Solution K24912512
Wednesday, December 4, 2019 10:23:23 PM CET

** SOLUTION **
Title

Volvo Chassis - Volvo Hill Start Assist System Information

Volvo Models
Volvo Model

VN

** SOLUTION **
Cause

Limited information in Trucks Dealer Portal ( TDP )

Volvo Hill Start Assist HILLSTA2

Solution

Hill start assist, function description
Impact Meta Data
Valid From

04.07.2019

Valid To

01.01.2099

PD string

VN, HILLSTA2

Hill start assist component location, dashboard
1

Instrument cluster (A03)

2

Hill start assist switch (S212)

3

Brake control unit (A12B)

4

VECU (Vehicle Electronic Control Unit) (A17)

5

GLU (Gear Lever Unit) switch (S171)

6

Accelerator pedal sensor (B25A)

7

Brake pedal valve switch (S59)

Control units, chassis

1

ECM (Engine Control Module) (A14)

2

TECU (Transmission Electronic Control Unit) (A13)

Function behavior

General information
Hill start assist variants

Variant symbol

Variant description

HILLSTA2

Hill start assist, all wheels and trailer

UHILLST

Without hill start assist

Description
Hill start assist is safety feature in the truck which helps the driver to avoid rolling back
while truck taking a gradient.This feature allows the truck to hold in a uphill with a
forward gear or in a down hill with reverse gear when the service brake is released. The
hill start assist program is activated when the driver releases the brake pedal and the
brake pressure being hold for 3 seconds until the truck is sufficient to move on without
roll back.
The brake pressure in the brake chambers being held by the hill start pneumatic solenoid
valve via brake control unit. The TECU calculates the required engine torque with load
dependency and releases the hill start assist program while truck starts move on without
rolling back.
The hill start assist program activates automatically in the truck with predefined
conditions.If the driver wants to deactivate the program by selecting the dashboard
disable switch.The program will re-enables when the ignition key covers a cycle of OFF–
ON or else the driver can chose the disable switch again to re-enable the program.
Functionality conditions
Conditions to hill start assist activation
Key switch in the ignition position and engine rpm is present
Vehicle speed is equal to 0 km/h
Vehicle is in a forward gear on an incline equal to or greater than 2%
Vehicle is in a reverse gear on an decline equal to or greater than 2%
The brake pressure should not below at 10 psi
Service brake is activated
Hill start assist disable switch is not selected
No brake failures with active J1939 communication faults
System is not in a downgraded mode
Conditions to hill start assist deactivation
Vehicle Speed is greater than 20 km/h (12 mph)
Hill start assist disable switch is in ON position

Note:
Hill start assist will not works (applies brake) when the system attains low brake
pressure. It
will activates when the brake pressure is at 10 psi or greater.
Pneumatic diagram

1

Primary tank

2

Secondary tank

3

Treadle valve (foot brake valve)

4

Central pressure control ATR-6 relay valve

5

Central pressure control ATR-3 relay valve for Hill start assist

6

Quick release valve for front brakes

7

Quick release valve for rear brakes

8

Relay valve for rear brakes

9

Trailer hand control valve

10

ABS Solenoid valves

11

Brake chamber, rear

12

Brake chamber, front

13

Trailer protection valve

14

Park control valve

15

Pedal plate bulkhead

Activation scenario, truck is stopped
1. The brake pedal is released and the brake pedal response information send via
foot brake valve switch (S59) and the signal received by brake control unit
(A12B).
2. The brake control unit (A12B) receives wheel rotation information via speed
sensor (B1).
3. The brake control unit (A12B) receives steering angle position via steering angle
sensor (S49) and lateral acceleration via yaw rate sensor (S50).
4. The VECU (A17) analyses the pre conditions of the truck via brake control unit
(A12B) to activate hill start assist.
5. The VECU (A17) send information to brake control unit (A12B) to activate hill
start assist.
6. The brake control unit (A12B) pulses the solenoid valve (Y106) to hold the
brake pressure for the set time.
7. The brake control unit (A12B) sends the status information to instrument cluster
(A03) about hill start assist program activation.

Deactivation scenario, truck is driving off

1. The accelerator pedal is depresses and the information send via accelerator
position sensor (B25A) and the signal received by VECU (A17) or when
deactivation switch (S212) is in ON position.
2. The VECU (A17) calculates the available engine torque as well as the driveline
torque from ECM (A14) and from TECU (A13) .
3. The TECU (A13) calculates the engine torque available from ECM (A14)
4. The VECU (A17) send the information to brake control unit (A12B) to release
the brake pressure from brake chambers via solenoid valve (Y106).
5. The VECU (A17) commands the TECU (A13) to move the truck with available
engine torque via ECM (A14).
6. The brake control unit (A12B) sends the status information to instrument cluster
(A03) about hill start assist program deactivation.
7. If the brake pressure is below 10 psi , the brake control unit (A12B) calculate the
brake pressure value and sends information to VECU (A17) and send
deactivation status to instrument cluster (A03).

Solution visibility

Dealer distribution

Function(s)/component(s) affected
Function affected

VECU , EECU , MID 128 – EMS , MID 130 – TECU , MID 144 – VECU , Braking
/Auxiliary Pneumatic Circuits , Diagnostic tool

Function Group
Function Group

5 brake

Customer effect
Main customer effect

efficiency/abnormal behavior , stop/start

Conditions
Vehicle operating mode

on start-up , uphill

Road conditions

very hilly

Administration
Author

A044013

Dealer ID

A044013

Last modified by

RU4469V

Creation date

21-10-2019 13:10

Date of last update

14-11-2019 14:11

Status

Published

UNCONTROLLED COPY. Printed document is for temporary use only and should not be retained.