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2004 Gmc 2500 HD 8.1L
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Discussion Starter · #1 ·
I am trying to convert my 5 speed Allison to 6 speed, I've read where it has been done for the 8.1L. I have read every post online that I can find, and the information seems to be contradictory. Today I pulled my pan and valve body out and compared it to one out of an 2006, other than the park neutral switch being internal on the new one it would fit. I have a TCM from an 2006 and my local transmission shop said they can reprogram it. I've read where the TCM operates on different speeds and can't be changed out to a 2006, but I've also read that it can be? One post said the TCM to transmission needs to be rewired, another said only one wire needed to be changed. I'm confused on how to move forward with installing this valve body and TCM, any feed back appreciated. Thanks
 

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2004 Gmc 2500 HD 8.1L
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Discussion Starter · #3 ·

You'll have the same allison as the duramax, no idea about the tcm programming.
Yes its the same T1000 transmission, I know that kits are made for the diesel with valve body and new tcm, but no one makes one for the 8.1 engine, I'm sure it doesn't have the same following as the diesel and the tcm program is much different for the gas engine. Also in 2006 the 8.1 had the 6 speed T1000.
 

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2004 Gmc 2500 HD 8.1L
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Discussion Starter · #4 ·

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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No, IDK anything about what's in that kit. From what I recall of reading about it on another forum, Allison rejigged the transmission a bit to make it a 6 speed, and people figured out what Allison changed and then put those changed bits into their transmission (and reprogrammed the tcm I think). I don't believe Allison made/supported these conversions.
 

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2015 High Country Duramax
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Might be worth emailing the guys at calibrated power.

See if they can tune for the 8.1. The trans is the same as the diesels except for the torque converter, so as long as they can tune foe the correct shift points so it's happy behind a gas motor, their kit should work.

 

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2004 Gmc 2500 HD 8.1L
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Discussion Starter · #7 ·
Well, finally might have some clarification on how to update my 5 speed without buying the entire kit from custom store like Suncoast. Nobody does one for the 8.1 anyway.
Have the newer 2006 valve body installed, you do need the the internal wiring harness so that you can change out ends because the solenoid connectors are not all the same. My truck is a 2004 which means it already has the G solenoid, but if you have and earlier than 2004 will need to get a internal harness with the G solenoid wiring. So far quite straight forward, swap out valve body and make 2004 harness fit new 2006 solenoid connectors by soldering.
Now what I have not understood from other posts is my TCM, since it is programmed for a 5 speed that is all it can do, but evidently it has the ability to be a 6 speed. Now I've checked in my town and nobody will do it, all they can do is download the program from the vin number, what is needed is someone who uses Efilive or similar program to be able to go into the TCM and actually enable the 6th gear engagement programming. This does not have to be done on the truck but only the TCM is needed for this part. Places like Suncoast have the TCM already programmed for the Duramax but not for the 8.1 and they will not do it, I have been told the 8.1 can't pull the 6th gear because it does not have enough power. I have found someone who lives out of state that knows how to use Efilive and has done the conversion on an older Duramax and he is confident he can update my 8.1 TCM so this is my next step is to send it off to him.
Tonight will install the new valve body, deep pan and fill up with oil and will see if my new valve body works through all 5 gears, then after camping season will pull TCM and ship off to get programmed.
 

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I can't hardly believe the claim that the 8.1 can't do 6th due to power. 6th gear isn't that much lower than 5th on the allison 1000s,band the 8.1 has plenty of low end torque.

Almost wonder if someone like black bear has messed with the Allison's at all. I would think one of the good diesel tuners could get it dialed in failry well with some trial and error. Especially since the 8.1 has a decent power curve that isn't a whole lot different than the old diesels. Obviously is different, but has it lower than the 6.0 does.

Did you reach out to calibrated power? Not sure I they'd touch it either, but might be worth a try. The Allison's are a super universal trans so I'm sure someone out there has tuned these for a gas engine. And come to think of it, the new International school busses use an allison 2000 (same as the 1000 for the most part) behind the 8.8 gas which is sort of based off the GM 8.1. So they may be able to use a base file off those to make your tune. Wouldn't be able to use it directly since they use the new 5th gen TCM, but they could at least look at the shift points and all and sort of copy that over for the older tcm
 

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2004 Gmc 2500 HD 8.1L
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Discussion Starter · #10 ·
Well and update:
came home and installed the 2006 valve body with the 2004 wiring harness. filled up with 4 gallons fluid, (wow) check engine light came on, so drove it around the block to make sure it wasn't something like air in system or something. Cleared the codes and then when I got home check them again and 30 stored codes for tcm. I"m sure its something to do with wiring but not sure how to proceed.
p0711,p0717'0721,0722,0731,0732,0733,0734,0735,0736,0741,0742,0763,0768,0773,0847,0870,0871,0872,0876,0747,0777,0746,0776,02810,0703,u1096,0743,2771,0880
Its like everything is wrong, checked fuses all ok. double check connector going into transmission, seated good. Kind of wondering if the internal harness is not matched up the same from 2004 to 2006 so all the electrical in the pan is in havoc?
Well crap sure hope it's something easy as have a camping trip planned with 5th wheel. really don't want to have to pull apart and try and resolder the wiring harness again for the old valve body.
 

