1995 Volvo 850 ABS and Traction Control Systems
When ignition is turned on, control module activates warning indicators on instrument panel and performs an initial diagnostic test which ensures there are no internal faults in control module. At same time, main relay portion of combination relay is activated so hydraulic modulator valves are supplied with power. See Fig. 1. Control module then checks operation of valves by activating them in sequence.
If no fault is detected, warning indicators go out about 2 seconds after engine is started. When vehicle is driven, control module checks that it is receiving signals from wheel sensors. At 20 MPH, it checks that hydraulic pump is operating by activating pump relay so pump is started. This is confirmed by a signal from rotation sensor. In addition to the initial diagnostic tests, control module continuously checks that all signals and components are operating, and there are no internal faults in control module.
ABS SYSTEM
As vehicle is driven, control module monitors wheel speeds by computing their acceleration and deceleration. During braking, signal from brake light puts control module into standby mode. If any wheels start to lock, control module activates hydraulic modulator which adjusts hydraulic pressure.
Control module also receives a brake pedal position signal from pedal sensor (in master cylinder) during braking. This information is used to control hydraulic modulator so it does not affect brake pedal position. See Fig. 2.
TRACS SYSTEM
TRACS is an optional traction control system that operates in conjunction with ABS system. If vehicle speed is less than 25 MPH and one of the drive wheels starts to slip, control module starts pump in hydraulic modulator, which pumps brake fluid to brake caliper of wheel that is slipping, applying braking force so both drive wheels have same speed. If brake is applied, brake light contact closes at same time as pressure switch breaks a circuit to control module. TRACS control stops and system goes into ABS control standby mode.
Because TRACS system is intended as an assist to starting on slippery surfaces, it is most effective at speeds less than 15 MPH, and less so between 15-25 MPH. TRACS is completely disengaged at speeds greater than 25 MPH. Hydraulic pump is in continuous operation while TRACS is engaged.
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