Have you ever asked yourself the question, “What exactly causes a U code?” A famous quote from the prison warden in the classic 1967 Paul Newman movie “Cool Hand Luke” answers the question best: “What we’re dealing with here is a failure to communicate!”
Which module is talking and which one isn’t? This Tech2 is trying to talk to the highlighted module.
When communications networks fail to communicate on today’s vehicles, the results can range from mysterious U codes in multiple modules to scan tools that are unable to display anything beyond “Check Vehicle Identification” or some other onerous message. The F for frustration in “Frustrating Ridiculous Electronic Device” (F.R.E.D. for short) usually starts in about the time these kinds of problems come into your bay. After all, your scan tool could help you diagnose the vehicle if it could only communicate with it. Don’t despair if this happens in your bay. Take a few minutes to understand how serial bus communication networks work, learn a few simple tricks on diagnosing them when they don’t work and we’ll help remove that F for frustration.
Evolution
Serial bus communications became common with General Motors in the early 1980s when the ALDL (DLC of yesterday) contained a pair of wires from the ECM sporting a pulsing square wave when 10,000 ohms of resistance between ground and the serial bus was put into play via the scan tool being plugged into the connector and powered up. This voltage drop on a circuit called Diagnostic Enable signaled the ECM to begin making the bus do its thing communicating with your scanner.
Before you tire of reading a tiny history lesson here, it is important to know how your scan tool still has a place in getting communications to happen when it’s plugged into the late model vehicles you work on today. Serial buses have evolved tremendously from those old GM Universal Asynchronous Receive Transmit (UART) networks running at a baud rate (binary bits per second) of 160 and 8192. (Think really slow dial-up Internet services from those days.)
No communications? Multiple networks? Check the gateway module, the network translator.
What we see in our bays today on U.S. domestic models primarily is a combination of two types of buses. The first type came into existence right around the onset of OBDII. Because the theme for this month’s issue is domestic vehicles, that means the J1850 SAE Standard bus used by GM, Ford and Chrysler. The J1850 bus ran right up into the 2007 model year before being totally replaced by the second type of serial bus call Controller Area Network (CAN). There are a few similarities for all data buses on post OBDII era vehicles and a lot more differences. We’ll cover some similarities to start off.
General Bus Similarities
First off, most all serial buses used for scan tool communications are shaped square waves. Modern communications, a.k.a. multiplexing, can be accomplished with analog voltage shifts, (steering wheel controls) slight current level shifts, (electronic impact sensors) and fiber optics but those communications don’t interface with your scan tool and are for communicating between modules when used. If you were wondering why I used the word “shaped” preceding the term square wave, it is because a perfect square wave is a perfect source of Electromechanical Interference (EMI) so many OEMs smooth out those sharp edges. If you use a lab scope in your daily diagnostic strategies, you might pay close attention exactly what the waveform looks like such as injector pintle bump, fuel pump current ramping, etc.
From my personal experience, the digital square wave patterns of communication networks don’t need that much scrutiny. Their patterns typically are either present or they aren’t and that is simply what you need to know in most cases. Another similarity for vehicle communication networks is the fact that if the bus your scan tool is attempting to communicate with the vehicle on is completely shorted to power or ground you won’t see any of those previously mentioned U codes on your scan tool. That would be like hearing a tornado warning from a radio that’s completely broken; it’s not going to happen.
In those cases, put the scanner aside and focus on your meter or scope. Finally, with the exception of Chrysler SCI buses, most all of the buses I’m aware of are bi-directional, meaning modules/scan tools can use the same circuit to both talk and listen.
Bus Differences
J1850 was applied to Ford vehicles in the mid 1990s in a two-wire protocol (technical term for network language) and assigned to pins 2 and 10 of the 16 pin OBDII J1962 compliant DLC. The two circuits make square waves that are mirror images of each other and typically are twisted pair wiring. Both the double wire application (as opposed to a single wire) and the twisted pair wiring add noise immunity (thereby increasing reliability) to this bus that Ford calls Standard Corporate Protocol (SCP). Each module has a pair of wires running to one of several harness splices. Pin 2 is labeled SCP + and pin 10 is labeled SCP - .
Don’t look too hard at bus communications patterns. It’s either a square wave or it’s not.
If only one of the two bus wires is open or shorted, the bus might communicate one module to another as well as with your scanner. All bets for reliable messages, proper module operations and a U code free environment are off when this occurs, however.
