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Discussion Starter · #1 ·
I am hoping that someone here can help me with a problem I have with my transmission. When I driving between 40-50 miles per hour and I go to WOT to pass someone or just speed up, the truck down shifts into the passing gear, but it hesitates and take a few seconds for it to actually engage and for the truck to accelerate.

I took it to my dealer to have it looked at and they told me that it was because of the bigger tires (295\65\20) I have and that they are throwing off the line pressure and that the transmission is looking for more line pressure when making the down shift. They said they reprogramed my ECU with the biggest tire size they could which was the stock 275\60 20. I already had the tire size recalibrated with my DSP, so I can't see how changing the tire size back to stock would make it any better. I did test with both with the stock tune and with the 91 octane DSP tune and it happens with both tunes.

I am fairly mechanically inclined, but I can't seem to make the connection between the bigger tires and the transmission's line pressure being low. Am I just missing something here or is the dealer feeding me a bunch of crap?

Thanks for the help
 

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Discussion Starter · #4 ·
Thanks for the input. The tires are the nitto trail grapples and it happens both with and without the DSP. It is more noticable though with the DSP tune installed. I have the MDS turned on with the DSP, so I can't imagine that is it.
 

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I have a suggestion, ditch the diablo and grab yourself a superchips cortex. With this you can purchase the tranny tuning and torque management and remap all your shift points where you want them and with the torque mang. shifts are firm and precise.I had just the normal superchips 93 tune and it was good but now with the tranny tunning and torque mang. it's a completely different animal and shifts and down shifts exactly where I want. I'm confident this will answer your prayers, good luck.
 

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Yes, the dealer is feeding you baloney about the line pressure. Larger tires will raise your shift points (since they effectively change your axle ratio, and the shift pattern is based on output RPM, not vehicle speed). But target line pressure is based on input torque, not on speed or tire size.

Can you explain in more detail what happens during the downshift? Does it take a few seconds for the downshift to complete (for engine speed to jump up)? Does the trans slip (like it's in Neutral) and then grab? Does engine speed come up, but with no power (and then later it kicks in)? Or does engine speed jump up some at first (then delay) and then jump up the rest of the way (with more power) after a delay?

If it's the last option, then I'd guess you are getting two events (TCC unlock, and trans downshift) that are happening sequentially, with a delay in between. Does it feel like it actually bogs down or drags during the delay? Could be one of your clutch volume index (CVI) values is off and needs to be re-learned. You might have your dealer do a quick-learn procedure (they do this with a scan tool), which resets all your trans adaptives to "factory" specs and re-learns all the CVI values. They could also make a recording of one of these downshifts (using a scan tool) and then inspect that recording to see exactly what's happening.
 

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Discussion Starter · #7 ·
Thanks for the reply Transengineer! To further explain though when it downshifts, I feel it begin the downshift and it takes off little bit, then there is a hesitation and I can feel the transmission shudder, then I feel it engage in gear and it takes off as it should. It does definitely feel like it is bogging down during the shift.

Should I take it back to the dealer and specifically ask for them to do the quick re-learn or would it be better to just take it to another local dealer?

Thanks!
 

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Yes, if you feel a shudder during the shift, then it probably needs to have the CVIs relearned. You could take it to either dealer. To run the quicklearn, they just plug in the scan tool and push a few buttons, so it's hard to screw it up. Drive it gently for a few days after the quicklearn. The quicklearn resets everything and does a "quick" relearn (duh), but the PCM will then fine-tune the CVIs as you continue to drive it.
 

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TransEngineer - I hate to bother you but in reading your other posts, you sound extremely knowledgeable about these trucks. If you don't mind I have a couple noises that you are probably are familiar with or know about. The first is a whining/hum that comes from the transmission between the 1000 to 1500 rpm range. It happens in 2nd and above, won't do it in N or P when revved up. The second noise I am hearing is what sounds like a dry start or overstart when starting the vehicle after it's been parked for more than a few hours. It sounds almost exactly like the brakes do when the vehicle is moving at slow speeds 1-5MPH.

Thanks for your help.
 

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TransEngineer - I hate to bother you but in reading your other posts, you sound extremely knowledgeable about these trucks. If you don't mind I have a couple noises that you are probably are familiar with or know about. The first is a whining/hum that comes from the transmission between the 1000 to 1500 rpm range. It happens in 2nd and above, won't do it in N or P when revved up. The second noise I am hearing is what sounds like a dry start or overstart when starting the vehicle after it's been parked for more than a few hours. It sounds almost exactly like the brakes do when the vehicle is moving at slow speeds 1-5MPH.

Thanks for your help.
Wow... I have no clue on either noise. Whines from the pump are typically at high RPM, not low RPM. Planetary gear whine is usually most noticeable in 1st gear at launch.

For the "dry start" noise, are you talking a scraping/grinding noise (like maybe rusty brake rotors), or a loud, low-pitched drone, or something else? Is this noise when it's still in Park, or when you shift into gear? Doesn't sound like anything I'm familiar with.

You might take it to your dealer and see if their trans tech can listen to it. Maybe he would recognize the noises.
 

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Yeah it is more like the rusty brake rotor sound. I have noticed that when the motor has been running and is up to temp, that it will start really well and rpms are really high and smooth. when cold the rpm and start-up is not as smooth and its as if something is being stripped. Thanks for your input.
 
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