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What is the signal level for PCM Pin 12 on the EEC connector? With no transfer case installed the signal path to ground, when the lever is in the Low range position, is not there. The 12V (High) signal that starts at Fuse 17 is still going through the instrument cluster Low range bulb/isolation diode. From there that 12V still continues on to PCM Pin 12.
This scenario is very odd for sure. I would never have expected PCM Pin 12 wire to not go anywhere. That implies there is a different harness for 2wd versus 4wd trucks.
So how would the device inside the computer that senses a High or Low input work with no external connection....Looking at the diagram some more I noticed and mentioned the isolation diode. I thought it was more of a steering diode function, but given your revelation there is no connection to anything for PCM Pin 12, my hypothesis leans towards there is a pull-up resistor inside the computer for that circuit. Meaning the High is internally generated. When there is a connection to the transfer case Low range switch, the Low/Ground turns on the Low range lamp and pulls down the input signal to the sensing circuit inside the computer to trigger the altered shift schedule.
Could be off base on the above. Just trying to see if anyone else has a different view or more to add.
All wires regarding 4x4 are terminated down inside the harness covering with these. The connector is populated but the wire just goes a few inches into the harness to nowhere
Originally Posted by rla2005
This scenario is very odd for sure. I would never have expected PCM Pin 12 wire to not go anywhere. That implies there is a different harness for 2wd versus 4wd trucks.
So how would the device inside the computer that senses a High or Low input work with no external connection....Looking at the diagram some more I noticed and mentioned the isolation diode. I thought it was more of a steering diode function, but given your revelation there is no connection to anything for PCM Pin 12, my hypothesis leans towards there is a pull-up resistor inside the computer for that circuit. Meaning the High is internally generated. When there is a connection to the transfer case Low range switch, the Low/Ground turns on the Low range lamp and pulls down the input signal to the sensing circuit inside the computer to trigger the altered shift schedule.
Could be off base on the above. Just trying to see if anyone else has a different view or more to add.
Even just for fun I brought Pin 12 high through a plain old incandescent test light to simulate the 4low light and no change
Originally Posted by rla2005
This scenario is very odd for sure. I would never have expected PCM Pin 12 wire to not go anywhere. That implies there is a different harness for 2wd versus 4wd trucks.
So how would the device inside the computer that senses a High or Low input work with no external connection....Looking at the diagram some more I noticed and mentioned the isolation diode. I thought it was more of a steering diode function, but given your revelation there is no connection to anything for PCM Pin 12, my hypothesis leans towards there is a pull-up resistor inside the computer for that circuit. Meaning the High is internally generated. When there is a connection to the transfer case Low range switch, the Low/Ground turns on the Low range lamp and pulls down the input signal to the sensing circuit inside the computer to trigger the altered shift schedule.
Could be off base on the above. Just trying to see if anyone else has a different view or more to add.
I did not see the reply above until today. So we have two trucks with the same issue and replacing the PCM has not changed the symptom. Same issue, different thread/OP: https://www.ford-trucks.com/forums/1...ode-633-a.html
That 4WD-Low signal is either a high or low. With no wire attached to PCM Pin 12 is would be plausible the signal could be in between and triggering the code. In the other thread there was a response stating to cut the 4WD-Low signal wire, which would be the same as what this current vehicle has from the factory. I am baffled on this one (and the other).
I did not see the reply above until today. So we have two trucks with the same issue and replacing the PCM has not changed the symptom. Same issue, different thread/OP: https://www.ford-trucks.com/forums/1...ode-633-a.html
That 4WD-Low signal is either a high or low. With no wire attached to PCM Pin 12 is would be plausible the signal could be in between and triggering the code. In the other thread there was a response stating to cut the 4WD-Low signal wire, which would be the same as what this current vehicle has from the factory. I am baffled on this one (and the other).
short to ground ground(or positive im not sure how that wire works) from the thing it plugs into? Conductive paths and all that. Connector cleaner and a visual inspection are wise
