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.
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.