Volvo Penta D4 Operator's Manual page 26

Instruments
24
Battery (orange indication)
The battery lamp lights up if the alternator is not
charging. Stop the engine if this lamp lights up during
operation. If the lamp lights up, this can be due to a
fault in the electrical system or because the alternator
drive belt is slack.
Check the alternator drive belts. Please refer to
"Maintenance: Engine, general".
Also check that there is no poor contact/broken wires.
WARNING! Do not continue operation if there is
any problem with the alternator drive belts. This
could cause serious engine damage.
Coolant temperature (red indication)
The coolant temperature lamp lights up when the
coolant temperature is too high. Stop the engine if this
lamp lights up during operation.
Check the coolant level. Please refer to "Maintenan-
ce: Fresh water system".
Check that the sea water filter is not blocked. Please
refer to "Maintenance: Sea water system"
Also check the impeller in the sea water pump.
Please refer to "Maintenance: Sea water system".
Please refer to the "In case of emergency" chapter, and
you will find detailed information about recommended
action in the "Diagnostic function" section.
WARNING! Do not open the coolant filler cap
when the engine is warm, except in emergencies.
Steam or hot fluid could spray out.