48re won't downshift at a stop.

I've been hesitant to say this because I'm working on memory and don't have my truck any longer.

1-2 governor pressure should be near 0psi.

The first pressure increase should take it to 2nd gear, but be less than 80psi. 3rd gear should be 80psi+.

IIRC, TV input and Gov counteract one another on the same spool valve. If TV is high, this creates more resistance to the Gov pressure, delaying the up shift.

If it is high during downshifts, it will prevent the 2-1 shift from occurring.

If Gov pressure exceeds a high voltage threshold (4.95V+?)for long enough, the electronics will go into limp mode, preventing shifting out of 3rd gear on down shifts until a key cycle.

Since this is a new build, and you only seem to have one symptom, I would drop the baseline on the throttle valve screw just a bit and see if you can get your 2-1 dialed in.






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Just talked to ATS. Gave them all the diagnostic data I've collected.
They are sending me a new GM solinoid, new transducer, and new valvebody harness.
And even though I'm out of warranty, it was free of charge (minus shipping), so that's pretty good.

When I gave the data on probing the case connector pins, I expressed my concerns about pin #5 only showing 1.2 volts, when the ATSG says it should be 8.3 volts.
They said there should NEVER be 8 volts in that circuit..
So I guess the ATSG was wrong on that one ?