GM and Chrysler took a slightly different route to adhere to the OBDII complaint J1850 protocol with a single wire bus in Pin 2 of the DLC. GM termed their bus Class 2, while Chrysler used a bit of imagination and termed its very similar bus Programmable Controller Interface (PCI). Chrysler uses a wiring connection strategy of one wire per module terminating in a common shorting bar/connector called a Diagnostic Junction Port (DJP). This connection typically is located near the DLC in the area of the knee bolster under the steering column. The connection allows for removal of one or more PCI wires making fault isolation much easier. GM’s equivalent single wire J1850 bus (Class 2) has a varied wiring connection strategy depending on the model line.
Some GM models use the single wire per module strategy where each wire connects to a single splice pack connector (called a star connector) in the same general area of the DLC. Termed the Star Configuration, the star connector usually is taped up in electrical tape near the DLC, but is easy to remove for fault isolation. Some models might have a second star connector on the opposite side of the IP area.
Another wiring strategy GM used with its Class 2 bus was called the loop configuration. One these models each module had two Class 2 bus circuits looping them together. Fault isolation is more difficult on the loop configuration. To eliminate a module as being the source of a bus short to power or ground you must locate each module one by one in order to disconnect them individually from the entire circuit.
However, with a loop configuration in the event of an open Class 2 bus circuit, bus communications from one module to the next will only cease to occur if there are two open circuit conditions. Some later GM Class 2 applications may use a combination of the loop and star configurations throughout the vehicle.
CAN – Controller Area Network
The CAN bus first appeared in European cars courtesy of its inventor Robert Bosch in the 1990s, but didn’t see the U.S. domestic market until around the 2003 model year. Gradually the American car makers along with the Asian car market adapted CAN on some models until the 2008 model year when federal regulations required every vehicle to be equipped with a high-speed CAN bus for powertrain and chassis modules.
Individual networks may or may not be wired to the DLC.
For powertrain and chassis module communications the CAN bus will have two circuits similar to the Ford SCP J1850 bus and are pinned out in pins 6 (CAN High +) and 14 at the DLC (CAN High -). Depending on the domestic OEM and model year, the CAN equipped vehicle might have one or two other speeds of CAN used for other types of module communications such as body electronics and graphical displays not requiring as high of communication speeds as powertrain and chassis. Much in the same way you don’t require a cable modem for a simple text email, the Body Control Module (BCM) does not require high-speed dual wire CAN to pop the trunk or honk the horn when other modules are involved in such a process as is the case much of the time.
Whether a J1850 bus or CAN bus, the network classifications are broken down into three speeds: Type A, (under 10Kbps), Type B (10-125 Kbps) and Type C. (125 Kbps and up). Pre-OBDII buses most often will be found in the Type A speed while OBDII J1850 buses will be Type B. GM calls its CAN GMLAN. Its dual wire bus is termed HS (High Speed) GMLAN used for powertrain and chassis communications (a Type C bus), and a single wire medium speed GMLAN bus (Type B) is reserved for other modules and functions on the vehicle. Ford simply calls its powertrain/chassis bus HS CAN (High Speed) and its Type B medium speed body electronic bus MS CAN (Medium Speed).
Some CAN-equipped Chrysler models have an IHS CAN bus (Intermediate High Speed) Type B bus in pins 3 and 11 for non-powertrain/chassis modules. We bring up letter designations because you frequently will see Chrysler use the CAN C term in its service information. It also is important to note that many Chryslers use a separate HS CAN bus (termed CAN C Diagnostic) to allow for all scan tool communications, through a single module while the rest of the modules on the vehicle communicate on either their dual wire HS CAN C bus (powertrain and chassis modules) or a single wire CAN B bus for most other modules. This strategy is not unique to Chrysler’s CAN bus vehicles.
.JPG?auto=format,compress&fit=max&q=45&w=250&width=250)
A DLC breakout box is a must have tool for bus troubleshooting.
Looking at their history, scan tool communications via a generic OBDII tool (Global OBDII) would on past Chrysler models (pre-CAN) be handled by another bus termed Serial Communications Interface (SCI) receive and SCI transmit for both the ECM, TCM and ABS, while the PCI (J1850) bus handles module-to-module communications and enhanced scan tool communications. Some European models (Mercedes) also take the approach of having the vehicle’s entire scan tool communications take place through a diagnostic CAN bus dedicated to a single module.
Your ‘Gateway’ to Logic
If there was ever one single point on a complex subject to pass along, it would be information about gateway modules. Both J1850 vehicles and CAN bus vehicles are known to use one or more gateway modules that each connect to multiple buses used on the vehicle. Besides J1850 and CAN, there are buses call Local Interconnect Network (LIN) that operate at a lower speed for modules not requiring super fast communications. Components such as door and seat modules, HVAC, etc. might use this less expensive bus, which then connects to a gateway module in order for other modules to use and share info with modules on the LIN bus.
