Allison 6sp: 2wd to 4wd Head Scratching

[IOWA LLY]

The main engine/tranny harness only connects to the "transfer case" harness with one connector. (two wires) I am pretty sure all those two wires are for is output speed. The ECM/TCM knows when your in 4-hi or 4-lo because there is another harness that runs from the fusebox down along the frame to the transfer case.

I see no reason why it wouldn't work to simply put the sensor somewhere that would read the output speed of the tranny and just forget about all the rest.

But if you want to do it right you will need the additional harness to go to the fuse box.

 

[Josh2002cc]

I have a spare wire harness you can have for free if I can find it.

 

[SmokeShow]

The main engine/tranny harness only connects to the "transfer case" harness with one connector. (two wires) I am pretty sure all those two wires are for is output speed. The ECM/TCM knows when your in 4-hi or 4-lo because there is another harness that runs from the fusebox down along the frame to the transfer case. even in a manually shifted transfer case? under hood fuse box?

I see no reason why it wouldn't work to simply put the sensor somewhere that would read the output speed of the tranny and just forget about all the rest. To clarify, do you mean you think the SS only needs to be on the output of the allison and not on the output of the transfer case? If so, how or would, I be able to use the 4wd tables within the TCM tuning?

But if you want to do it right you will need the additional harness to go to the fuse box.

Thanks.



I have no doubts that Mike knows as much as there is to know about these things as anyone and if he says put it on the output of the xcase then most likely it probably needs to go there BUT, I'd like to know why he says that. It's not a personal thing or that I don't trust him or that I'm trying to be rude by "questioning him". I'm not questioning the validity of his statement (which I could see as being taken as rude but that's NOT what I'm doing), but rather just to understand the logic of the allison and why it would need to be there instead of somewhere else like the output of the alli or on the rear axle or wherever else someone could dream up to put the thing. So mike, if your absense from this thread since your initial reply is because I "questioned" you and think I'm some rude ass now, that's not the case. I just would like to understand why. Would you mind answering that? If not in public for some reason, at least in private? Over the phone?




Josh, if you find the extra harness, that would be AWESOME! Let me know and I'll get you a shipping address and money for shipping it to me.



Thanks guys!!

 

[THEFERMANATOR]

I think this post from another forum will help out on this subject. Yes you can put the OSS in the rear of the transmission with a transfer case mounted to it. The OSS when mounted in the rear of the transfer case works just like it does when in the rear of the transmission. The TCM simply needs to know what the ratios are. So if you do it like the link shows on page 3 then the trans will shift normal all the time, and needs no input as to when it is in 4 low. If it is in the rear of the transfer case, then you have to hook-up the 4 low sense wire to the TCM. The stock 261 uses a 3 wire switch on it, one wire is power another one engages the front axle when in 4X4 and the last one sends a signal to the TCM and ECM when you engage 4 low so the TCM knows to account for the 2.72:1 gear reduction. Obviously if you use the DMAX guage cluster, your speedo won't read correct when in 4 low.

http://www.pirate4x4.com/forum/showthread.php?t=633965&page=3

I'm in the process of doing a DMAX swap into a 95 SUBURBAN, so I've been doing ALOT of research into these areas. And according to a swap check, the 2 wheel drive and 4 wheel drive TCM's are the same. And I'm not certain on the later models, but EFILIVE will program in different low range ratios into the TCM on earlier TCM's.

 

[SmokeShow]

You'll see that I've got the last word in on that thread. Still never have heard back from the guy on whether its working or not. Tried PMing him... nada.

Thanks though. Your logic makes since to me.


I'm still really interested to hear what Mike's reasoning is because I respect him and his knowledge as he is the smartest on these Allisons that I know to get in touch with and he says no, put it on the output of the xcase. Just would like to know why that is.

 

[Mike L.]

