Thank you,I actually purchased this off a friend to fix and sell. Nothing against gm or blazers but someone else will appreciate it more.

 

The P1860 is the main code to address as the others will piggy back on it. Transmission may exhibit symptoms of failing to shift and/or dropping into or remaining in 3rd gear when gearshift lever is in "OD" range. This condition may set additional DTCs P0740, P0753, P0758 and P0785. This may be caused by a poor terminal crimp on transmission solenoid battery feed circuit between instrument panel fuse block and transmission solenoids. Check terminal crimp at bulkhead connector where circuit passes through firewall.

Inspect wiring for poor connections at control module and transmission 20-pin in-line harness connector. Check for bent, backed out, deformed or damaged terminals. Check for weak terminal tension. Check for chafed wires that could short to bare metal or other wiring. Check for broken wire inside insulation. If diagnosing for intermittent short or open condition, move wiring harness while observing scan tool for value changes.

 

I finally fixed it today,yesterday i mocked together a harness and tested it out with wheels lifted and it shifted and all codes are gone.i made a new harness today amd ran it along the passenger side fram rail,under the ac condensor. Its shifting like a new new truck now. So if anyone ever has a bunch of codes,and youve replaced ignition,made sure you have 12v at e wire on external witing harness the the 4l60e. And youve had the transmission out before its quite possible the wires got pinched.so in my opinion easiest solution is make a new wiring harness and simply follow pinouts on pcm and connector.

 

can you send me the pin out for this cuz I have the exact same problem you do and all the same codes

 

See post #10 that I wrote above.

 

Does any of this info relate to a 97 Blazer. I'm new to the Blazer and just bought one and all these codes are showing up
1870
0740
0785
1860

 

The diagrams were specific to the vehicle being discussed in the OP (2000). I do not have access to the pinouts for a 97 to check for differences. If this is a vehicle you plan to keep, you may want to look at investing in an AllDataDIY subscription for it. You'll get these diagrams and much more with a subscription. I was able to find the code '55DIY' that will give you a 1yr sub for $19.95 and a 3yr sub for $29.95. Very cheap considering that you are basically getting the same info that the factory service manuals have plus all technical service bulletins. Or maybe someone else can post up the relevant diagrams...