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page 35
T-12-UT
Maintenance
Electric Brake System Troubleshooting
Most electric brake system malfunctions that cannot be corrected by either brake
adjustments or synchronization adjustments can generally be traced to electrical sys-
tem failure. Mechanical causes are ordinarily obvious, i.e. bent or broken parts, worn
out linings or magnets, seized lever arms or shoes, scored drums, loose parts, etc. A
voltmeter and ammeter are essential tools for proper troubleshooting of electric brakes.
Measuring Voltage
System voltage is measured at the magnets by connecting the voltmeter between
the two magnet lead wires at any brake. This may be accomplished by using a pin
probe inserted through the insulation of the wires leading from the trailer frame to the
brake assembly, or if necessary by cutting the wires. The engine of the tow vehicle
should be running when checking the voltage so that a low battery will not affect the
readings.
Voltage in the system should begin at zero volts and as the controller bar is slowly
actuated should increase gradually to about 12 volts. This gradual increase of voltage
is referred to as modulation. No modulation means that when the controller begins to
apply voltage to the brakes it applies an immediate high voltage (acting more like an
On-Off switch), which causes the brakes to apply instantaneous maximum power.
The threshold voltage of a controller is the voltage that is applied to the brakes
when the controller first energizes. The lower the threshold the smoother the brakes
will operate. Too high a threshold voltage (in excess of 2 volts as quite often found in
heavy duty controllers) an cause grabby, harsh brakes.
M
A
I
N
T
E
N
A
N
C
E
INCREASING PEDAL PRESSURE
GR
AD
UAL
VO
LT
AG
E
RI
SE
ABR
UPT
VO
LT
AG
E
RI
SE
GOOD
MODULATION
POOR
MODULATION
IN
C
R
EASI
N
G
VO
L
T
AG
E
ZERO VOLTS
2 VOLT
(THRESHOLD)
12 VOLTS