I think the only problem you will have taking your output speed signal is when the TC is in 4Lo. The TCM will be reading signal off output shaft on Alli and not the 1.96 ( or whatever ) on TC. According to Ben, Duratothemax, you can address this with EFILive but you will still have to let the TCM know when you shift into 4Lo. This can be done with a switch to you energize when you do this.

 

[duratothemax]

correct.

Pin 42 of the TCM connector is the 4wd low signal wire. When its grounded, the TCM switches to 4wd low mode and then uses the 2.72 (or in your case, 1.96 because thats what a 205 low range is) as a multiplier to calculate the proper output speed reading.

basically, mount the output speed sensor on the output shaft on the xfer case (same as its setup from the factory). Go into the TCM file in EFILive and change the xfer case ratio to 1.96. (D5061) Then it should work perfectly.

thats my final answer. It will work

ben

 

[THEFERMANATOR]

I'm unfamiliar wit how the ALLISON TCM's logic works, but the only issue I could see with having the OSS on the trans output shaft instead of the T-cases(besides the speedo) would be the torque shifting tables(or whatever they are called) inside the TCM. The TCM needs to know output shaft speed of the transmission so it knows gear ratio of the transmission. When in 4 low it has to multiply by the low range to get the correct ratio. If you have the speed sensor in front of the transfer case, the TCM would think it's in 2WD all the time. This would be no different than those that have put the ALLISON(or any of the GM electronic autos) with a divorced transfer case and left the transmission as a 2WD unit utilizing the OSS in the tranny. I know ADVANCE ADAPTERS reccomends putting the speed sensor in front of the transfer case when utilizing a GM electronic auto.

The issue with the torque shifting tables would be that when in 4 low it would take less engine torque to push the vehicle and the tTCM may hold the shifts out to long because of the decreased torque required. The TCM simply needs to know output shaft speed, so if in 4 low the TCM has to be notified. If it is reading actual output shaft speed all the time, then there would be no reason for the multiplication factor when in low range. I know this is how DODGE does it with there transmissions as the RE's had the OSS mounted in the tailhousing of the tranny in both 2 and 4 wheel drive applications.

 

[duratothemax]

this is different than a dodge.........

do it the way I explained and it will work fine.

 

[SmokeShow]

Thanks a lot guys. I understand why it won't work on the alli output. It's like mike said, because in 4lo, the tcm would think its spinning the tires at say 40mph but do to downstream gear reduction the tires would only be spinning, let's just say 20mph ( I don't know exactly what they'd spinning with doing some more calcs but not necessary for diminstration purposes). So it would be shifting the trans way too early when in 4lo. So nope, ss on the alli output won't give good results in 4lo.

I've decided not to go with an np205 anymore. Too expensive and impractical to make one hold. Will be using an nv261 or 263 from a d/a truck and switching the front diff to a ford d60.

Thanks a bunch guys!

 

[SmokeShow]

correct.

Pin 42 of the TCM connector is the 4wd low signal wire. When its grounded, the TCM switches to 4wd low mode and then uses the 2.72 (or in your case, 1.96 because thats what a 205 low range is) as a multiplier to calculate the proper output speed reading.

basically, mount the output speed sensor on the output shaft on the xfer case (same as its setup from the factory). Go into the TCM file in EFILive and change the xfer case ratio to 1.96. (D5061) Then it should work perfectly.

thats my final answer. It will work

ben

just to be sure, that's the correct pin on the LBZ?


Ican put this on a rocker switch or something so that when I pull the lever back into 4lo, the "2wd" tcm will start reading and acting according to the 4wd tables or do I need a 4wd TCM???



Thanks again guys! Feel better about this now.

 

[Mike L.]

Thanks a lot guys. I understand why it won't work on the alli output. It's like mike said, because in 4lo, the tcm would think its spinning the tires at say 40mph but do to downstream gear reduction the tires would only be spinning, let's just say 20mph ( I don't know exactly what they'd spinning with doing some more calcs but not necessary for diminstration purposes). So it would be shifting the trans way too early when in 4lo. So nope, ss on the alli output won't give good results in 4lo.

