Hello Im new to this form, I have read many posts to try and figure out what the problem is with my F-350 6.0. I have an intermittent problem with my Speedo reading zero while I'm doing 70mph on the fwy. since the speedo won't work the truck goes into limp mode, the turbo will not function at optimum level. I have replaced the diff. sensor, the trani sensor, the a.b.s.sensors, the instrument cluster to a reman cluster. Then when I had a local mechanic plug a laptop dianostic program on it as he was running through the system inadvertently the radio cut out, so I found that who ever installed the after market stereo they cut a few wires to the harness, I went to the junk yard found the harness end and replaced the end. so with all that said and done no one knows what is causing this speedo not to work and limp mode. about 4 years ago the oil pump stopped working and ruined the turbo so the whole front end of that glorious 6.0 was rebuilt the problem started about a year after that. Any suggestions????

 

Do an electrical check on the wiring harness between the VSS (on top of the rear differential) and the PCM. You most likely have an intermittent open or short there.

 

the mechanic with the laptop tested all the sensors from the plug under the steering wheel said they are all reading just not showing on the dash then I purchased the cluster have not retested the system. I know where the VSS is, not sure where the PCM is?

 

The PCM is on the left hand inner front fender, next to the battery.