So I've made some improvements in the 4LO shifting. The fundamental problem is that the GM computer was getting the VSS signal from the transmission tail shaft, so in 4LO, it thinks it is going much faster then it is, and so shifts immediately. My solution was to get a Dakota Digital SGI-100BT Universal Tach & Speedometer Interface. I used this to translate the 4000 PPM Toyota VSS signal from the transfer case output shaft to the 128K PPM expected by the GM PCM (Pin 21 Green). I then connected the Toyota speedometer to the 4000 PPM output from the PCM (Pin 50 Green). I also used the Dakota Digital unit to handle the tachometer and correct the speedometer.
I had it out on some very steep stuff today (Metberry & Longwater Gulches), and it shifted well. I'd like it to hold 1st a little longer, but overall i'm happy with it. I still need to adjust my linkage so I manually shift to 1st.
I also added a relay to ground Pin 16 based on the transfer case 4LO switch, but it doesn't seem to have any effect. I talked to support at HP Tuners and got a beta version of their software that exposes the 4LO shift tables, but nothing I do there makes any difference. So I wonder if something failed in the PCM with that P2771 code. That code hasn't come back.
Finally, I got the Toyota stock ECT switch to control the Tow/Haul mode (aka Performance Shift) on the GM PCM. The GM PCM needs a momentary ground on Pin 71 Blue to engage/disengage tow/haul mode. However, the Toyota ECT button is a latching switch providing a constant 12V when engaged.
To convert this to a momentary ground on change, I went here:
https://timers.shop/Multi-Functional-Timers_c_15.html and got a programmable relay: “Multi-Functional 3V-18V Time Delay Relay (Timer) V7” which can be programmed to provide a momentary + signal on change of the input trigger (Function 25 Output on Trigger Change, Trigger Configuration 2).
To change the output + signal to a ground signal, I also got their sink module: “Sink Adapter - Positive To Sink”
This is working great. It also works properly to engage tow/haul on vehicle start, if the button was left engaged when you shut it off. I was concerned that maybe the signal would be to soon, before the PCM was ready, but HP Tuners shows it engaging correctly every time.