2003 duramax Allison 1000 p0776 and stuck in 1st gear intermittent

Trae

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Apr 7, 2021
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2003 Silverado 2500hd extended cab short bed. 240k miles. One day I pulled out and it shifted from first to second then slammed into first and was stuck in first for the rest of the drive. Hooked the tech2 up and got codes ; p0776 and p1621 and service brake system came on. So I figured the trans was on the way out so I bought a used built trans and before I put the built trans in I put all new speed sensors including the transfer case, I put new NSBU, went through the entire trans harness, even taking the loom and tape off. Checked every pin and every connection including tcm and ecm. Truck is still having the same issue. I’ve had the harness on and off a hundred times hoping I missed something. But it does it intermittently. But when it does shift up through all five gears it is harsh and downshifts aren't pleasing either. I have reset all taps and have tried fast learn. Nothing works or helps . Fluid level is perfect and new along with both filters . I also replaced internal harness. And also replaced tcm. I’m at complete loss , hopefully someone has the cure. Mike L hopefully you can help
 
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Chevy1925

don't know sh!t about IFS
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Oct 21, 2009
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What are all the codes now? We need everything including abs codes. Bet it’s a t-case issue
 

Trae

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Apr 7, 2021
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Current codes are p0776 p0571and one time I got U1301 but It may have been from me unhooking plugs. Then sometimes instead of getting stuck in first it will get stuck in 2nd then the code p0746 came but mostly I get p0776 more often. Never had a abs code ( also note that I bypassed the abs. Still hooked up tho)
 

Trae

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Apr 7, 2021
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These were all taking at idle in parked or in drive with foot holding on brake at dead stop. Do they look normal
 

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Trae

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Apr 7, 2021
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So I’m thinking it’s my transfer case , because the output shaft has play up and down and in and out … but no pump rub abs zero leaks . The output speed sensor looks as if it’s been rubbing on the gear . So I ordered a reman tcase and it has the same amount of play. Are they supposed to have play like that? Any info will help thanks
 

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