A 1999 Blazer 4 door with a three button 4WD.

Won't shift after the 2WD/4WD shift buttons have been cycled twice. If in 4WD, and I push the 2WD button, the 2WD light flashes once and the TCCM relay clicks. If I unplug the TCCM, everything works perfectly for exactly two cycles, then does the click and flash with no shift. Reading the DTC code for the TCCM I get fault code 3.

I had a spare transfer case, so I swapped the vacuum switch and the encoder motor with no change. Vacuum lines are all good.

I'm thinking there is a fault with the TCCM.

Tim