JL Audio M400/4-24V Owner's Manual page 17

"My amplifier shuts off once in a while, usually at higher volumes."
"My amplifier turns on, but there is no output."
Check your voltage source and grounding point. The power
supply of the M400/4-24V will operate with charging
system voltages down to 20V. Shutdown problems at higher
volume levels can occur when the charging system voltage
(or remote turn-on voltage) momentarily drops below 20V.
These dips can be of very short duration making them
extremely difficult to detect with a common DC voltmeter.
To ensure proper voltage, inspect all wiring and termination
points. It may also be necessary to upgrade the ground
wire connecting the battery to the vehicle/vessel's electrical
ground and the power wire connecting the alternator to
the battery. Many vehicles/vessels employ small (10 AWG - 6
AWG) wire to ground the battery and to connect the alternator
to the battery. To prevent voltage drops, these wires should be
upgraded to 4 AWG when installing amplifier systems with
main fuse ratings above 60A. Grounding problems are
the leading cause of misdiagnosed amplifier "failures."
Check the input signal using an AC voltmeter to measure the
voltage from the source unit while an appropriate test tone is
played through the source unit (disconnect the input cables
from the amplifier prior to this test). The frequency used
should be in the range that is to be amplified by the amplifier
(example: 50 Hz for a sub bass application or 1 kHz for a full
range / high-pass application). A steady, sufficient voltage
(between 0.1 and 4.0-volts) should be present at the output of
the signal cables.
Check the output of the amplifier. Using the procedure explained in
the previous check item (after plugging the input cables back
into the amplifier) test for output at the speaker outputs of
the amplifier. Remove the speaker wires from the amplifier
while doing this to prevent unpleasant noise and possible
speaker damage. Turn the volume up approximately half
way. 5V AC or more should be measured at the speaker
outputs. This output level can vary greatly between
amplifiers but it should not be in the millivolt range with
the source unit at half volume. If you are reading sufficient
voltage, check your speaker connections as explained below.
Check to ensure that the speaker wires are making a good
connection with the metal inside the terminal block. The
speaker wire connectors are designed to accept up to 8 AWG
wire. Make sure to strip the wire to allow for a sufficient
connection with the metal inside the terminal block.
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