Stephen,
Thanks for the reply. I have done a bit of internet research on this over the past couple of days. Here is what I found out. Remember this is internet knowledge. If I am wrong about something I hope somebody will correct me. I did not look into the old TBI computer logic, so this info may not apply to those vehicles. I do want to correct myself about something I posted earlier. I believe the switch on the shift rail of the 91 NP205 case is only used for a light in the dash and not used for a signal to the TBI computer. I have seen the switch on much older 205's. Also this may not apply to aftermarket trans controllers. My K5 has a 2004 8.1 with a 4l85 trans.
For the newer LS stuff with a 4l60/4l65/4l80/4l85 transmission, there is a speed sensor on the trans post torque converter and at the trans output for 2wd or at the t-case output for 4wd. The sensor at the 2wd output or the 4wd t-case output is only used to verify the trans internal clutches are not slipping and for the speedometer. The signal is compared to the post tc signal and the trans knows what gear it is in. Transmission shift points are based on throttle position, engine rpm, and mph. When the transfer case is put into low gear, a switch lets the computer know it is in low. The transfer case ratio is programmed into the computer, and it can continue to monitor internal clutch slippage and provide a correct mph signal to the speedometer. What it also does is use a low range shift table. Shift points have been programmed to work properly with the lower transfer case gear.
On these computers/transmissions, if you add additional transfer case ratios you must use a speed sensor at the tailshaft of the transmission. If you use a speed sensor at the transfer case output, you will not be able to program the computer for the additional ratios and the computer will go into limp mode because the input shaft rpm vs the output shaft rpm will not be correct to the computer. As far as I can tell, you can only program for one transfer case low gear, and not 3. Maybe if you are a computer geek, you could figure out how to add additional shift tables selected via switches on transfer case levers. If you can do this, let me know.
If you use the speed sensor at the tailshaft of the transmission, and you do not ever tell the transmission that the transfer case is in low gear it will work, but may not work to your liking. The reason I say not to your liking is because it will use the same shift table that was used for high gear in the transfer case. In low gear, the mph signal will be incorrect for obvious reasons. Remember, the transmission shift points are based on throttle position, engine rpm and mph. In order to upshift, the computer must see a pre-determined engine rpm and a pre-determined mph. Those values will change with different throttle positions. So, the way I see it, the mph requirement for an upshift will be met sooner than expected, but it will shift when the engine rpm reaches the preset number. My guess is that gm programs the trans to shift at higher rpm in low gear depending on the throttle position. If I were to use a speed sensor in the tailshaft of the trans, I would let the computer know I was in 2:1 (original doubler) or 2.7:1/2:1 (magnum) by tripping a switch on the transfer case shift linkage. The actual ratio programmed in to the computer would be 1:1 for this. This would allow for the use of the secondary shift table. Then for the 4:1 (original doubler) or 5.3:1 (magnum), I would shift the trans manually. The way I see it this gear would only be used for real technical obstacles, so manual shifting would be preferred anyways???
Here is my dilemma: my 4l85 does not appear to have the speed sensor ring installed. Somebody tell me I am wrong. I took this pic today of the sensor hole after I removed the plastic plug:
Not exactly sure how I would install a sensor ring, but I am pretty sure I don't have the correct tools to remove the guts of the trans myself. Does anybody know how many labor hours would be charged by a shop to do the work, and how much the sensor ring costs? At this point I am seriously considering the Lomax instead of a doubler. I am looking for opinions from people who actually wheel their truck to tell my why I shouldn't go with the Lomax. I live in Colorado, so I will be wheeling lots of rocks. My truck will be full bodied and I don't want to rip it to shreds, so I wont be doing any trails that would do that. I have 5.13 gears in the axles, and 39.5 swampers that measure out to about 38.5". Stephen, I remember you guys were Lomax distributors. Do you know anything about the limited December production run they are doing on this case and gears? Do I need to order this month in order to get one? Do you know if the case and gears are made in the USA? I already have a NP205 tailhousing with sensor, so this swap would be easy. The 3:1 of the Lomax could be programmed into the trans computer, and it is very close to the 2.7 that the computer is now programmed for. Low gear shift table should be very close to ideal.
I appreciate any info. Thanks guys.
I'm not sure how smart the pcm actually is and how that varies through the years. I imagine newer computers are smarter than old ones but I don't know for sure if they really know that 2:1 or 4:1 are not 2.7:1. It's very possible we're overthinking this.
What we know works is running the trans with the output drum speed sensor and never telling it when we go to low gear. This is the system we use in our "Cousin Willard" crewcab with a compushift controller and it works fine. We do tend to shift it manually more than others might when we're offroad and the controller does have a full manual mode that we use sometimes.
I do have a couple customers (that finished and drove the projects and got back to us with some results) that have run the computer totally off a tone sensor in the back of the 205 and report no problems with the trans controller freaking out over the low ratios not matching up.
Our safe answer has been to run the speed sensor on the back of the trans and just never tell it that the t-case is in low gear. On a 4L60 you have to use an adapter with a speed sensor port or add a speed sensor port in the back of the trans to do this. Or use a manual controller.
