I was reading the description on how this system works, and if the speedometer works, the speed sensor has to be working. The speed sensor develops a pulsing signal that goes only to the instrument cluster. That signal is what runs the speedometer, and that's how we know the sensor is working.
The instrument cluster "cleans up" that signal, then sends it to the Engine Computer. That signal is used, along with throttle position and load, to calculate transmission shift points. The Engine Computer is what will set the fault codes for missing signals, but you don't know where it went missing. Everything in the diagnostic charts talks about a problem with the Engine Computer, (if the speedometer is okay), but then they suggest other causes like corroded connector terminals. Logic would dictate too there could be a problem in the instrument cluster in the circuitry that sends the speed signal out to the Engine Computer. The problem now becomes how much do you want to spend on parts to "try"?
We are never in favor of throwing random parts at a problem in hopes one proves to be the solution, and this is especially true when those parts are expensive. It's different if you have a good used Engine Computer laying around, but if you have to buy one, you'll be better off dollar-wise to pay a mechanic to diagnose the cause of the problem.
Sometimes you can resort to trying parts on your own when they cost less than an hour of the mechanic's time, and when they're a good suspect, as in this speed sensor, but you have to be careful here too because you can still spend a lot of wasted money on multiple unneeded parts, and with each one you add another variable and a potential mistake that can add more problems.
The next step in the diagnostics is to connect a scanner to view live data and see what the Engine Computer is seeing for speed. If it sees correct road speed, you know the speed signal is getting to it, but if it still sets that fault code, it's not acting on that signal and the computer is suspect. My bigger concern is the buzzing noise you originally mentioned. That has to be caused by a mechanical issue in the transmission. A computer or electrical problem won't cause that, but a mechanical problem CAN result in setting a fault code that refers to an electrical problem.
Tuesday, April 7th, 2015 AT 7:49 PM