batteries. The long cable run to the meter
often causes significant charging system
voltage drops and resultant undercharging.
Another, more noticeable problem is that
the heavy current-carrying cables are run
with other cables and this is a cause of
radio interference; often you will see other
meters “twitching” or pulsing.
The shunt ammeter is essentially a
resistance or shunt inserted in the charging
cable. The twisted pair of meter wires are
connected to the shunt and can be run to
any meter location without voltage drop
problems, as the output is in millivolts.
Operating hour-meters and counters
are necessary to track engine hours for
service and maintenance. The meter and
counter are only activated when the engine
operates. The ignition switch is probably
the most practical method of activating the
meter, which is simply connected across
the ignition positive and a negative so
that it operates when the engine is running. The oil-pressure switch-activation
method is now uncommon although some
older installations activate through the oil-pressure switch.
Remove oil-pressure sensor units from
the engine every year. Clean oil sludge or
deposits out of the sensor orifice, which
tend to clog, causing incorrect readings.
When sensors are badly grounded on the
engine block, often Teflon tape is incorrectly applied to the threads. Don’t use Teflon
tape; this creates a high-resistance contact,
causing incorrect or no meter readings. The
same applies to temperature sensors.
Check that tachometer sensors that
pick up from the engine flywheel are
secure. A common cause of failure is damage to the head caused by striking the flywheel when it comes loose. Check that wiring is secure, not chafing or rubbing on any
surfaces. Check that the sensor terminal
and wire connections are tight and clean.
For mechanically activated gauges, check
capillary tubes for any signs of rubbing or
chafing and ensure that tubing is secure.
John Payne is the author of The Marine
Electrical and Electronics Bible, and
Motorboat Electrical and Electronics
Manual. This article first appeared in Mad
Mariner’s DIY Boat Owner Magazine.
Water temperature meter circuit.
+ Direction of current
Key switch Fuse To starter
Voltmeter connection circuit.