Code 31 on obd 1

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Always parting 80s
Jul 27, 2008
Dublin GA
So other than replacing the air flow meter or ECM is there anything else I could try? I have searched all day and very little to be found about this code.

I have looked over most of the wiring but see nothing obviously wrong. The truck barely idles and has no power.
Yeah, it's fine. I replaced it with a spare but it didn't change anything
a little update. keeping this updated for anyone that might have issues and finds this thread.

checked the VFM with the FSM and it checks out... i guess. the FSM shows two different diagrams for the pin out. really hard to tell if i was doing it correctly at first but i did get readings that line up including the wave pattern.

next step is to check the wiring harness but its cold and raining today so will put this off. if no bad wires i will move to ECM. this is an early 93 truck so the ECM is already suspect. does anyone have the part numbers for the good and bad OBD1 ECMs? want them in this thread for future help to others.
You hava an early 93 (built before May 93) with an engine ECU #89661-60170 or 89661-60220 (Ca emissions). You have TCM-ECM communication problems. In order to correct it you will need to replace the engine ECU with one numbered 89661-60221. You may be able to find a used one out of a late 93 or a 94. A new one lists for $1,575.79


Quoted from another thread...
Addendum to the above post. New ECMs are long-gone so used is the only venue at this time.
Thanks guys for the info, hard to cut and paste on this phone. Not sure what ECU is in the truck but it came with r12 so I know it is an early model.

I will pull the ECU tonight, shove the harness back into the engine bay and then check for continuity from the afm to ECU. I will also post a part number for the ECU I have but I do not have the three transmission codes (yet) so maybe it has already been swapped out.
An R12 truck would have originally had a 60170 or a 60220 if it was CA emissions. If it has a 60221 somebody replaced it along the way.
can someone help me out with the pin out on the VFM just to make sure i am doing this right.

the FSM i got from here shows 3 different things. all of these are from the same EG-289 section of the book. i am unsure which pins i should be checking. in our reading of the wiring at work (3 of us that are pretty good at wiring) we cant figure out which pin is which since they all seem to point to different places.



my ECU is 89661-60170

i checked the plugin for the VFM and it is fine to the back but i was unable to test the wiring to the ECU. it was rather hard to reach around under the dash and to the hood. i will see if i can get someone to help me tomorrow over lunch. i am still confused as to which wire is which in the VFM harness but i will post my findings here once i get it all sorted out. the FSM is not very clear on this.
so after hours of playing around with it i can say that this is the correct pin out after taking a spare harness and setting it out to check the wires.


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