Code P1870 & Speedometer error
The vehicle: 1996 Bonneville SE, 3.8 L engine
Transmission is 7BX W from a donor vehicle
PCM is correct for 1996 Bonneville, P/N 16232094 GM update unknown
The problem: Transmission shift points are uniformly too high in all gears.
TCC lock up point is 55MPH consistently
Speedometer is uniformly reading higher than GPS tested speed
Fuel consumption higher than others with same engine and GM FWD cars.
Fuel gage unreliable as it is intermittent, dropping to 0 and setting off check gauge warning when it does. Refuel after 245 miles on odometer as work around.
Around town and on short drives, under 45 minutes car is fine except for above items.
After driving more than 45 minutes or so many times will set MIL and P1870 code and all correct actions occur that are associated with that code. In hot weather this can occur sooner than 45 minutes.
MIL can be reset with scanner and transmission resumes normal operation until next longer trip.
Recent engine problems due to MAP sensor resolved with replacement.
Checked: transmission code is correct for car, 3.06 ratio, standard pitch output shell, 35/35 gear set, 30 tooth VSS output rotor, RPO was FW2 from both this car and donor.
PCM is correct P/N for 96 Bonneville with this RPO.
Alternator Voltage is normal 14.5 +/- .3
Grounds near drivers seat all clean and dry no corrosion
Checked resistance of VSS from original and donor, 860/870 ohms, changed and no difference on speed readings.
Suspected grounds but ohmmeter checked and all points seem good, .001 ohms or less, within error range of DVM.
Have factory manuals but not scan tech of high end testors.
I am stumped on what this could be, it i obviously a speed related issue and I believe this is setting off the P01870 code but for the life of me an can't find common issue unless it is the speed signal generator in the PCM.
Does anybody have any ideas?