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2004 Gmc 2500 HD 8.1L
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Discussion Starter · #11 ·
after sleeping on it, I forgot to swap over the electrical module on vb that has the 12 pin connector, hopefully didn't hurt my tcm. 3 steps forward 2 back.
I had 3 gallons of transynd for this job, well had to buy another gallon last night and I was still about a quart low. so looking like 17 quarts when pulling vb with a deep pan and new spin on filter. O'reileys had 3 gallons, $57 each, ouch.
 

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2004 Gmc 2500 HD 8.1L
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Discussion Starter · #12 ·
That's comical... my '06 pulls 6th perfectly fine. :ROFLMAO::LOL:
I've called every place I can find online and the answer is always the same. Even reached out to killer bee and same response..
Me -- Hello,
do you have any TCM for a 2004 2500 hd trying to convert my 5 speed to 6 speed, I have a valve body already out of a 2006, I believe I can use my wiring harness would just need to install proper ends for the solenoids. Was reading from forum Duramax from panserwillie but his was a diesel and he purchased a tcm from you for his truck. I'm assuming the tcm is from that year vehicle and just reformatted, or would any 2006 tcm work for this update? So I'm trying to find a tcm to fit an 8.1 gasser.

Thanks

Killer Bee -- Hi Daryl. To my knowledge there is no 6-speed upgrade TCM for the 8.1. We do not offer one, mainly because the performance needs of the towing vehicle are not supported at lower rpm in a normally aspirated gas engine.

Me-----In 2006 the 8.1 had the 6 speed, and I have read where a couple of people have ungraded to the 6 speed. For towing it probably would not work with big trailer but with small trailer pulling an atv don't see why it would not go into 6. Is there anyway to get a tcm set up for this truck? I'm assuming it has to be for a 2004 era and not a 2006, I've read where they operate differently and are not compatible with the older trucks. So then are you saying that it's not available because no one makes the correct TCM with programming for the older style tcm with a 6th gear option?

Killer Bee------The 01-05 AL5 was never produced with a 6-speed operating system for the 8.1. The 06 and on models were.

That was our conversation, so unless he is saying that the tcm cannot be reprogrammed on the 8.1 because it is different from the Duramax, (thought they were exactly the same, but I'm assuming from what have read on these forums) I'm not understanding the logic?





 

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For fluid check truck shops next time. I work for international and we sell transynd for $37 a gallon. Allison spin on filters are $7. That's what I used for mine initially. Now it's even cheaper since I used regular old dex3 fluid
 

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Discussion Starter · #14 ·
Well came home and swapped out the pressure switch assembly on the valve body. Now have the dreaded P0872 code which looks like stuck E valve. Guess will pull VB off tomorrow and clean valve body especially the E valve. Anything specific that I should look at or for when pulling it apart? Should it be cleaned with brake cleaner and blown out?
 

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Discussion Starter · #15 ·
What an adventure this has been, Saturday pulled valve body back out after getting the p0872, looked up what the most common issue that causes it and pulled my vb all apart. The E valve did not freely slide out of bore, so cleaned and went over both leading and trailing edges with a scotch brite scubby ow so gently. Finally it would slide in bore freely, so looked at valve next to it and did the same, they really were not bad and did not see any debris in the vb. I was skeptical if it would help so put all back together. Then turned over vb and there was this little screen filter, ow crap where did this come from? looked online, nothing. Split vb again and found it's home, put back together retorqued, installed filled, started and on comes my light again. p0871 what the hell, I was ready to throw away the newer valve body and go back to the old one. Drained and pulled pan again, looked at my wiring and one of my connectors was not plugged in. Refill started up very skeptical again, but this time no light. Put in gear and clunk in R and clunk in D, so very carefully went out and drove, it shifted terribly so drove all over town and it learned and not its pretty good, still hasn't relearned but it's running again. Whew.

Take aways when trying to convert to 6 speed, have to use old wiring harness and pressure switch, swap out the connectors, soldering them on the solenoids that old wiring harness does not fit onto the new. (not much of a take away but I had to pull mine out a few times to get work, if you are going to do this and use a deep pan like I did then plan on 4 gallons of fluid, it's cheaper to buy online than at local store.)
Once I get tcm done will repost on how it goes.
 
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