Solution for P0500 VSS speed sensor problem on Nissan Sentra 96
Problem: P0500 error code on Nissan Sentra 96. Speedometer works fine.
I'm posting the solution to this problem to help other Nissan owners on the Internet.
Solution: Since speedometer works fine, the problem is NOT the speed sensor (which is on the trans-axle and difficult to access and pull out). The solution turned out to be an intermittent open at a terminal on the back of the instrument cluster. Wiggling the wires to terminals 22 (R/Y wire) and 23 (R wire) and 21 (PU/R wire fixed the problem.
In retrospect, the loose terminal wire problem was probably caused by a person who hit me in rear with enough force to jiggle wire and slosh brain around in skull, but not enough to damage the resilient Nissan rear bumper.
To diagnose the problem, I did the following tests:
I first tested the PU/R wire running to ECM (engine control module. This wire carries the VSS signal. (There is a harness connector just prior to ECM that is easy to access on passenger side near ECM). Take multimeter and set to DC or AC volts; connect multimeter between ground (I use cigarette lighter outside metal for ground) and harness connector female terminal 1 with PU/R wire. Start up car and drive around a bit; voltmeter should read nonzero when moving and should increase with increasing speed.
If read zero, then VSS signal not reaching ECM. Gain access to back of instrument cluster (easy to do, consult Haynes manual) and perform same voltage test except connect between ground and terminal 21 (PU/R wire ) on back of instrument cluster. If get nonzero reading, then you have an open between speedometer and ECM.
NOTE that the voltage tests were done with all harness connectors connected. You can connect to wires from rear of harness connectors.
Verify by checking for continuity of PU/R wire running from speedometer and ECM (the two terminal you just did voltage tests on). Remember to disconnect harness connectors at both ends to isolate from rest of circuit. If continuity checks out, then possible PU/R wire is being grounded. Check for continuity between PU/R wire and ground (remember to reattach all harness connectors but disconnect battery. If connected to ground, then isolate location of grounding. Reconnect battery.
For me, continuity checked just fine between ECM and speedometer. I was going to replace speedometer until I decided to measure voltages at instrument cluster which was enough to jiggle wire back into place.
To finish up, I reset the OBD error code (use OBDII software on portable computer). Then use OBDII software to monitor VSS. It was nonzero while driving around, indicating all was fine with VSS.