while driving 110 km the tranny started to downshift on its own all the way to 1 st quite a quick stop. tranny shop said no electrical issue and when I picked it up is started to down shift in about 10 miles when warm. I changed the filter ,oil and shift module as o seals looked distorted. I noticed the tac wavering just before down shift. Now the speedometer is reading less than the speed and bouncing all over, the tranny seems to shift OK but have not gone to far because I was thinking speedo may have something to do with the tranny problem

 

2wd or 4wd?

 

sorry 4 wd

 

From what you are describing I would lean towards a failing speed sensor. Do you have a CEL on? Not sure which TC your blazer has,but here is where the sensors are located. The part time case has only one sensor. The full time has 3 that are all the same. If you have a full time case just remember that the fluid is the AutoTrac type. Good luck

 

Thank you will try speed sensor in April broke down 300 miles away and had to leave it.
What does CEL stand for. Part time as I have to use switch on dash 2wd 4 wd 4wd low

 

CEL = Check Engine Light = SES = Service Engine Soon light

 

Thanks for the follow up Les. The sensor you need is GM part #is 15547452. Shop around as GM is really proud of that part. Lol.

 

Thank you again for your help it was a speed sensor, do not know why 2 different tranny shops did not know right away, 5 minute fix and Dorman parts is suppose to be improved to avoid future problems. The old girl lives to fight another day.