Check the following.
Open circuit in between tach and ECM or between feedback pot and ECM
Defective tach
No output from ECM to steer motor
Feedback pot defect
Check tach input. Test voltage between K1-4 and TP3 when tach is spun. Test voltage between K1-6 and TP3 when tach is spun. If voltage present then bad ECM.
Master drive unit out of range. Jack up truck and turn drive unit back into range.
Defective steer motor
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