I have a 96 blazer that runs excellent, but for some reason just recently the speedo does not always work. when the speedometer does not work the transmission is not shifting properly. sometimes i start the blazer, put it in gear, and the speedometer works correctly, and the transmission works fine too. When the speedometer does not work the transmission shifts hard at a high rpm into 2nd, but will not go into 3rd. when speedometer is working the transmission shifts at the proper rpm, and has normal shift. I assume that this must be some kind of an electrical issue as it is intermittant, but I have no idea where to start looking, any advice would be extremely helpful, Thank you

I have been researching this for hours online now, and seems i will first try cleaning the vss on tailshaft of the transfer case. Am i on the right track ? my problem is intermittant, and never happens while driving, just depends on the day, it either starts and works right, or starts and speedo, tranny problems.
more info :
96 blazer 4wd - 4.3 litre - the abs light is on, but has been on since i bought the blazer last year. Also, i noticed in another post that someone had similar issues after installing a new radio. I bought the blazer with no radio, but installed a factory radio out of a 95 s10 sonoma, used all factory harnesses, no wires were cut, however, the radio does not have constant power. when i shut the truck off, the time resets, radio goes back to am station, and cannot save any preset radio channels. not sure if this is related or not though.

 

I could have literally written your first post word for word. I am currently having the exact same experience, but mine has gotten worse and worse and now it will not shift at all and the speedo doesn't work at all either. It used to start working after it had warmed up a bit. I have to get this fixed! HELP!

 

you're on track. Clean the electrical connector to the vss sensor, and fill them with dielectric grease. If that doesnt do it, replace the sensors.

For the radio power problem go to 12volt.com and look up your radio wiring diagram, That will tell you what wire to look for.

 

I had this problem with a '96 not long ago. The PCM was going bad. Replaced it and no problems since.