In the past, we could conclude that with a vehicle no start condition, the PCM was active if the check engine light came ON with the ignition switch in the ON position.
Since 2008 vehicles use a CAN BUS as a line of communication that controls the check engine light as well as all the other instrument panel lights and indicators.
If the instrument panel does not receive a communication from the PCM, it will most likely turn the check engine light ON regard- less of whether the PCM is active or not.
Enhance your diagnostic strategy
When you are dealing with a starting prob- lem with no communication with the PCM (No Start/No Scan Data), begin by follow- ing this unique chart, which will speed up and simplify your diagnostic process. This chart is valid for all the makes of vehicles. To view the diagnostic chart, refer to the Diagnostic Chart—Training Tip 001.
The ve-volt regulator is the heart of the PCM. This voltage must range between 4.9 and 5.1 volts. The microprocessor, which serves as the PCM’s brain, requires the ve volt regulator. Without this ve- volt supply, the microprocessor is not able to function.
When this happens, it becomes impos- sible for the microprocessor to communi- cate with the scanner or to carry out all other tasks such as ordering the open- ing of the injectors or triggering the fuel pump relay.
Note: With Drive-by-Wire throttle technologies PCMs use two independ- ent five-volt regulators that should be tested separately.
Diagnostic chart description
1) If the voltage between the engine ground and the ve-volt supply of any three wires’ sensor such as TPS, MAP or all other sensors that use this ve-volt supply, is lower than 4.8 volts, it is likely that the ignition voltage sup- ply or the battery stay alive memory voltage of the PCM is too low. Warning—if the voltage supply of the PCM is displaying as normal, a shorted sensor could be the cause of a low ve-volt supply and in certain cases, could cause permanent damage to the PCM. Disconnect all sensors related to this circuit while monitoring the ve volts supply to nd the faulty sensor. Before replacing a PCM with a suspected five-volt issue, make sure you disconnect the ve-volt output circuit feed of the PCM and with the ignition ON. Check to see if the ve-volt output supply is present at the PCM or if the PCM is still faulty.
2) A voltage higher than 5.2 volts is gene- rally caused by a bad PCM ground. The complete absence of a ground to the PCM would result in a reading close to 12 volts on the ve-volts reference. A voltage drop of the PCM ground circuit will be propor- tional to the increase of the ve-volt refe- rence. A voltage drop of 1 volt on the ground circuit will result in a reading of six volts on the ve-volt reference.
3) If the ve-volt supply is in spec (between 4.9 to 5.1 volts) and your scanner has no communication, attempt to communicate with other modules or different protocols via Global OBDII or OEM. A short in the commu- nication line could also be a possible cause.
Other tests
Check for spark, fuel pressure, injection pulse, engine mechanical and scan data, etc. Also, try another scanner to see wheth- er it is possible to access the PCM data and perform a detailed analysis of all PIDs.
