In 1996, all new cars or light trucks had to have a standardized diagnostic connector under the dash called an OBDII connector or Diagnostic Logic Connector (DLC). The connector was designed to test emissions and create a standardized method and language of communicating with the emission control system.
The OBDII 16-pin trapezoidal connector obeys an SAE/ISO standard. Nine of the connections or pins are the same for all vehicles, while seven are open for use by the OEM as they see fit or for future technologies they could not anticipate 20 years ago. For a top technician, it is a portal into many vehicle systems and circuits.
What Is In There? How Do I Probe It?
The connections in the OBDII DLC represent networks, power and sensor (signal) grounds. The connector can be probed from the front or back with the appropriate test leads.
The best bet is to use an OBDII DLC breakout box to prevent damage to one of the most used connectors on the vehicle. These boxes can also be used with a scan tool. These boxes have banana plug connections to the 16 pins. Some models even have lights that flash when a circuit is active or transmitting information.
Meter + Breakout Box: Power, Grounds and Resistance
Pin 16 in all OBDII connectors is battery voltage that is hot all the time. Pin 4 is always chassis ground and pin 5 is signal ground. The signal ground can be used to diagnose problems with the engine control module’s ground. A weak ground can be diagnosed by measuring the resistance between either pin 4 or 5 and the negative battery terminal. Pin 16 can be used to measure battery voltage from inside the vehicle.
Almost every SAE/ISO standardized serial data bus terminates in the DLC. If you need to see if a serial data bus is healthy, you can measure the resistance using the two connections in the DLC using a meter.
If you want to test if there is a short or open in a network, you can measure the resistance between the lines in the DLC with the battery disconnected. Most serial data buses use terminating resistors in one or more of the modules. The resistance should match the specification in the service information.
For our example, we will look at the CAN bus in pins 6 and 14. The typical CAN bus has a resistance value of 60 ohms when measured between pins 6 and 14. This value is because most networks have two 120-ohm resistors in parallel.
If there is no resistance reading, there is an open connection in a module or wiring. If there is too little resistance, it is a sign there is a short to ground in one of the modules or the wiring.
If the reading is around 120 ohms, it is an indication that one of the terminating resistors is missing. This simple test can be performed without having to lift the hood with the help of a breakout box.
Scope + Breakout Box: Signal Quality
When you are looking at a serial data bus signal on a scope, you are never going to be able to decipher a series of 1s and 0s into commands. What it can tell you is if the bus is active and healthy. But, with a scope, it is possible to see the voltages toggling as modules are communicating on the bus.
A CAN bus uses two dedicated wires for communication. The wires are called CAN high (pin 6) and CAN low (pin 14). Both lines carry 2.5V when the bus is awake. When data is transmitted in 1s and 0s, the CAN high line pulls the voltage up to 3.75V and the CAN low pulls the voltage down to 1.25V. This pattern can be seen with a scope. If you set up a two-channel scope with the correct time and scale values, you can see the voltages mirroring each other. The patterns should be clean with no spike above or below the defined values.
If the signals do not correspond or are above the preset values, it can indicate a problem with a module or wiring that is shorted or open.
Parasitic Draw Testing + Breakout Box
The lights on an OBDII breakout box can help you to solve parasitic draw problems. Most modern vehicles do not completely power off when the key is removed. The modules will perform housekeeping chores like reposition HVAC doors, controlling interior lights and even cycling oil and coolant in the engine. The modules and networks will eventually go to sleep. But, sometimes a serial data bus might stay awake due to a problem with one of the modules.
If you connect the breakout box to a vehicle and shut the door, you can observe the serial data buses going to sleep. This can help you focus your additional diagnostic tests.