These modules are called gateway modules and pull together the different networks on the vehicle. They act as a translator for total inter-vehicle communications as well as scan tool communications. This is essential knowledge for the tech. If a service manual theory of operation or vehicle network communications schematic shows a module with more than one bus protocol connected to it, consider that module a gateway.
This means if a module you want to look at PIDS or DTCs isn’t connected to a network that is pinned out in the DLC, your scanner is going to be going through a gateway module to communicate with that particular module. In the case of some Chrysler CAN bus vehicles, the Totally Integrated Power Module (TIPM) under the hood is the exclusive communications link between the only network pinned in the DLC (Pins 6 and 14 called CAN C Diagnostic) and your scanner.
A resistance of 120 ohms indicates an open terminating resistor or an open in the circuit leading to it.
On GM and Ford CAN bus vehicles, the DLC might contain other circuits for other networks such as a single wire CAN. Late-model GM vehicles, which fall into its Global A architecture, might add an additional dual wire CAN bus called a chassis expansion bus. This bus is for modules such as electric power steering and intelligent stability control sensors. This bus will be pinned out in the DLC in pins 12 and 13 allowing some scan tools to communicate with that bus directly.
The modules on that bus can still communicate to the modules on the main dual wire CAN bus (pins 6 and 14) via (you guessed it) a gateway module function of the ABS module.
Knowing which module is the gateway(s) not only helps you determine the path communications is taking when your scan tool attempts to obtain information, it is also helpful in determining the most likely cause when there are multiple bus problems.
Diagnosing F.R.E.D.s That Aren’t Communicating
Your first order of business is knowing what networks are on the vehicle, which networks are involved in modules related to your vehicle’s symptoms, which networks are pinned in the DLC and which network your scan tool is attempting to communicate with if it eventually says No Communications. Everything mentioned except for that last item is easy enough to determine by studying the vehicle communications schematics. That last item might not be hard to determine depending on the scan tool.
Typically, older platforms of tools go to the network the scanner “thinks” that vehicle’s system should have (according to how you’ve built the vehicle when setting up the tool) and sends a message out on that network essentially saying, “Hey, I’m another node (engineer talk for module) on the bus and please send me some data.” You might have no idea which network(s) the scanner is trying to address. Some of the newer scan tools will show which network the tool is attempting to communicate on. If only one module is not communicating on a bus while others are OK, check for a blown fuse first followed by a lack of good power and ground to that module’s connector. You can’t communicate when you don’t have power or ground.
Next look for an open circuit in the network between the proper DLC pin and that module or between the gateway module your scanner is attempting to communicate through and that non-communicating module. Keep in mind your scan tool (unless it is a factory scan tool) simply might not be up to the task of communicating with certain body and chassis modules. If U codes are present in other modules, follow published diagnostics. As a general rule, the modules with U codes that point to failure to communicate in general (U1000 on J1850 networks for example) are the last to suspect as being the culprit.
This is especially true for a module specific U code. Let’s say you do a DTC scan of a CAN bus equipped vehicle and find DTC U2111 (Electronic Steering Control Module Not Communicating) in several chassis related modules. Don’t suspect those modules. Suspect the module they are unable to communicate with by the definition of that code. CAN networks have an abundance of very definitive DTCs when the bus has voltage level problems such as intermittent shorts to power or ground, open circuits and accumulated errors in communications that can temporarily cause a module to pull itself offline.
You can accidentally set the stage for a U code by coding a new module with incorrect information.
Another really important thing to remember when interpreting U codes on CAN bus vehicles are non-consequential history codes that set. Sometimes when there has been a service procedure where a module has been w/o power codes will set in history. They mean nothing if not accompanied by a real complaint with the vehicle. Simply record the DTCs for possible referral in the future then clear them. Some scan tools might cause U codes to set in select modules when they are unplugged from the DLC while still in a linked-up communication condition. This isn’t a condemnation of the car or the tool, just a reaction to two parties not wishing to part ways yet. Avoid this anomaly by simply backing out of the communications screens with your tool prior to disconnecting.