I've decided not to go with an np205 anymore. Too expensive and impractical to make one hold. Will be using an nv261 or 263 from a d/a truck and switching the front diff to a ford d60.

Thanks a bunch guys!

I was not ignoring you through this post. I had some opinions of my own amd called my experts for theirs and we had a bunch of different opinions. I decided they were wrong and called Ben to get his opine and we settled on the above answers. I have never done this swap, yet, but am anxious to help and get involved.

 

[THEFERMANATOR]

If going with a SFA, I would opt for a 241 transfer case. You might not get enough travel in the splines in the transfer case on a 261/263. The 241 from an earlier CHEVY uses the same output speed sensor arrangement that the 261/263 does, and also utilizes the 3 wire switch for 4 wheel drive. And from what I have researched, a late model DODGE input shaft from a 98+ NV4500 can be had in the 29 spline configuration like that of the ALLISON and dropped into the 241 to make it mate up correctly.

 

[SmokeShow]

what do you mean by travel in splines in the transfer case???


thanks again Mike... I'm sure we'll talk some more when I get to diving into this thing.

 

[duratothemax]

what do you mean by travel in splines in the transfer case???
.

hes referring to the front slip yoke on the NP261/263's

 

[SmokeShow]

and there is concern that it might slip too much and separate from the splines due to running a SFA???


wonder if there is a SYE for the 261/263? Kinda doubt that one. and on the off chance that there is one, would it be a legitimate resolve to this possible problem?

 

[05smoker]

correct.

Pin 42 of the TCM connector is the 4wd low signal wire. When its grounded, the TCM switches to 4wd low mode and then uses the 2.72 (or in your case, 1.96 because thats what a 205 low range is) as a multiplier to calculate the proper output speed reading.

basically, mount the output speed sensor on the output shaft on the xfer case (same as its setup from the factory). Go into the TCM file in EFILive and change the xfer case ratio to 1.96. (D5061) Then it should work perfectly.

thats my final answer. It will work

ben

Good info Ben - didn't know you could work around it that way.

Thanks

 

[IOWA LLY]

The main engine/tranny harness only connects to the "transfer case" harness with one connector. (two wires) I am pretty sure all those two wires are for is output speed. The ECM/TCM knows when your in 4-hi or 4-lo because there is another harness that runs from the fusebox down along the frame to the transfer case.even in a manually shifted transfer case? under hood fuse box?


Yes, even a manually shifted T-case has this harness. It is what sends the signal back to let the TCM know what mode its in.

I see no reason why it wouldn't work to simply put the sensor somewhere that would read the output speed of the tranny and just forget about all the rest.

But if you want to do it right you will need the additional harness to go to the fuse box.

I still don't see why you couldn't just mount the sensor on the output of the tranny. I think you could tune the TCM shift points to work fine in 4-lo. However as has already been said you would not have multiple tables to choose from so you might have to compromise between 4-lo shifting and 4hi shifting.

What are you building the truck for? In my original post I assumed you were building a dedicated puller.

 

[SmokeShow]

yes, it'll pretty much just be a pulling truck but I don't want the possibility of driving it on the street from time to time to be out of reach. At least not yet.

Since I think I'll go with the 261/263 xcase, its gonna be much easier to put the ss in the xcase since it's already there rather than putting it on the output of the transmission where there is no provision for it with the 4wd tailhousing. If, for some reason, I did decide to put it on the output of the alli instead, I guess I might be able to adjust the T/H tables to be my "4lo" tables so that when I engage 4lo and hit the T/H button, it would shift appropriately. non-T/H mode would still be suitable for 2wd and 4HI shifting I think; I just wouldn't have a traditional T/H button while in 2wd/4HI unless I reflashed before and after pulling everytime.



C-ya

 

[duratothemax]

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