4L80E problems

Hi peeps, I have a 1992 g30 sportvan 350ci 4L80E transmission. first I lost overdrive then I lost 2nd and 3rd it is stuck in 1st however if I keep accelerating past the shift point it will shift into second and
sometimes 3rd like I have a radical shift kit. I was told there is a fuse trhat controls this but I cant find one anywhere for the trans..no listing on all data either so im wondering how accurate the info was on a 96 its in the fuse box. I was also told that the brake switch can cause this problem but I cant figure out how it would affect the trans so again im wondering how accurate that info is. I would appreciate any info you can give me. Thank you MUDDA
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Needs to be scanned with a trans capable scanner this reveal circuit/solenoid problems as the scanner can command and confirm shifts.

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i had one do this, just exactly like you're describing, & the cause was a faulty throttle position sensor. the sensor was telling the computer that the throttle was open like 95% of the way, so it would never shift out of 1st gear unless you had the pedal to the floor & it sounded like the motor was going to blow..... then it would slam hard into second...... replaced the tps, & all was good
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