Total bus failures like shorts to power or ground by a network circuit or module on the network can occur on a CAN HS network. This can prevent the vehicle from starting, (immobilizer) cause the entire PRNDL to light up all at once or induce multiple IPC telltale lights to illuminate. A short to power might be 12 volts measured at DLC pin 6 or 14, but don’t assume a short to ground is 0 volt or 0.1 ohms between ground and the network pin. I’ve seen 50 ohms of resistance between ground and a network circuit due to a faulty aftermarket accessory wired into one of the bus wires causing communications failures with some modules while others still would communicate. My scope showed a perfect pattern on that vehicle too but a quick measurement between ground and both network circuits revealed several thousand ohms of resistance on one of the two wires but only 50 ohms between the bus and ground on the other wire.
Aftermarket radios quite frequently are the culprit on my problem vehicle. Always consult the service manual for exact specs, but a rule of thumb for a short circuit by definition on a data bus network circuit is anything less than 350 ohms measured between ground and each network circuit.
End of the Line - Terminating the Bus
Some networks including CAN buses require termination resistances at each end of the bus. If the bus is a vehicle wide bus such as a GM or Ford HS dual wire CAN bus, there will be a pair of 120 ohm resistors either in two modules or a single resistor in one module and the other resistor wired in the network harness somewhere. If the vehicle uses one resistor in the harness as opposed to locating them internal to two modules, the resistor might be a simple stick lead resistor soldered into the harness and taped up with electrical tape or it may be in a small plastic housing.
I have seen cases with GM where the terminating resistor was in a black plastic housing about the size of a small relay and located under the vehicle near the fuel tank. The reasoning for this location was for the resistor to be near the end of the line of the HS CAN bus where the fuel pump control module is located. This has led to corrosion in the field which means resistance build-up. The resistors are there for noise immunity (EMI) so if the total resistance isn’t right – reliable bus communications suffer and phantom U codes and strange symptoms can be the result.
Remember, a network failure between an immobilizer module and the ECM can cause a no start complaint.
Because these two terminating resistors are 120 ohms each and they are wired into parallel, the total resistance should be 60 ohms +/- five percent. Because you are using your ohm meter, make sure the vehicle and the bus are asleep meaning no current draw and no bus activity. Measuring this terminating resistance is simple; apply your ohm meter between pins 6 and 14 and look for 60 ohms. If you see 120 ohms, there is a module unplugged or another type of open circuit in the network. If you see some other resistance much higher than 120 ohms, you have a complete open on the bus.
f you are using a DLC break out box (they are invaluable), check to see if a DLC pin has backed out as this has happened before. If you see something between 60 and 120 ohms, look for corrosion in places such as at that undercar resistor location (if used) or even a connector in the network being dripped on courtesy HVAC condensation.
On the subject of bus terminators, a note specific to Chryslers with CAN bus networks involving the use of that Diagnostic CAN C network as the exclusive bus for scan tool communications must be pointed out. These vehicles use a single 60 ohm resistor in the gateway module (usually the TIPM) as opposed to two 120 ohms resistances in two separate locations wired in parallel. The resistance measured at the DLC is the same: 60 ohms. To get to the terminating resistors total resistance on the regular Chrysler CAN C bus, you must back probe both CAN C bus circuits at a module or in the harness somewhere as there are no pins for this bus actually in the DLC.
Other Tips and Tricks
I’ve had good luck with scoping various networks to look for flat lining modules and wiring shorts to ground that were intermittent. Tap test, heat and freeze modules and connections until you see the pattern drop out and stay dropped out. Keep in mind some networks won’t just sit there active with the key on making your scope toggle a square wave. Some vehicles require a scan tool to initiate conversation with another module on the network before activity begins on some buses.
Most J1850 and CAN bus vehicles will show active square waves on the bus when the ignition is on or something is activated which leads me to a couple of parting tech tips. One is regarding the bus that won’t go to sleep. If after several minutes the networks on a vehicle don’t go flat line on a scope after the ignition is switched off and RAP functions have timed out, look for some kind of BCM input (door jamb switch, keyless entry, etc.) that might be keeping the network awake, which as you might imagine can lead to battery drains.
If you have a DLC break out box plugged in, make sure you don’t have a scan tool plugged into the break out box as it can keep the bus active. On the flip side of the bus that stays too active with the key off, I also have found a quick way of diagnosing power locks as far as whether the key fob or the receiver is bad. If my scope pattern on the low speed CAN bus or J1850 bus (body electronics) goes active when I press a button for lock or unlock, the key fob is good and the problem is somewhere upstream from the remote lock receiver.
Keep on increasing your knowledge about communication networks and how to apply your scan tool, meter and scope to troubleshoot those instances where a module is having a Failure to Communicate, and you’ll remove a lot of frustration from your diagnostics on the ever increasing numbers of F.R.E.D.s rolling into your service bay.
Subscribe to Motor Age and receive articles like this every month…absolutely free. Click here
