Nissan Repair Questions? Ask a Mechanic for Answers ASAP
Does the truck fine otherwise?
Well, in all fairness I must admit its hard to try and diagnose a problem when the proble is not actually happening.
The torque converter lock/unlock function depends on a few things including TPS signals and solenoid and pressure readings.
If the truck is NOT downshifting it SHOULD throw a code indeed, but it's not. You should get a TCC operation code or something or a shift function code.
The only thing you can do aside form that would be to communicate with the TCM and ECM while driving. Make sure the TCM is receiving the proper signals and check solenoid function. It wouldn't hurt to watch the revolution sensor readings from the shaft either.