I've got a bad speed sensor(s) in my 1998 XJ Sport with 4 speed AW4 transmission. My mechanic was nice enough to diagnose the problem for me and said it was an easy fix to swap out on my own.
From what I can tell, there is are 2 speed sensors an input and output. I can find what is specifically described as an output sensor but, can find anything specifically described as an input sensor.
I'm going to swap out both since I'm down there. Question is: Are the input and output the same sensor or are they different part numbers?

 

You have a 4WD. The speedo sensor is in the back drivers side of the xfer case, not the transmission. The input and output speed sensors in the transmission itself are for the transmission computer to determine shifting and not the speedometer. First, check that the speedo sensor in the xfer case is actually connected. Note the rotational position before you pull it out, as it can be installed in different positions to accommodate different size speedo gears.

Example Youtube on changing the gear:

 

Thanks for the reply but, it is the transmission speed sensors that I need to replace. I'm not having any issues with my speedometer.

 

Before you fire the parts canon, what are the symptoms, and did you pull any codes? A bad input sensor will throw a code. A bad rear output sensor will cause it to not upshift out of first with the shifter in D, if it doesn't think the vehicle is moving. Those sensors are pretty reliable, and typically it's a wiring issue. I would check the wiring along the drivers side of the transmision and clean any connectors as it heads up the firewall and into the cabin. You can test the sensors if you have a voltmeter. It should register a/c voltage as the output shaft is turning at speed - for example put the xfer case in neutral and trans in D. On a 98, it's 4 a/c pulses per rev, with voltage proportional to shaft speed. From memory it was under a volt at very low rpm, and get up to almost 30-volts spikes at 4000 rpm measuring the sensor directly without being connected to the computer. (Side note for other listeners, the older AW4 used a magnetic relay type switch that closed once per revolution)

To answer the original question, yes the input and output sensors are the same part number. In fact you can could swap them to confirm a diagnosis of a bad sensor. The rear one is tucked behind the xfer case linkage bracket and is harder to get to.

 

Resurrecting an older thread - so for sure the speed sensor on the back of the transfer case is only used for the speedometer and not as an input for any engine or transmission functions?

 

I found that the FSM states that for fuel injector control, the vehicle speedometer is used by PCM to understand whether or not the vehicle is moving. Presumably its used for cruise control as well, but I don't see any info in the FSM about this. It does not appear to be used by the TCM.

 

It definitely plays no role in the transmission. The engine computer might look at it though.