Pontiac Fiero 1988 Service Manual page 1030

C R U I S E C O N T R O L
V6 VIN 9 AND L 4 V I N R
CIRCUIT OPERATION
The Cruise Control System operates a me-
chanical linkage to the throttle by means of a
Vacuum Motor. This is a diaphragm moved by
a vacuum applied to one side. A solenoid oper-
ated valve connects the Vacuum Motor to a
Vacuum Tank. Another solenoid valve vents
the vacuum to reduce the suction. The Cruise
Control Module (V6 VIN 9) or Electronic
Control Module (L4 VIN R) controls the Vac-
uum Motor and the throttle by pulsing these
solenoid valves on and off.
One input to the module is the vehicle speed.
This input comes from the Vehicle Speed Sen-
sor Buffer. If the actual speed signal is dif-
ferent from the speed that was set into and
remembered by the module, the module gener-
ates pulses to change the vacuum and return
the vehicle to the speed set. Other inputs to
the module are from the Cruise Switch and the
Set Switch. A disconnect input to the module
comes from a switch on the brake pedal. A
separate vacuum shut-down of the Cruise Con-
trol comes from the Cruise Brake Switch on
the brake pedal.
The two outputs of the Cruise Control Mod-
ule operate the coils of the Vacuum Valve and
the Vent Valve. Both valves are located in the
Cruise Control Servo. These valves move the
throttle by means of the Vacuum Motor.
With V6 VIN 9 engine, the Servo Position
Sensor coil senses the position and motion of
the Vacuum Motor. It feeds this information
back to the module to provide smooth acceler-
ation while the vehicle is in Cruise Control.
With the L4 VIN R
f
the Throttle Position Sen-
sor in the Throttle Body senses the position and
motion of the vacuum motor. It feeds informa-
tion back to the module to provide smooth
throttle changes while the vehicle is in Cruise
Control.
With the L4 VIN R a number of additional
safety features are included in the system. If
the ECM diagnostics detect a fault with the
throttle position sensor (codes 21 and 22), with
the vehicle speed sensor (code 24) or with a
constant check engine light that will not dis-
play a code cruise is inhibited. Cruise will also
be inhibited if cruise is requested above 90
MPH; or with automatic transaxle, if the trans-
axle is shifted to NEUTRAL, resulting in a
very rapid change in RPM; or with manual
transaxle, if the engine exceeds red line RPM.
NOTE: With manual transaxle, changing the
transaxle speedometer gear to a new gear ratio
may adversely affect cruise control in lower
gears.