speedo gone wild!

I understand not trusting any garages, asking if any one else has seen this problem can be risky too but effective. Just a thought take it to another well recommended shop for a second opinion, but dont tell them it was looked at by another place or what they thought was wrong. See if opinions match, lots of parts changers these days ecms are to high to throw out a hell mary.

 

from speedo run extra ground under dash, go 2 j/yard get extra one. I had W/star jumpin 4 years then Quit one day. you can send off 4 repair

 

It may sound a stupid question, but does your tank have baffles? Does it do it when your empty of just full or both?

If no baffles it could be the surge of liquid causing it.

 

i'm NOT getting this?

 

Baffles are plates with holes in either top to bottom or top to 3/4 down a tank. depending on the tank size will depend on the amount of Baffles. Average baffles are 3. When a Vehicle brakes the liquid surge will push the truck forward and backwards , the baffles stop the surge but allow the liquid to move within the tank.

Container Tanks don't have Baffles, so leaving a bigger space between vehicles and breaking sooner than normal should be a standard operating procedure. Multi fuel tanks don't generally have baffles, but each tanks is divided in to leak proof compartments.

 

mine did that some time ago. ended up being the speed sensor on trans. going bad. it corroded from inside.

 

check to tone ring and yoke, make sure they are tight, next would be connections from the speed senor at trans onto the ecm, if ok check the sensor as said before can be going bad, if it still is acting up run new wiring from trans sensor to ecm with shielded wire. we have seen lots of issues like you described from the wiring picking up outside signals from oil lines or rubbed bare spots and air lines (ie speedo jumps when range shifting trans when parked), the shielded wiring has always worked for us.