Working on a 1993 Full size Blazer and replaced the heater core. I did not mess with the drivers side dash as the repair book suggested for some reason.
Heater core is in and now the truck will not shift and I lost the speedometer and odometer.
Everything was working fine before the heater core and the auto transmission gave no signs of a problem.
Someone suggested I look at the VSS buffer to see if I moved a wire when doing the heater core. I have power there and I ran wires down to the transmission sensor and hooked them into the buffer to see if the harness was bad. No change.
I suspect something like a vacuum line is the problem but have looked everywhere with no problems seen.
The truck starts and runs fine, the auto trans will take off in OD but will not shift.
If I move the lever to 2 it will shift and the 3 and OD position it just revs.
I did use a paper clip on the OBD connector and it flashed code 16
1993 K-5 Blazer 4x4 5.7 Auto Trans
Any Clues

 

There is nothing vacuum operated about the speedometer/odometer. You have an electrical issue somewhere. The VSS sensor is used by all of the components that are inoperative (speedo/odo/trans shifting). I would bet that there is something disconnected or possibly a bent pin on a connector...

Did you remove the ECU when you replaced the heater core?

 

No, I did not have to remove the ECU as it was above and to the right of the heater box.
I ordered a new VSS that should be here this afternoon and hope that might fix the problem. Your deduction of the no vacuum problem is true and I have look at every wire I can see.
One thing you might know about, the heater core leaked anti freeze on the pass floor so I took up the carpet and side kick panel cover to clean up the mess. In the panel where several clusters of wires that I have no clue what they go to. One of the clusters of wires has 6 connectors on it and it was starting to get the green corrosion so I cleaned it as well as I could. Any ideas what wires would be in that location.
I appreciate your reply. I have read thousands of posts in this group and learned a lot from the numerous posts you have made. I wave my flag at you because your an asset to the automotive do it your selfer Thank You & Happy Thanksgiving

 

Replaced speed sensor on transmission and no change, still no shift, no 3&OD gear,no speedometer