Page 112
TROUBLESHOOTING
The individual boost pressure readings should be
approximately the same as recorded in steps 3
and 4.
5. Compare the maximum boost pressure with
the boost pressures in the table on page
111. A low reading indicates a possible
problem. Readings at sea level should be
within 3 psi of the values shown (at higher
altitudes, slightly lower boost pressure
readings are normal). Readings within 3 psi
of the values shown indicate proper
operation of the engine brake.
Electrical Troubleshooting
ASET™ engines have been equipped with a
V-MAC
®
control feature that automatically
provides a level of protection by activating the
engine brake only after the recommended
minimum oil temperature has been reached. Be
sure that the engine is sufficiently warmed before
conducting tests.
The MACK PowerLeash™ engine brake is
activated by the V-MAC system. When the
ignition switch is turned ON, the engine brake
solenoids are supplied a constant 12-volt direct
current with current flow increasing whenever the
engine brake is activated. To properly diagnose
electrical problems with the brake, an ammeter
may be required.
Some automated manual transmissions prevent
engine brake operation when the transmission is
in neutral.
If insufficient electrical power is reaching the
engine brake, perform the preliminary checks
outlined below. For more information, see the
V-MAC
®
III Service Manual, 8-211.
NO ENGINE BRAKE OPERATION
1. Verify that the clutch pedal is not depressed
and that the clutch switch is working
properly.
2. Check for a blown fuse or circuit breaker.
3. With electrical power OFF, check the control
system for a short to ground. Check systems
separately to isolate where the short is
occurring. If the control system is OK up to
the engine brake valve cover/spacer
connection, measure the resistance to the
solenoid valve. High resistance means an
open circuit in the solenoid or solenoid wire.
ONLY ONE ROCKER SHAFT OPERATING
1. Determine which rocker shaft is not
operating by closing all the switches and
checking the power at the wires leading to
the solenoid valves (front and rear).
2. Remove the wire to the solenoid valve at the
cylinder head cover/spacer and check for
resistance (refer to the table on page 113).
No reading indicates an open circuit in the
wire or solenoid coil. A low resistance
reading indicates a short to ground in the
solenoid wire or solenoid coil.
A constant 12-volt low amperage signal is
supplied to the engine brake solenoids by the
V-MAC III module when the ignition switch is
turned on. During an engine braking event,
V-MAC increases the current to the solenoids to
activate the engine brake. The most accurate
method of checking electrical function of the
engine brake solenoid circuits is by using an
ammeter to measure current at the solenoids
when the engine brake is activated. When the
engine brake is activated, current should be
approximately 1.59 amps. For additional
information, consult the V-MAC
®
III Service
Manual, 8-211.
3. Test the solenoid by checking for current
draw and pull-in voltage per specifications
listed in the table on page 113.
5-111.bk Page 112 Monday, July 10, 2006 2:26 PM
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