Allison 6sp: 2wd to 4wd Head Scratching

[THEFERMANATOR]

There is a fixed yoke kit available for the front output shaft on the 261/263, says it's available from PRO-COMP. This site may have some useful info for you, he's building a similiar rig with an LLY for pulling.

http://www.lonniesperformance.com/82chevyduramax.htm

 

[othrgrl]

I'm not disagreeing with Mike or Ben but just throwing this out there for reference and more for people interested in running a non-stock or divorced t-case:

If you use the trans output shaft for the speed sensor it will shift fine in 4-Low. Your TCM is set up to shift based on speed at part thottle and based on speed and RPM at WOT. If you get the tuning right for 2-High it will work fine for both with no extra wires to hook up to the TCM. Your speedo will read about double what you are actually going and your ECM/TCM will also think that you are going double your actual speed. For example in 2HI WOT you would shift into 2nd at 3200 RPM and 18mph - in 4Lo you would shift into 2nd at 3200RPM and your speedo would say 18mph but you would only actually be going about 9mph.

All that being said if you can easily hook it up so that the SS is at the t-case output and tell the TCM when it is in 4WD I would do it that way. Also looking at the TCM tune from the 2WD LMM that we just did it has all the 4WD tables and parameters in it and matches up with the tune from a 4WD LBZ I've tuned so your TCM should be just fine.

 

[Big Block 88]

There is a fixed yoke kit available for the front output shaft on the 261/263, says it's available from PRO-COMP. This site may have some useful info for you, he's building a similiar rig with an LLY for pulling.

http://www.lonniesperformance.com/82chevyduramax.htm

You beat me to it.

 

[duratothemax]

I'm not disagreeing with Mike or Ben but just throwing this out there for reference and more for people interested in running a non-stock or divorced t-case:

If you use the trans output shaft for the speed sensor it will shift fine in 4-Low.

I realize that. But its just nice to have access to the different 4wd low shift points and converter lockup, etc... Why is this such a big argument, there is no reason/advantage to run the output speed sensor this way (pre-transfercase, or some other botched setup). No matter what transfer case you are running, stock, NP205, BorgWarner, whatever.....mount the speed sensor in the stock fashion, and adjust the TCM tables accordingly. Win-win-win situation if you do it this way, everything works the way its supposed to, you get the 4wd low tables enabled, etc...

You guys are all making way more work for yourselves and way overthinking all of this, when the simple factory solution is already in place, and works perfectly fine (once you change the TCM tables in efilive). The original question was how to set it up in the easiest way and make it work seamlessly, just like the factory setup. I provided the answer, how to enable 4wd low on the TCM (ground pin 42) and now people are STILL discussing how to do it differently as if there is a better way to set it up than like GM did from the factory............

and as a bonus, the last of the NP205's (early 90's GM HD's) even had a factory VSS with the correct 40 tooth tone ring...so you literally have to do ZERO modifications. Completely plug and play.

 

[SmokeShow]

Mitch why would there be ANY reason to not use the stock speed sensor location on the NP261/263 case????? Its a no brainer to make it factory...

if its a manual shift xfer case it will even have the proper switch on it to put the TCM in 4wd low mode as well......or you can just use a toggle switch as I stated before

That's what I've decided to do and for that reason. Thanks for your help in deciding that for me. :beer


I was just agreeing with the others that there could be a "back door way" to do it with the ss on the output of the alli using the t/h mode to achieve suitable shifting when it's in 4lo. No longer considering that method though. Just understanding that it's a possibility I guess.

 

[SmokeShow]

There is a fixed yoke kit available for the front output shaft on the 261/263, says it's available from PRO-COMP. This site may have some useful info for you, he's building a similiar rig with an LLY for pulling.

http://www.lonniesperformance.com/82chevyduramax.htm

And do you think that SYE kit is the cure to the situation you were referring to earlier about slipping off the splines because I'll be running an SFA???



Yes, I'm aware of Lonnie's build. He and I have spoken via this site. He ahs a really nice build thread going on about it right now. I didn't, however, realize he was building it for pulling. I thought it was more a show truck with some more just looks - it'll have plenty of power as well? Either way, he is doing a far better build than mine will likely be. It's nice!

Thanks again for the suggestions. :beer

 

[THEFERMANATOR]

I had read in one of his threads on another site that he wanted to do some pulling with it as well as show it. The slip yoke eliminator would eliminate the problem of travel as you could get a driveshaft with extra long splines in it to work with the added travel.

duratothemax: You say you have to ground the TCM pin on the ALLISON TCM? I believe my 95 powers from it's switch. I'm just trying to iron out all the details before I embark on my swap. I'm hoping to go look at a donor tommorrow.

 

[pugster]

I have a four wheel drive 1000 and I am looking for a two wheel drive output shaft and housing . I can trade the four wheel housing and shaft I did not get the transfer case with trans. I can be reached at [email protected] please leave phone number and I will call back. Need parts asap as I am building a 47 GMC pug car hauler with a Duramax

 

[Tucsontoy]

2wd to 4wd... update

I thought I would add to this. I had a 2wd Allison that I had Mike L. switch over to 4wd. My conversion included the modified tail housing that Suncoast now has on their web site. After a lot of testing and modifing the TCM trying to see the correct VSS from the tail of the transfercase, I changed dirrection. I had the tail housing on the transmission machined to fit the VSS and added a 40 tooth gear to match it. Now the ECM/TCM is always in 2wd and does not know what is happening after the tranny. My issues were compounded with a four speed transfercase and being able to run in front wheel drive only during some 4x4 manuvers. The only issue in doing this is the speedometer is off in any of the lower gears. So what! It's a buggy. It is correct for the street...and I no long get slip codes. Ask me if you have any detailed questions.
Roger

 

[THEFERMANATOR]

How are the shifts in 4 low? I know the concerns before with doing this was that the shifts may be off in 4 low as it would change the engines load VS speed and make the adaptives go haywire. In theory you now have 3 different trans modes as well if you wanted since the 4 low can be tuned seperately of normal and tow/haul. You could go in and set the transfer case ratio to 1:1 and have a 3rd shift mode on a toggle switch.

 

[Tucsontoy]

If this buggy is in low, the trail is rough and I do not think about shifting gears. I have not done a sled pull, yet, in low where you would actually feel the shifts. I have only used low/low to hold up without brakes on steep down hill trails. So far I have not noticed shifting issue but I'll pay more attention on the next 4x4 adventure.

 

[cwylie]

Bringing a old one back.......

Which pin is pin 42 on the TCM? Does anyone know where I can get the pig tail for the sensor on the NP261xhd transfer case?

 

[cwylie]

Someone here has to know how to locate pin 42.

 

[IOWA LLY]

Once you open up the back of the connector the pin locations are numbered.

Sent from my SM-N920V using Tapatalk