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Re-program an 04 GMC/Duramax for larger tires?

LT4247

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LT4247 Enthusiast

LT4247

Posted
Posted

Have a new 04 GMC 2500HD 4x4 with the latest model Duramax. I had the dealer put on new 265-75-16 tires at time of sale. The change from the 245's put the speedo at about 3% slow. This translates to almost 73 mph with an indicated 70 mph which is verified by a Garmin GPS unit. Not good. Could be the difference from getting a ticket and not getting one! Anyway, dealer says they tried to reprogram the truck for the 265's but said it wouldn't take. They ordered another computer thing and installed it and it did the same thing. They say they did some checking and now have a bulletin that the Duramax trucks can't be reprogramed by them for this situation. Sounds like BS but I would like to believe them. I would also like to have my truck to show something other than

"slower than I'm going" speed. Anybody run into this? My dealer full of prunes?

Any and all comments appreciated.

 

LT in Georgia

TPB Enthusiast

TPB

Posted
Posted

LT,

 

I had the dealer install 265's on my '05 GMC w/ DMax when I purchased it last week. They told me they reprogramed the speedometer, & they didn't mention having any problems. However, I don't have a GPS to verify the accuracy.

 

Todd

c5hd Enthusiast

c5hd

Posted
Posted

I tried to reprogram the speedo with my predator and it wouldn't take either. I tried every which way, nothing. So there might be something to that bulletin Diablo sport said they have never heard of this problem. I am just living with it now. I am not converned about the speedo really, it is easy enough to remember the difference and adjust my speed accordingly.

banshee42096 Enthusiast

banshee42096

Posted
Posted

just got back from dealer.they told me 265-75r16

is as much as they can reprogram for d/a combo

told the shift points change to much with bigger tires

. only on the d/a combo he said .dont know how true this is

good luck

linda Enthusiast

linda

Posted
Posted

well, i didnt send Lyndon my Tranny PCM, so how is he able to change the speedo? Westers has not had a problem with his reprogramming that i know of, so why should the dealer? I didnt think the speedo was derived from the allison PCM...i know for a fact that the VSS (vehicle speed sensor) comes directly (or at least passes through) from the PCM at connector "C1" pin "50".

scematic001.jpg

bigdogs Senior Enthusiast

bigdogs

Posted
Posted

My dealer says the only thing they can program for is what came stock or as an option on a truck. They didn't seem to even want to do 265s. Does the Tech2 go in and reprogram the Allison or does it just change the speedo?

Machine Enthusiast

Machine

Posted
Posted
The Allison has it's own seperate computer and must be programmed seperatly from the PCM.

Hi -

 

Help me understand why my Diablow and Quadzilla are able to reprogram my speedo for 305's. My speedo has been dead-on since I adjusted it for 285's and then the 305's that I am currently running. I've pretty much verified the accuracy via my Garmin 76S

Butch 02 Denali Enthusiast

Butch 02 Denali

Posted
Posted
The Allison has it's own seperate computer and must be programmed seperatly from the PCM.

Hi -

 

Help me understand why my Diablow and Quadzilla are able to reprogram my speedo for 305's. My speedo has been dead-on since I adjusted it for 285's and then the 305's that I am currently running. I've pretty much verified the accuracy via my Garmin 76S

It looks like the speedo doesn't go through the seperate Allison computer by the diagram, so yup, I guess the speedo is programmed in the PCM. I guess all the other functions are programmed in the Allison computer like shift points and shift firmness. Now that I think about it, when Tom from Tbyrne was showing me his truck (8.1/Ally), he did say Lyndon programmed it for his big tires!

Firetiger(MN) Enthusiast

Firetiger(MN)

Posted
Posted

The dealer can't do it, but a programmer like the Quadzilla Stealth can. That's one of the key points in my decision to get the Stealth. I ended up buying the Juice with Attitude later (it wasn't available at the time when I bought the Stealth) and it will show the corrected speed on the Attitude display. Kinda sucks that it doesn't correct the odometer or speedometer in the dash, but it gives me the accurate speed nonetheless (and a lot more of it too :thumbs: ).

 

Oh, and there is a problem getting the speedo corrected on the '01s. Stealth can't do that, but word is that Quad is working on it and has been for some time......

bigdogs Senior Enthusiast

bigdogs

Posted
Posted

The Juice/Attitude does not fix the speedo for bigger tires??? Hmmm I thought that it did, thanks for the heads up. I don't plan on lifting mine (been down that road many years ago with a Blazer) but would like to level it out and put some 265s on it with some more aggressive tread. Will the Tech2 reprogram the Allison PCM as well as the Speedo and ABS computers? Like I said my dealer does not really want to do this so getting any info about this is next to impossible.

bigdogs Senior Enthusiast

bigdogs

Posted
Posted

Well my dog (in my avatar) is a 190lb Mastiff so think he would rather reprogram the truck than that :thumbs: . I've asked them a few times and they said they can't program it (aren't allowed actually) for anything that did not come on a GM truck. If I can find the RPO code the 265s I could maybe see if they will do that.. When the dealer uses the Tech2 does it reprogram just the speedo or does it also go in and change it for the ABS and shift points in the Allison?

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