Electrical Overview

Electrical overview





Control module
The control module compares selected speed with actual speed. If necessary the control module activates a vacuum pump or regulator, which in turn control a vacuum servo. This servo is connected to the accelerator pedal (AP) and therefore controls vehicle speed.
The selected speed is stored in the control module memory until the cruise control is disengaged by moving the switch on the direction indicator stalk to the OFF position, or until the speed stored in memory is changed by using SET+ or SET-.
The control module incorporates several safety functions to prevent damage due to system failures. Vehicle speed must be above
35 km/h (22 mph); the selector switch must be in the ON position, and the SET button must be pressed in before the control module can operate the vacuum pump relay. If the car has automatic transmission the control module is switched off when the gear selector lever is in the P or N positions.
In addition to electrical safety functions there are several pneumatic safety devices, see Vacuum control.
The control module monitors the system and disengages cruise control during abnormal acceleration (wheel-spin) or deceleration (wheels locking). The control module also disengages the system if vehicle speed drops below 75% of the selected speed. This may occur when driving up steep slopes when the engine cannot maintain a constant speed.
Another important control module function is to store diagnostic trouble codes (DTCs) allowing fault-tracing using the Volvo Scan Tool (ST).
In order to operate correctly the control module requires several sensors. These are described below with their functions.





Stop (brake) light switch
The stop (brake) light switch acts as an extra safety device in case the brake pedal is depressed and the vacuum valve does not open. When the brakes are used the control module is no longer grounded via the stop (brake) light bulbs and the system is disengaged. If the bulbs are defective cruise control will not operate. Always check the stop (brake) light bulbs first if there is a fault in the system.




From 1995- there is a new type of stop (brake) light switch, however, its function is the same.





Pedal valves (electrical)
These are located above the brake pedal and clutch pedal (manual transmission) and have the same function as the stop (brake) light switch. They cut off the supply to the pump and regulator in the pump unit. This opens the pump regulator and allows air into the vacuum system. Normally when the pedal is up the valve is closed.
The pedal valves are combined electrical/vacuum valves. When the pedal is depressed the vacuum circuit is broken.

Location of Components, Cruise Control (Passenger Compartment - Cruise Control, Switch):

Selector switch
The switch incorporates:
- a switch for switching the system off and on
- two buttons to increase and decrease vehicle speed.
- a sliding switch to resume the selected speed.





Vacuum pump and regulator
If vehicle speed drops the vacuum pump is turned on and the vacuum in the servo increases. The servo presses down the accelerator pedal (AP) a little and vehicle speed increases again. If vehicle speed increases the regulator allows air into the vacuum system reducing the vacuum. The vacuum pump is driven by an electric motor. This is switched on when the SET+ button is pressed in and the control module relay is operated. The motor drives the pump until the vacuum servo has a sufficient vacuum to maintain the selected speed.
When extra power is required, when driving up a hill for example, the vacuum pump is switched on again in order to increase the system vacuum until the selected speed is achieved. When driving downhill the regulator vacuum valve is opened to allow air into the system, reducing the vacuum to the vacuum servo.

Location of Components, Cruise Control (Passenger Compartment - Park/Neutral Position (PNP) Switch, Automatic Transmission):

Park/Neutral Position Switch (PNP)
The control module is switched off when the automatic transmission gear selector lever is in the P or N positions. However, the selected speed is stored in memory.





Vehicle speed signal (VSS)
The cruise control module requires information about the actual vehicle speed. This information is obtained from the speedometer. In cars with manual transmission the mechanical movement of the speedometer cable is converted into a speed-related pulsed frequency via a reed element switch in the instrument panel. This signal is then transmitted to the control module. In cars with automatic transmission this information is provided by the Aisin Warner transmission. This cable is routed to the instrument panel.
Data from all these sensors allow the control module to control vehicle speed. Actual speed control is carried out using a vacuum servo next to the accelerator pedal (AP). The vacuum for this servo is governed by a pump or control valve, the regulator.