I need the transmission speed sensor for my truck and I am getting a confusing array of Motorcraft and Ford part numbers. I have Motorcraft numbers DY1251 (Turbine Shaft Speed Sensor Assembly) and DY1229 (Transmission Speed Sensor). Not sure what the difference is between the two.

I've been having a recurring problem where, when my speedometer starts wavering, the truck will sometimes jerk, like maybe it's trying to down shift or a stumble of some sort. I've replaced the VSS in the rear diff (with Motorcraft) which cleared up the worst of it, but this problem still persists.

 

Could you try pulling the diff sensor out and see if it is covered with metallic particles?

 

Yes, I was going to do that as well. When I changed the Dorman part for the Motorcraft part, I didn't see anything. I was going to pull the fill plug on the diff, isn't it magnetic?

 

I don't have your cover so I can't say. I've seen a number of cases where the sensor collects enough iron debris to screw up its ability and gives an inconsistent reading. While I realize it's a long shot, I would do it just to eliminate the possibility and check off the box.

Even if the plug was magnetic, my typical inspection has shown a much larger accumulation of debris in the bottom sump of the differential housing, enough that when we changed fluid on vehicles I made sure there a good effort to get all of the muck out of the housings bottom.

Again, maybe not your issue but it's a no cost item to check, one that has a history.