Which speed sensor?

[Copper93]

I have a 98 Expy and the A4WD light just flashes when the knob is turned. The 4WD and 4WD low seem to work. The speedometer works also. The transmission changes a little hard from 2 to 3 but that maybe another issue. Pulled codes with FORScan and got back an 1845 code, if I remember correctly. It stated a Hall effect sensor shorted to ground. How many speed sensors are there and where are they located. Everything I’ve looked up gives from the front of the transmission, rear of the transmission, transfer case and differential. Which sensor controls what?

 

[Copper93]

Any suggestions?

 

[Joseph3]

Well I don't know if this helps much, but if your speedometer is working then the differential sensor should be good. I know from personal experience that when that sensor fails your speedometer will go wacky. Also cruise control won't work and the o/d light may flash. Since you don't have those symptoms, I don't think it's the differential speed sensor.

If a sensor shorted to ground it may indeed be causing your hard shifts too. Should be easy to detect with an ohmeter, just unplug the sensor and check each pin (on the installed sensor) to ground. If either pin is shorted to ground, the sensor is bad. This is assuming the sensors are accessible. Since your auto 4wd light is flashing, it might mean it can't detect the difference in speed between front and rear (that's what activates the front wheels in auto 4wd).