MiaSSmaro98

iTrader: (9)

Hey guys long time since i have posted,so i bought a 2001 gmc sierra 1500 5.3l 2wd 200k miles.the tranny was stuck in third gear.scan for codes and the 3-2 downshift came up.replaced all shift soleniods,new fuild exc.when you start up the truck and drive off it will shift from 1-2 then 2-3 and stay stuck on third gear.never hits overdrive or come back down to first.the only code i have now is p0785 i believe.read on couple forums it can be my ignition switch or my pcm?need all the help i can get thanks.

RevGTO

iTrader: (12)

Good info in this thread: https://ls1tech.com/forums/automatic...785-4l60e.html

MiaSSmaro98

iTrader: (9)

some good info,but i have already change the 1-2,2-3 up shift and the 3-2 down shift solenoid and still have the problem.

RevGTO

iTrader: (12)

That's the point of a couple of somewhat unclear posts in the thread. He did all that and ended up having to change the ECM.

MiaSSmaro98

iTrader: (9)

i have also read it can be the ignition switch.now dumb question can the neutral safety switch on the transmission have an effect on the problem?since its adjustable?

Big Reade

That was gonna be my guess Rev, computer related.

MiaSSmaro98

iTrader: (9)

i hate swapping parts and hoping it fixes the problem,a remand ecm 120 dollars,factory ignition switch 130 dollars,nuetral safety switch 55 dollars.so i hope some one can help in fixing my problem

RevGTO

iTrader: (12)

I agree in hating to throw parts at a problem, but a certain point, there's nothing else you can do. You have all the information on hand as to what causes this problem. There's not much more to be said.

You've tried the basics (solenoids) and it hasn't worked. I'd follow the example of the guy who did everything you did, then swapped the ECM, and solved his problem.

MiaSSmaro98

iTrader: (9)

alldata was a procedure/test to check if i have correct voltage/ground at the transmission plug.i will check it over next weekend and post results.basically no power bad ignition switch,no ground bad ecm

RevGTO

iTrader: (12)

Sounds like the right way to address it!

MiaSSmaro98

iTrader: (9)

found my problem.the wires going to the main tranny harness basically broke off from the leads on the plug.i had four broken,so i went to the junk yard found a Healthy plug from a 2001 astro van, so i weld and heat shrink the harnees and problem sloved.all my gears work but the truck slips for like a second between shifts what can that be?????????????

MiaSSmaro98

iTrader: (9)

anyone?

mrvedit

Slippage could be due to low line pressure or excessive wear.
Low line pressure can be due to:
Low fluid level
Defective EPC Solenoid
PCM tuning issues
Engine sensor problems (TPS, MAF, MAP)
Worn pump
Significantly worn valve body

You could start by replacing the EPC since a bad one can be difficult to diagnose. Hook up a pressure gauge to the trans and note the line pressure when the shifts take place.

MiaSSmaro98

iTrader: (9)

epc solenoid ?how can i check the line pressure?were can i tap into?

joecar

Line tap is on LHS of case above range selector shaft.

MiaSSmaro98

iTrader: (9)

thank you sir what can of psi should i be looking for?also is there a gauge they sell for this or make my own?