Originally Posted by Varilux

Okay, the info on the function of the speed sensor makes sense given the readings I had on the OHC today.

1. Started the drive home from work with a cold engine- as the info suggested, the correct temp displayed on the OHC.
2. On the way home, I washed the truck- and sprayed some cold water on the sensor. On restart a few minutes later, the temp was indeed lower than the previous reading- so it displayed a lower temp.
3. A mile later, I stopped to run into the grocery store. When I came back a few minutes later, the temp read the same as it did when I stopped (because the sensor was reading a HIGHER temp than before now). As the info said, since I don't have the speed wire connected, after a few minutes the temp started going up by 1 degree per minute.
4. Here's where it got interesting. I disconnected and reconnected the sensor with the truck running- which caused it to default back to -19F. Drove around a bit, and the temp started climbing one degree per minute from -19. That was annoying, but then I read the info and realized the temp would reset after 2 hours.
5. Sure enough, I let the truck sit for 3 hours, went out and started the truck and it read the correct temp.

So, even though hooking up the speed sensor wire (gray/black) IS optional, if you stop and restart the truck it will cause the temp to not read correctly for some time. Fortunately, once the truck sits for 2 hours it resets.