I flip a switch, and then it goes into "manual" mode. When I press the - button it shifts to 4th gear, then when I press the - button again it shifts to 3rd gear and the PRND321 display changes to highlight "3". Press it again, and the PRND321 display changes to "2".
It does not show the factory-style 2006+ tap-shift display in the DIC.
what I did was build a mini computer/logic circuit that mimic's the NSBU and basically just fools the TCM into thinking you physically moved the shifter.
The allison is a completely electronically controlled transmission; once you move the shifter out of park, basically nothing physical happens in the valvebody (unless the trans is in hydraulic-default limp). The TCM relies completely on the NSBU to know which position you moved the shifter too.
If you want a demonstration that you can do on your own truck, go drive down the highway and shift into reverse. Nothing happens; the trans just goes into neutral.
Or sit there in a parking lot and floor the gas pedal in neutral and do a "neutral drop". Once again, nothing happens, the trans stays in neutral, TCM requests a torque reduction, the ECM overrides the what your right foot is doing, brings the engine down to idle, then says to the TCM "ok you're good to go, ive taken control away from this idiot driver", the TCM then engages "drive", and then the ECM lets you have throttle control back.
Its a fun trick to mess with tailgaters; if someone wont get off my ass on the highway I just throw it in reverse. The reverse lights still do come on even tho the trans just goes to neutral, so they dive for the other lane pretty quickly.
Thats all because yes, you did shift to "reverse" or move the shift lever, but because with an Allison (thats not in hydraulic-default) you arent actually physically changing anything or redirecting any fluid, the TCM has the final say in what the trans actually does.
Now if the trans is in limp mode, then the TCM cuts ALL electrical power going to the trans. The solenoids are setup in a specific normally closed/normally open logic so that when they all have no power going to them, it IS possible for fluid to flow so the truck can still move under its own power if there was a complete electrical or TCM failure. The shifter is connected to a manual valve that can actuate 3rd gear, reverse, and park/neutral when there is no power going to the trans. This is called "hydraulic-default"; the trans is not being controlled electronically; it is being controlled with a manual valve connected to the shifter like other automatics. This is "limp mode". You only get third gear and reverse.
Thats why, If the trans is in limp mode with the check engine light on/stuck in 3rd gear, do not try to do the fun neutral drop or clever "reverse at 40mph" trick because IT WILL WORK!
So when I flip the "manual" switch, my circuit disconnects the NSBU on the trans and "plumbs in" its own logic generator to the NSBU signal wires that go to the TCM. Kinda like a rail-road track switch, except for electrical signals.
yes, it only costs about 30 dollars in electronics parts, plus the 37$ shifter (I know, I was surprised it was that cheap too, you'd think with GM's ridiculous parts prices it would be like 300 bucks). But it is a LOT of work to wire it in and takes forever to solder together.
If you want the techno mumbo-jumbo...You are basically designing/building a simple computer with logic gates, up/down counters, voltage comparators, and debouncing circuitry. I also used an ELM-415 human interface chip to operate the up/down counter and BCD to decimal decoder. And finally 4 mini DPDT relays and 1 4PDT relay to do the switching of the NSBU signal wires.
If anyone knows about programming micro-controllers, it would be VERY simple to make this work in a small fraction of the time and using much less parts. I dont; so I had to build it/figure it out the complicated "old fashioned" rube-goldberg way.
I never did (and probably never will do) a DIY how-to thread because this isnt just wiring in an aftermarket stereo. The amount of PM's and help-me-it-doesnt-work-what-did-i-do-wrong questions I would receive would drive me on insanity..
ben
