Abstract: A constant speed travel control system uses a time period Q which is set such that, if a motor of the actuator is not locked but normally rotated, actuator opening becomes almost certainly fully closed within that time period with the result that a first limit switch is made open. If the motor is normal, the processing operation proceeds to a normal constant speed travel control by which an electric current to the motor ceases and "motor open" condition is detected. When no "motor open" state is detected, even after the elapse of the time period Q, under the condition that the vehicle speed is higher than a reference speed, a determination that the motor is in the locked condition is made and the operation proceeds to a speed control cancellation process.

Abstract: When an "off" state of an IDLE switch is detected even once after an ignition switch of an internal combustion engine is turned on, a cruise ECU determines that the IDLE switch is normal, and performs constant-speed cruise control if the start thereof is instructed. Because the constant-speed cruise control is normally instructed while a vehicle is running, if the IDLE switch is normal, the IDLE switch becomes the "off" state at least one time during the time period until the start of the constant-speed cruise control is instructed after the ignition switch is turned on. For this reason, if the IDLE switch does not become the "off" state even once, the cruise ECU determines that the IDLE switch has failed and interrupts the constant-speed cruise control.

Abstract: Engine-racing control is performed in which a throttle valve is temporarily driven to the open side at start of cruise control. The engine-racing control is forcibly stopped when skip vehicle speed Vsk serving as an estimated vehicle speed after a predetermined time becomes higher than storage vehicle speed (target traveling speed) Vm. For example, on a downward slope where a vehicle is naturally accelerated due to gravitational acceleration, the vehicle is excessively accelerated due to the engine-racing control same as that on a flat road. However, because the behavior of the vehicle on the downward slope is reflected on the skip vehicle speed Vsk, it is possible to stop the engine-racing control early compared to the case in which the vehicle travels on the flat road and to avoid excessive acceleration.

Abstract: When an overdrive cut is performed to an automatic transmission control when climbing uphill slopes, the actuator position An is stored in overdrive cut aperture storage value C with overdrive cut demands to the automatic transmission executed until the actuator position changes by a first predetermined value Ct1. If the condition C-An>Ct1 is satisfied for a predetermined period of time T0 until timer counter TMR becomes zero, then restoration to overdrive is allowed. Accordingly, even when cruising along long uphill roads, up-shifts to overdrive are prohibited during this time and because there are no up-shifts to overdrive during this time, vehicular speed does not change in fixed periods and thus, a high-level of passenger comfort is achieved.

Abstract: When an IDLE switch goes on and so a condition of strong possibility of fuel cutoff is obtained, a throttle degree of opening is immediately opened slightly and an IDLE switch goes off, and thereby actual entry of an electronic-control fuel-injection device into fuel cutoff processing can be prevented. Moreover regulation of the throttle degree of opening to a closed side is blunted by enlargement of an insensitive zone of a difference between target cruising speed and vehicle speed, and so the throttle degree of opening does not change and a rise of vehicle speed Vn continues for a while. Because of this, a large cycle of vehicle change is repeated and so physical sensations do not deteriorate.