99 Silverado 4x4

[JIC_Bro]

Hey I have a question, I am having problems with 99 silverados and GMC's shifting weird after tuning these trucks. In 2wd and 4 high it shifts fine but when you put it in 4 low in less than 10 yards it shifts from 1 to 2 to 3 to 4 . If I floor it it does the same, it shifts right away and feel very heavy. If I shift it manually it works like it should. things that I have tried, raising the rpm shifting points, raising the speed and grounding pin 16 and none have worked .Any other ideas of why it would shift that way?

[1364838952]

Okay.. Let's back up a little bit.. You do know that 4 LOW is running through a planetary set inside the T-case to give you a "granny" type gear ratio right? It is PERFECTLY NORMAL for the trans to upshift VERY quickly when in 4 low.. Are you trying to drive this vehicle on concrete/pavement/black top in 4L????? 4L is for SOFT surfaces ONLY. If you attempt to run 4L on pavement, you are going to have severe issues turning the front wheels.. The vehicle will bind, and pop, as the tires are trying to all maintain the same speed, even when they cannot. Trying to drive on pavement in 4L is going to result in some serious binding that may make the vehicle feel "heavy" as you put it because the vehicle will be fighting itself to move. Another thing that really exacerbates this condition is if you have tires that are of different sizes (heights) DO NOT just look at the tire size on the sidewall of the tire and assume that because the #'s are the same, that the tire height is the same.. You need to measure all 4 tires for HEIGHT, and compare them to each other. They must ALL be within 1/2" of each other. If one, or more, tires are of a different height, the vehicle IS going to bind up in 4L and make one heck of a racket when you try to move, or drive, it.

I think, and not being able to see and experience what you are seeing yourself, but I THINK there is not a problem here except you are missing the point that 4L IS A GRANNY GEAR arrangement inside the t-case. Imagine having 10.90's for gears in your rear differential... 4L on your t-case is doing exactly that.. (I fudged the # just to make a point)... In case I was not clear above DO NOT EVER PUT YOUR T-CASE INTO 4L ON HARD SURFACES.

[JIC_Bro]

Thank you for your reply, let me give a bit more detail. I totally understand what you are telling me about driving on hard surface is a bad idea. I am doing this in sand. All four tires are the same type size and inflated the same. To what I am referring is that at 5% tps and under 5mph I'm running on 4th gear, if I try to floor it well it's not going to do much and it won't down shift. When I decided to floor it to see if at high rpm it would change anything (I know I'm giving the tranny a beating so I only did it 2 times) let me see if I can post a scanned to show you what I'm referring to.

[1364838952]

Simple solution: Put it in 4 high if you want to punch it and throw some sand! I promise you it will shift later, and will downshift if you leave it in 4H..

Post the log when you can and I'll look at it.

[JIC_Bro]

Just and update, Well after looking at the harness wire by wire I noticed something that could be wrong but cant pinpoint what it could be. Well when the key is off position and I engage 4low, I test pin 16 and I have a ground. As soon as the key is placed in the ACC position the front differential motor engages and the pin 16 ground stops sending the ground. I cut pin 16 wire and grounded it to see if it would shift correct but it did not. Any Ideas?

[JIC_Bro]

onittoday.hptshifting.hpl