We have been having trouble with our 4wd on the latest 2001 we bought. The service 4wd light came on a few weeks back when we first tried to use 4wd. When it comes on the 4wd auto lights up even if though no buttons have been pressed on the dash. This has happened both sitting at idle and when driving. At least twice we started the blazer and put it in reverse and the car felt like it was trying to back up with the front brakes on, steering was vary hard also. Today when it happen the 4wd auto lit up on its own again.

Things I have done. Swapped ABS module from first 2001 4x4 blazer when ABS code indicated module needs rebuilt. Everything was good for a few months after this swap. I hope the ABS module isn't something thats needs flashed for the VIN.

Checked for corrosion at large plug on encoder motor. Did not unplug smaller plug yet. Removed TCCM. Cleaned the connections of TCCM and removed wires from plugs to clean. Found corrosion was only on ends were the wire goes in the metal connectors. Looked like the corrosion was bridging the wires across the corrosion on the plastic comb. Cleaned all connections and combs. This was last week. Service 4wd light back on once since then,went away on its own.

Resoldered the pins that go out to the plug in the dash switch.

Borrowed Genisys SPX scan tool today and no 4wd codes found. Power should not have been off to TCCM since light was last on several days ago.

P0101B in history, most likely cause when I left the PCV hose off of intake air duct last week. Service engine light came on, light back off after hose put back.

P0174B in history and current, Bank 2 lean.

B1001- Wrong/used SDM.

Done a lot of reading here and elsewhere, don't seem to see anyone saying the 4wd auto was coming on by itself. Do now see the specs to ohm out dash switch so will start there next.

Thanks for any help you all can offer.
Blazeme