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Isuzu Rodeo won't shift out of first gear

I have a 1994 Rodeo that won’t shift out of first gear.

It will physically shift through all gears but only engages P, R, N and then D through L get first only. When shifting from 2 to L you can feel the transmission kick down even though it is obviously still in the same gear.

V6, Auto Trans.

Here’s what’s been done so far:

Tranny replaced by prior owner.

Range sensor cleaned (it was not in bad shape).

Connectors checked.

Other potentially related symptoms:

No dash lights for transmission indicator.

Dead battery when I bought the car. It charged O.K. though.

Do you still get engine braking when in 2 as opposed to L? Not that I have much input for you, just trying to explain the mechanical notation you have between 1st in LOW versus 1st in 2.

Yeah it is confusing. It doesn’t do much engine braking, almost like it is in neutral except when I press the gas.

I checked the alternator output at the battery: 14.29V unloaded and 14.19V under load which seems in spec. I’d heard that a bad alternator can cause transmission issues in these models.

Next conundrum is that pulling the TCM wiring harness does nothing to change the operation. Leads me to believe that the tranny isn’t getting signals from the TCM or the TCM is dead.

Any thoughts?