Well, it sounds like the pump's really not working. What is working though is the diff temp sensor, and that's good news. Can Forscan find the diff temp PID? If you can't find it, then it's masquerading under a new name. In the back of my mind, I'm wondering if Ford's engineers brought across a control software component originally built for another system, maybe a transfer case or something else in the Ford product line on another vehicle.Another update - my son just got back from driving the car and got it up to 210 on the street. I got under the car and the pump was not running as far as we could tell (no noise, no vibration when I touched it). We then put a heat gun on the diff cover near the temp sensor and got it to read 275 on the dash. Again, no activity from the pump whatsoever.
I'm not sure if the pump is designed to only run when the car is moving, but it definitely wasn't on when we were testing it.
I also went through all of the pids in Forscan while the car was running, and couldn't find anything for the diff.
If there are no DTC's other than the overheat one (P188A) then the ECU either thinks the pump's working fine or it's not thinking about the pump at all (wrong software). Thing is, just because Forscan can't find a PID doesn't mean it's not there. If a genuine Ford (Bosch) IDS system can't find it, then it's really not there. If you know a shop with a high-end Snap On system or similar, they could probably diagnose it too.