Problems Documented

Hunting
Hard shifting Hunting at steady speeds (Martin Chung)

Symptom: When travelling at a steady speed on level ground, the transmission may hunt (shift between gears for no real reason) with very slight throttle movement.

Possible cause:  The throttle position sensor may be faulty, or it may be misaligned.  I had diagnosed mine as a bad TP sensor and it was replaced by Nissan under warranty, however the problem still didn't go away.  I then checked the alignment of the sensor relative to the actual throttle plate position and found it was off, so the new sensor wasn't reporting the correct throttle position and it was misinterpreting the amount of throttle I was actually giving (it thought I was giving more throttle so it downshifted when it didn't have to).  I guess the mechanic didn't align it properly.

The shop manual gives the procedure to align the TP sensor properly.

Hard Shifting when Hot (Martin Chung)

Symptom: Transmission shifts smoothly when cold or warm; however, when it has warmed up sufficiently, it sometimes shifts hard with a jarring bump or kick.  When it cools down a bit, it shifts fine again.

Possible cause:  The transmission fluid temperature sensor may be faulty.  This sensor adjusts the accumulator pressure which in turn adjusts the smoothness of the clutch engagement.   Cold fluid is thicker and flows slower, while hot fluid flows faster.  The accumulator therefore has to adjust the amount of fluid flow accordingly.  Therefore if the sensor is faulty, it might report that the fluid is cold when it is actually hot.

In my case, the sensor was failing when the transmission got to its operating temperature.  I was able to check the voltage from the sensor to find that it would jump from reporting normal temperature (170 degrees F or so) to cold (50 degrees F) all in an instant!

Be aware that this is a tricky part to replace as you have to drop the transmission pan to get at the sensor, which is attached to the bottom of the valve body.  For later model W/D21's (say '92 or '93 and up), the sensor isn't individually replaceable but you have to replace it and the entire wiring harness to the engine compartment near the battery.  The shop manual for my '94 is incorrect in showing the sensor as a separate part.