What would cause this car no start and no scanner communication on the bus network at DLC? These are what I have done; checked power and ground supplies to pcm with pcm disconnected, checked the resistance at DLC pin 6 and pin 14 with KOEO and it showed 60 ohms. All these readings showed the wirings are fine but why is my PCM burnt inside?
When there is no communication for any modules in the bus network, most likely the wiring is shorted. However, since you got 60 ohms when PCM is connected, then the shorted wire must be in the PCM circuit itself. Testing varies from verifying the vref 5 v at TPS or 8 v supply at the crank sensor at KOEO.
You could have a huge voltage drop outside the computer circuit. Did you check your starting circuit? This is the common part that carries large voltage besides your secondary ignition system. You should check the integrity of the power or ground supplies to see if you find anything unusual.
Check this approximate 2004 Nissan Maxima power wiring supply circuit below:
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As you can see, power comes to the PCM from the IPDM module, from ignition switch then finally from the fuse. However, since we suspect of a shorted ground wire, you can check the voltage drop by comparing the voltages at KOEO when the voltage is measured at the FRAME of the vehicle and at ECM ground pins 115 or 116 as shown above. There should no more than 0.250v at KOEO. If higher, there is short to the ground supply. To verify, this test should be done with ECM disconnected. If PCM is plugged in, then the shorted wires will kill it.
To test voltage drop for the ground, measure the voltage between negative battery post to the pcm ground connection. You should get no higher than 0.250 volt . If higher, go over the ground connection from the battery upto the ground wire at pins 115 and 116 at pcm.
I found almost 12 volts at pin 115 but none at the frame. Checking the cable closer, it barely held to the battery terminal. Putting a new one fixed it!
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