Maybe this info may save some in unnecessarily replacing the speedo head. The speedo head contains circuitry which converts the ac signal of the VSS to a 0-10 Volt square wave and then routes it to the ECU. If your speedometor works, but your getting 0104, the ECU is not getting the square wave signal.
1995 Maxima 0104 VSS, DTC0504 A/T Control
Subject: 1995 Maxima DTC0504 A/T Control
Over the past year, I have had an intermittent CEL 0104 (vehicle speed
sensor), along with 0504 (a/t control) and 0304 (knock sensor). The
speed sensor checked out, plus my speedometer was functional. My guess
was that the signal path from the speedo head to the ECM was
intermittent. I was able to borrow a portable o-scope and monitor pin
29 (vehicle speed sensor) of the ECM. The signal during normal
operation was a 0-10V square wave where the freq would vary with
vehicle speed. A few times the signal would be stuck at 10V or 0V.
When this would happen, the CEL would trigger. I banged on the dash
near the speedometer and the square wave returned. I later took the
dash apart and cleaned the connections to the speedo head with contact
cleaner. I continued to monitor pin 29 and for about a month now the
signal has behaved normally and no CEL.
A few days ago I checked for non-CEL DTC's. I had both 0504 and 0304.
The FSM continutity test of the 5 wires to/from the ECM and A/T control
unit passed. The short to ground test also passed. The FSM is vague
on any more detail on the specific function of these 5 signal lines, so
although I could monitor them, I'm not sure what to look for.
I've also been monitoring pin 64 (knock sensor), it's voltage varies
from 2V to 3.3V. Is this normal operation for this sensor? I was
expecting the voltage to increase if I accelerated hard, but have not
yet noticed a consistent pattern.
Part of my impetus for checking for DTC's was that I noticed a slight
hard shift from 1st to 2nd.
Appreciate any helpful thoughts anyone might have.
I use premium gas, usually Shell 93. 159K miles. Maxima SE.