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OBD Pin #5 Sensor Ground High Resistance (4-6 Ohm) & PCM Communication Issue Troubleshooting

305 Views 3 Replies 2 Participants Last post by  Patrick Feeley, 
OBS-MD
I've been dealing with communication issues between my OBDLink MX+ and my 1997 F250 via the data link connector (OBD). While doing a pin out check, I verified I'm getting 12V on Pin #16 and my chassis ground (Pin #4) is showing 0.000 Ohms. However, my sensor ground (Pin #5) is showing 4.0 Ohms with Key Off and 6.0 Ohms with Key On when probing between the pin and a good chassis ground point. I know anything above 0.5 Ohms is not good and I'm thinking this high resistance is why my scan tool can't communicate with the PCM.

Unlike a chassis ground that's easy to fix with another ground point added, I don't know what do to with the high sensor ground resistance. Is this implying one of my sensors connected to the PCM is bad and causing a high resistance scenario preventing the PCM from communicating? Should I be measuring this resistance differently (to a sensor versus chassis ground)? Could the fact my ignition doesn't need the key in place to turn the ignition/truck on causing high resistance and preventing the communication? I would love to know how to fix this issue and get my PCM to talk to my scan tool.
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Patrick Feeley
The ground path goes from C227 though C202 Pin 63 to G100 located on the LH front of engine compartment, on upper radiator support. There is no sensor in the path unless ground G100 attachment is bad. You may need to trace it from end-to-end (I would take a look at G100 first to see if the attachment may be corroded of loose). See the attached for more info. Cheers!

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OBS-MD
Patrick Feeley said:
The ground path goes from C227 though C202 Pin 63 to G100 located on the LH front of engine compartment, on upper radiator support. There is no sensor in the path unless ground G100 attachment is bad. You may need to trace it from end-to-end (I would take a look at G100 first to see if the attachment may be corroded of loose). See the attached for more info. Cheers!
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Thank you VERY much, Patrick! I really appreciate you going out of your way to upload all of those PDFs. They have been very helpful (albeit a little confusing). Dumb question, but where is C202 located? It looks like both Pin 63 and 26 are both part of the 570 circuit. Speaking of, what is the 570 circuit? Either way, sounds like my source of the higher than expected resistance along that B/W wire is the G100 shared chassis ground. That PDF graphic is hard to read, so can you tell me if this it the ground I should be focused on? It's not quite on the radiator support like you described.

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Patrick Feeley
C202 is the harness feedthrough connector on the driver's side of the firewall. Sorry, I missed pin 26! The only things I find for 570 wires on C202 are the C227 Pin 5 wire and another running from the Throttle Position Sensor (TPS) to the PCM. I guess it could be either pin, but I would just check the continuity between the C227 Pin 5 and both pins and you should get your answer which one it is. The 570 circuit is just a ground circuit number. I wish the EVTM provided circuit descriptions! That also looks like G100 (and that is part of the core support). Cheers!
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