short battery voltage into a bus communications circuit.
If your scan tool isn’t communicating, use a 10-megohm impedance voltmeter grounded to the body to test for B+ at pin #16. If you don’t have B+ at pin #16, use your voltmeter to test both inside and underhood fuses using the vehicle owner’s manual as a guide. If you don’t have a ground at pins #4 or #5, consult a schematic for ground locations. Last, if you’re doing a lot of scan tool diagnosis, it would pay to invest in a DLC breakout box as shown to the right with LED indicators that show pin #16 voltage, pins #4 and #5 ground function and bus communications activity.
MODULES AND NODES
In general, we refer to large modules as “computers” or as PCMs, ECMs, BCMs, TCMs, etc. A control module is a mini-computer that has a logic and memory function built into it. By “logic” I mean the ability to process data, and by “memory” I mean the ability to store basic information and codes. We might also see the term “node,” which can include your scan tool when it’s connected to the DLC and sending and receiving information via the bus network. Nodes can also be built into switches and other small components. Remember that many electronic aftermarket accessories can create communications compatibility problems when connected to the DLC or bus communications system.
Since it’s physically impossible to accommodate the large wiring harnesses required by conventional “hard-wired” systems in a modern chassis, engineers now use control modules to perform routine tasks, such as activating interior and exterior lighting.
In modern electrical architecture, the light switch might signal the light control module to turn the exterior headlamps, marker lights and interior lighting on or off. In the same manner, the dimmer switch might signal the light control module to switch from the high-beam to the low-beam headlamp circuit. A control module needs only battery voltage, a good ground and a bus communications system to operate any specific system.
“Multiplexing” is the method or “topology” of connecting modules in series or parallel configurations. Most early bus systems are referred to as “multiplexed” systems. “Gateway” modules are used to translate the different protocols used in these systems. Terms like “hub,” “master” and “slave” are also used to describe module dominance functions. Since topologies can become quite complex, it’s important to use schematics and service information to understand their relationships within their bus communications circuits.
NETWORKS AND NETWORKING
While terminology varies among manufacturers, a “network” is generally a series of control modules that receives requests and shares information by receiving and transmitting a digital language over one or more bus communication circuits. “Networking” is the process of modules sharing information through different communications protocols or “languages.” Protocols on conventional, non-CAN systems usually switch from zero volts to a specified voltage to create an on/off signal that makes up a communications protocol.
CAN systems differ from earlier multiplexed systems because there are no dominant master/slave relationships among modules. Instead, all modules receive all transmitted messages and each module “decodes” or recovers its own message from a common bus communications network. If a bus system includes a slower protocol, the PCM might, for example, act as a gateway module to translate the two protocols.
In CAN systems, the signals switch from a floating bias voltage. CAN+ rides on a 2.5-volt bias, which means it toggles from 2.5 bias volts to 3.5 volts. CAN- toggles from the 2.5-volt bias to 1.5 volts. CAN+ and CAN- added together equals a reference voltage of 5.0 volts. When both are displayed on a labscope, a CAN- waveform is the upside down, mirror image of the CAN+ waveform.
While a labscope can identify the amplitude of a bus signal, the actual waveform varies widely among applications. While CAN+ is usually found on pin #6 and CAN- is found on pin #14, these pins might serve different functions in different vehicles and with different scan tools. Slower protocols, such as UART, might also be included with the CAN system, but are not necessarily connected to the DLC. And, to further confuse the issue, multiple CAN systems are often identified as “CAN A,”“CAN B,” “CAN C,” etc. Remember that the labeling of different CAN systems varies widely among manufacturers, so consult original equipment (OE) service information for application-specific terminology.
POLLING THE MODULES
Most OE and aftermarket enhanced scan tools automatically poll or ping the modules or perform a “health check” to confirm that all modules are online (see image below). If your scan tool doesn’t automatically communicate with all modules, you must manually poll each module to determine if that module is reporting. If your scan tool indicates “no communication” with a specific module, then you must diagnose the reason for the non-communication.
