Breakout the diagnostic box

Gary Lovett of Snap-on Diagnostics shares his thoughts on the 'no communications' message.

Published:  19 September, 2012

We hav all seen the dreaded 'no communications' notification but proving where the fault lies can be difficult as there are many reasons this can happen. These can include, but are not limited to, poor connections or wiring, software or a defective ECU.

Some faults reside on the vehicle, so blaming the scan tool for 'no-comms' is not always correct. Being able to look at the signals or voltages going to and from the scan tool can assist in correct diagnosis.

The 16-pin diagnostic connector has been in use for many years now. Generally, it provides an access point to specific ECUs or to the vehicle network. The connector design, contact allocation and electrical requirements are defined by the SAE J1962 standard.

Standard or not, engine management EOBD/OBD-II compliant vehicles can use any of five communication protocols. The protocol defines the 'type' of communication - namely the computer language used - as well as the speed of communication and the pins to use. EOBD/OBD-II uses the pins in an order as laid out by a number of international standards, namely J1850 PWM, J1850 VPW, ISO9141-2, ISO14230-4 and ISO15765-4/SAE J2480.

On the other hand, the older EOBD uses pins 7, 15, 10, 2, 5, 6, 14, 16 and 4. Effectively only half of the 16 pins are used!

The technician cannot always gain easy access to the diagnostic connector for testing purposes due to its location - identifying which pins are in use when upside down in the foot-well can be a challenge. Here, a breakout box is a definite advantage, especially if the box is on a long lead.

Typically the first step will be to confirm Pins 16 and 4 are providing power to the scan tool (live and chassis earth respectively). Low voltages or poor ground connections can be indications of electrical problems or even just a blown fuse on the vehicle.

It has been seen on many occasions that poor connections at the diagnostic connector has caused communications to the scan tool to fail. It is also possible for pins to push out of the vehicle connector or pins to get bent as the diagnostic tool is plugged in, again resulting in no communication. These cannot be seen and you need to test - it is not acceptable to blame the scan tool for no communications without first confirming good connectivity.

This protocol is commonly used for vehicle diagnosis since 2008, if there is a fault on the network, it potentially could stop ALL communications.  It is important that we check the CAN signal is being seen at the diagnostic connector with an oscilloscope. It might sound like a wrong way around, but it is important to check there are no problems with the connectors and signals PRIOR to connecting the scan tool. The reason for this is that scan tools have been damaged when there has been a vehicle wiring fault and battery voltage is present on a pin not designed for battery voltage.

It's also important to check if any signal is being seen at the connector as there could be a wiring fault between the ECU and the connector. Again, if these are not checked, the user will only get the dreaded "no communication" message.

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