If the scan tool does not receive data from an OBD II vehicle
TEST 1: Connect the positive DMM lead to DLC pin 4 and the negative DMM lead to the negative battery terminal, using jumper wires if necessary. Do not connect to the body or chassis ground but to the battery terminal directly.
With the ignition on, the voltage drop across the ground terminal should be 0.1 Volts or less
TEST 2: Repeat for DLC pin 5
NOTE: An open circuit or high ground resistance means that the PCM will never enter diagnostic mode.
TEST 3: Voltage check on DLC pin 2. (does not apply to a Mazda MPV)
PCM transmits data on this pin
Connect the positive DMM lead to the class 2 Serial Data pin and the negative DMM lead to either pin 4 or pin 5 or a known good ground
With the Ignition on, the voltage on class 2 serial data pin should be 5V or fluctuating between 3.5V and 5.0V, If open, PCM cannot transmit data to the scan tool.
Connect positive DMM lead to DLC Battery + terminal Connect negative DMM lead to battery ground terminal
It should read battery voltage
This tests the ECM Voltage supply as no power may be caused by a blown lighter fuse
TEST 5: Connect positive DMM lead to either DLC bias terminals Connect negative DMM lead to the ground terminal
With the ignition key ON and no bus activity, BUS + should read 0V BUS should read 5V
With the ignition key ON and bus activity, the voltage should vary from 0 to 5V depending on the amount of bus activity!
ADDITIONAL NOTES PCM receives battery voltage (B+) through a fuse or fusible link. Check for an open circuit. An open ground or power (B+) circuit on a fuel-injected engine removes power from the ECM and prevents the engine from starting.
ADDITIONAL INFORMATION: Old PCM is fried, see the attached picture when attempts are made to communicate after fixing the wiring problem.
The new PCM works fine.
Check the fuse box (Power Panel), it might be the reason why PCM isn’t able to receive power as well as communicate.