Volvo Engine Brake VE D12, D12A, D12B. Manual - part 3

Group 25

Design and Function

Engine Braking

Control System, D12 and D12 A

With the complete release of the accelerator pedal, the
engine brake is activated according to the selection made
on the dash board, but only if certain conditions are met:

•

Accelerator pedal must be fully released (fuel injec-
tion must not occur).

•

Engine speed must be greater than 1100 rpm.

•

Clutch pedal must not be depressed.

•

Boost pressure must be lower than 150 kPa (22 psi).

•

Constant engine speed mode (PTO) must not be
activated.

•

Engine brake disabled, input must not be activated
(ABS).

•

Vehicle speed must be greater than 3.2 km/h (2
mph).

•

Engine temperature must be higher than the follow-
ing:

Part number of Tempera-
ture Switch

Temperature measured at
outlet of cooling pump

p/n 8155799

C (110

p/n 3985349

C (158

As an additional option, the engine brake may be en-
abled automatically if the vehicle exceeds a specified
cruise control limit. If this option is selected, the engine
brake may be applied at a selectable speed from 6.4 to
32 km/h (4 to 20 mph) above the cruise control set
speed.

W2000877

Fig. 10: Engine brake logic schematic, D12 and D12 A.

Group 25

Design and Function

Switches

On the dash the switches for the engine brake have dif-
ferent configurations. With the two dual-position
switches, the brake system is activated by placing one of
the switches in ON position. The HI/LO switch, in the HI
position, selects both the exhaust brake and the com-
pression brake (Volvo engine brake or VEB). In the LO
position, only the exhaust brake is selected.

W3002827

Fig. 11: Dual-position switches for Engine brake

The engine brake switches are on some truck models re-
placed by a single three-position switch. As with the
switches above it also regulates the engine brake for the
cruise control.

The three position switch has the following selection:

No engine brake engaged

Exhaust brake, EPG

Exhaust brake and compression brake

T2006953

Fig. 12: Three-position switch for engine brake

Exhaust Brake

During exhaust braking, control pressure of about 752
kPa (109 psi) activates the exhaust pressure governor.
The exhaust pressure governor plunger is activated and
the shutter connected to the plunger restricts the flow of
exhaust gases out of the cylinders.

Restricting the flow of exhaust gases creates a back
pressure between the shutter and the piston crowns.
This provides a braking effect on the pistons as the ex-
haust valves open.

The higher the engine speed during exhaust braking, the
greater the braking effect.

T2006832

Fig. 13: Exhaust pressure governor

Group 25

Design and Function

Compression Brake

The exhaust brake is always engaged with the com-
pression brake. This is because the compression brake
induction phase fully utilizes the backpressure created
by the exhaust brake in the exhaust manifold.

Induction Phase

The induction phase begins at the end of the intake
stroke and continues slightly into the compression stroke.

The piston travels downward toward bottom dead center
and the camshaft induction lobe opens the exhaust
valves long enough to fill the cylinder with the backpres-
sure created by the exhaust brake in the exhaust
manifold.

When the induction lobe closes the exhaust valves, the
cylinder has a backpressure at the start of the compres-
sion stroke. This backpressure increases compression
considerably during the compression stroke which, in
turn, creates a powerful braking effect when the piston
moves upward.

T2006830

Fig. 14: Induction phase

Decompression pPhase

At the end of the compression stroke, as the piston ap-
proaches top dead center, the camshaft decompression
lobe opens the exhaust valves and releases the pressure
from the cylinder. Shortly before bottom dead center, the
ordinary exhaust lobe opens the exhaust valves. During
the exhaust stroke, a backpressure is created in the ex-
haust manifold. This has a braking effect because the
exhaust pressure governor shutter is closed.

T2006831

Fig. 15: Decompression phase

Group 25

Troubleshooting

Troubleshooting

Engine Brake, Fault Tracing

Before working on a vehicle, set the parking brakes,
place the transmission in neutral, and block the
wheels. Failure to do so can result in unexpected ve-
hicle movement and can cause serious personal
injury or death.

When operating a vehicle on streets and highways,
during data collection procedure, it is mandatory to
have a second person drive while a technician col-
lects the data.

Never disconnect an air system component unless all
system pressure has been depleted. Failure to de-
plete system pressure before disconnecting hoses or
components may result in their violent separation and
can cause serious bodily injury.

WARNING

HOT ENGINE! Keep yourself and your test equipment
clear of all moving or hot engine parts. A hot engine
can cause serious burns or can permanently damage
test equipment.

WARNING

Always wear appropriate eye protection to prevent the
risk of eye injury due to contact with engine debris or
fluids.

CAUTION

Always clean the engine before servicing and keep
the compression brake components clean during all
service and repair procedures. Lack of cleanliness
during service and repair work could result in a fault
or malfunction of the compression brake system.

Basic knowledge of the system description and operation
is necessary to perform the troubleshooting procedures
in this manual. For basic system knowledge, refer to:

Service
Manuals

V776–200–820SM
Specifications, VE D12
284–610, PV776–TSP128229
Engine Control System, VE D12

VEB Diagnosis and Troubleshoot-
ing

The D12 engine is equipped with a diagnostic system to
simplify troubleshooting.

The first step to take with all troubleshooting should be
to check whether the diagnostic system indicates any
fault codes.

If fault codes are present, these should be corrected be-
fore proceeding further with the troubleshooting, even if
the fault codes do not indicate a fault that would affect
the engine brake system.

If the truck is equipped with ABS, verify that information
code 61 is not displayed during the check.

For details about how the fault codes and information
codes are interpreted, refer to:

Service
Manual

280–600, PV776–TSP105620/1
Fault Codes, Engine Electronic Control
Unit

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