THE “U” CODE
A bus communication failure is indicated when a module stores a “U” code.The last three digits of a “UXXXX” code indicate which module is experiencing communications problems. I’ve found that U-code diagnostics should be approached with caution because a simple problem like a worn ignition switch can set “U” codes by intermittently reducing voltage to the modules. Bad module grounds can also create intermittent problems. Manufacturers are now using much smaller pin connections, which can easily be damaged when probing the connector. In fact, pushed-out pins in the connectors cause many bus failures. In many cases, the OE manufacturer supplies connector-testing kits that eliminate the need for intrusive diagnostic techniques.
As mentioned previously, a CAN– waveform is an upside down mirror image of a CAN+ waveform. Two 120-ohm resistors, one located at each end of the plus/minus circuits, connect the positive and negative sides of CAN. These resistors are usually embedded in modules. When the CAN network is shut down (ignition off, key removed, no voltage at CAN+ or CAN-), a resistance of 60 ohms should be measured between pins #6 and #14. A 120-ohm measurement would indicate an open in one of the resistors or CAN circuits.
For this month’s Real World case, we will attempt to provide a plan of attack for communication issues on General Motors products with the Class 2 Protocol.
Our diagnostic journey begins with a 2004 Chevrolet Tahoe.
This vehicle was tested at the EPA test facility in our area and it was determined that there is no communication with the PCM.
Our subject vehicle is taken to a local repair facility to be evaluated. The first step there is to confirm the no communication issue.
The technician uses a Tech 2 scan tool to access data and the scan tool is able to communicate with the Tahoe without a problem.
So, he calls the EPA test facility to report that the vehicle does not have a communication problem.
The state responds in a very pleasant manner and advises the tech that the vehicle must communicate with their equipment in order to pass the emissions test.
It’s interesting to note at this time that the state communicates on a generic level.
The tech takes out his generic scan tool and finds that the vehicle does not communicate with it. The problem has been confirmed.
The first step in our diagnosis is to review schematics for the communication lines. Figure 1 denotes the circuitry.
The PCM is denoted in Figure 2.
Now we have a complete picture of what our testing will entail.
A comb device will be removed out of the splice pack to take all of the modules on the schematic offline momentarily.
See Figure 3.
The modules will then be placed back online one by one.
A lab scope is placed on the data line to view the quality of the signal.
A jumper wire will be used to place each module back online one by one to view the serial data quality for each one. The serial data line is a 0 to 7 volt pulse, which is pulse width modulated.
Figure 4 is an example of a known-good pattern for your review.
The jumper wire was used to bring each module back online one by one, a pattern showing 0-7 volts was seen by all of the modules except one.
If your guess was the PCM, you are incorrect!
An example of the bad pattern is shown in Figure 5 for your review and analysis.
This pattern showed a range of 0 to 5.8 volts, this was below the threshold needed in order for proper communication to take place on this vehicle.
The bad pattern occurred on the circuitry for the SDM or sensing and diagnostic module (airbag module). The airbag module circuitry was loading the circuit to this value, causing the PCM not to communicate with the generic scan tool.
The pin for the airbag module was taken off the bus and generic communication was restored.
The vehicle then passed the emissions test, and the repair shop was advised to repair the airbag circuit.
This Pulling Codes case is now closed.
Note: If you’re interested in reviewing websites that provide examples of known-good waveform libraries on a variety vehicles and vehicle systems, you can contact me at [email protected] and I will provide you additional information.
– By Carlton Banks, contributor, Tech Shop magazine
In this Tip Clip Dave Hobbs talks about EVAP and FREDS, you know, Frustrating Ridiculous Electronic Devices, things like PCM’s and ECM’s. He talks about the logic of how these devices control the operation of the vehicle’s evap system. He discusses how the ‘brain’ of the evap system handles things like the operational nature of the evap system such as purge. He also goes into detail regarding how the FRED also handles the regulatory side of things such as EPA guidelines and leak tests.