Tought Leader OBD II: What You Need to Know WRITTEN BY MARC ELLISON M
ost have seen it. Some people know why it exists. Others don’t care. But we all have heard the name—the On-
Board Diagnostic connector—and we all should know what it can do for your vehicle, both the positives and negatives. Tat little, black trapezoidal connector, the OBD II, as it is commonly referred to, is typically located under the dash near your left knee and serves as the gateway to chassis data for cars, light-duty trucks and vans. It was originally designed as a portal for testing station to obtain emissions data. Since the advent of the original OBD back in the 1970s, the OBD II port has become so much more than just an emis- sions data port. Every OEM uses this as the point of entry for scan tool diagnostics. Starting in the mid-2000s, it became a
portal for aftermarket companies to use to read and write to the Controller Area Net- work (CAN) bus to make upfitter compo- nents work better, smarter and sometimes more efficiently. On the positive side of using the OBD II connector, access to chassis data via the connector means fewer or sometimes even no other connections to the vehicle’s wiring system are necessary. Oftentimes splicing or tapping OEM wires is a contributor to vehicle electrical problems and can lead to vehicle downtime, or worse. Simply clicking into the OBD II to make the connection eliminates those issues. Te ability of after- market companies to read, and sometimes write on the CAN bus has opened a world of new, more reliable possibilities.
• A partial list of vehicle data available on the CAN bus includes:
• Odometer • Doors locked/unlocked, open/closed • Vehicle speed • Trottle position • Transmission position (PRNDL) • Engine running • Turn signals
Te list of what data is available is driven
by whether an aftermarket company reads the data actively (requesting info from the PCM) or passively (finding it without an obtrusive request for data). Te former is fairly common place for companies that connect devices to the OBD II, and it is how a scan tool functions. Te companies that can read CAN data passively are at an advantage because more data is available on the network passively and active commands, if requested too frequently, can interfere with vehicle operations. Writing to the network (commanding an action) is not commonplace because it requires a deeper knowledge of the vehicle’s electrical architecture and how command- ing actions can impact the vehicle’s other functions. So how is CAN data used? Here are a
few examples of vehicle functions that can be controlled or improved by utilizing the OBD II:
• Engage the shift lock (i.e., lock the vehicle in “Park” if the wheelchair lift door is open).
• Command a higher RPM level in “Park” for better air conditioner or alternator performance.
• Automatically stop/start the engine for idle reduction.
• Connect (sync) secondary signal lights to OEM turn signal lights.
• Automatically activate the stop arm and/or stop sign if the vehicle is in “Park” and passenger door is open.
While there are a lot of positives to using the OBD II, there are several things to keep in mind which will help keep you out of trouble:
Tere is no positive lock on the connector. Always use a zip tie when connecting a harness to the vehicle’s OBD II. Otherwise, you risk vehicle vibrations separating the connection.
38 School Transportation News • OCTOBER 2016
Marc Ellison is vice president of sales and marketing for LGS Group, an automotive technologies firm that is the master distributor for InterMotive
Vehicle Controls. He may be contacted at
mellison@lgs-group.com.
Writing to the CAN bus, when done
incorrectly, can cause issues with the Body Control Module—eventually causing it to lock up and requiring costly replacement.
Communicating on the CAN bus too
frequently. Some systems request data at frequencies that can be detrimental to the vehicle network or other devices. Keep in mind that the network has a limited bandwidth and excessive communication (e.g. requesting vehicle speed 40 times a second, even in Park) may affect the ability of the vehicle or other aftermarket systems to function properly. Te OBD II was never made for multi- ple devices to be connected and communi- cating at the same time. Tere are products available, such as Safe-T-Net, which can be plugged into the OBD II that will allow multiple devices to “share” the network.
Using the OBD II connector to read and write on the CAN bus can be a powerful tool to make your vehicle and equipment work better but it requires that you under- stand the benefits and the potential pitfalls. Te best way to know…is to ask ques- tions of your bus builder and their vendors. Who knows, you may learn about a feature you never thought was possible while avoiding other potential problems. ●
CELEBRATING25YEARS
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