97 SHO does speedo, speed control, and trans. shifting based on vehicle speed read by Wheel Speed Sensors and sent out by ABS module. No VSS on Trans. Here's the test procedure:
G1 Check Speedometer
Road test vehicle
Is speedometer operating?
Go to G2
Go to G5
G2 Check The Odometer Reading
drive vehicle over a known distance
Is odometer reading inaccurately?
Go to G3
Go to G4
G3 Check Speedometer Accuracy
Drive vehicle at a steady speed over a 0 km (16 mile) distance and record the total distance travelled.
Does speedometer meet speedometer calibration tolerance specification?
REPLACE speedometer assembly
G4 Check SARC Vehicle Speed PID
connect Rotunda New Generation Star Tester or equivalent to data link connector C229 beneath instrument cluster.
Access SARC PID: VSS_SRC. Refer to Suspension Control for procedure and entre PID list.
Observe vehicle speed via PID VS_SRC
Does vehicle speed indicated in PID: VSS_SRC reflect approximate vehicle speed?
Go to G5
SERVICE Circuit 679 (GY/BK). Go to Antilock Brakes / Traction Control Systems.
G5 Check Circuit 679 (GY/BK) For Open Circuit
Using an ohmmeter connect one lead to Circuit 679 (GY/BK) at anti-lock brake control module connector Pin C1057 and the other lead to instrument cluster Pin C251, Circuit 679 (GY/BK).
Is resistance less than 5 ohms?
GO to G6.
SERVICE Circuit 679 (GY/BK) for open circuit. RESTORE vehicle. RETEST system.
G6 Check Circuit 398 (BK/Y) Ground
Connect ohmmeter to instrument cluster connector Pin C250 , Circuit 398 (BK/Y).
Measure resistance to known good ground.
Is resistance 5 ohms or less?
GO to G7.
SERVICE Circuit 398 (BK/Y) for open circuit. RESTORE vehicle. RETEST system.
G7 Check Clip To Cluster Circuit Connector
Disconnect instrument cluster connectors C250 and C251
Using an ohmmeter, measure resistance of speedometer indicator terminals to printed circuit where they seat.
Is resistance 1 ohm or less at each indicator terminal?
GO to G8.
REPLACE contact clips. RESTORE vehicle. RETEST system.
G8 Check Printed Circuit
Reconnect cluster connectors.
Using an ohmmeter, measure resistance from speedometer contact clips to instrument cluster connector according to chart.
C251 Power Input (RUN)
C251 Vehicle Speed Input Signal
Is resistance less than 1 ohm?
REPLACE speedometer assembly. RESTORE vehicle. RETEST system.
REPLACE instrument cluster printed circuit. RESTORE vehicle. RETEST system.
A note, the ABS Module is under the battery tray, make sure there is no acid or corrosion on the module connector terminals.