GMC 4000 Series Service Manual page 324

b. If gauge shows high temperature under all
conditions, wire leading from gauge to engine unit
is shorted to ground.
c. If gauge registers a low temperature under
all conditions, wire between gauge and engine unit
is broken.
Do not attempt to repair either the engine unit
or the gauge. When installing new engine unit, do
not use thread compound on unit threads, as this
will increase electrical resistance of unit and
cause faulty reading on gauge.
ENGINE OVERHEAT ALARM
Overheat alarm switch is located near the
water temperature indicator, engine unit. Refer to
figure 2 or 3 for location of engine overheat alarm.
NOTE: Electrical wire end of overheat switch
is retained by screw, whereas, wire end for temp­
erature indicator is retained by a nut.
The alarm system is interconnected to a te ll­
tale light and, on some vehicles, to a buzzer. The
purpose of this system is to signal the driver when
engine temperature exceeds a safe range. Refer
to current wiring diagram in "Wiring Diagrams"
booklet for alarm system circuit.
Overheat temperature switch is not repairable
and must be replaced as an assembly or unit.
TESTING ENGINE UNIT AND CIRCUIT
NOTE: Before testing overheat switch, make
the following circuit tests:
Circuit Tests
1. Disconnect wire at engine unit.
2. Connect a test light consisting of a 2-
candlepower 12-volt bulb and a pair of test leads
in circuit by clipping one lead to a hot terminal
and other lead to body of engine unit. If bulb does
not light, check for presence of sealing compound,
oil or paint around threads of unit. Remove insul­
ating substance and repeat test. Make sure unit is
properly grounded before proceeding with tests.
3. Remove test lead from body of engine unit
and connect lead to terminal of unit. Then provid­
ing engine water temperature is not exceeding
switch setting, and the bulb lights, engine unit is
internally short-circuited and should be replaced.
Refer to "Specifications" for switch setting.
4. Remove test light and reinstall wire on en­
gine unit.
5. If alarm buzzer sounds and/or tell-tale
lights under all conditions with engine control
switch in "ON" position; wire leading from buzzer
or light to engine unit is shorted to ground.
6. With engine control switch in "ON" posi­
tion, and light does not come on when terminal at
temperature switch is shorted to ground, a break
G M C S E R V I C E M A N U A L
Temperature G a u g e — Toro-Flow Diesel Engine (Typical)
o r short in wire between engine unit and engine
control switch is indicated.
thread compound on unit threads, as this will in­
crease electrical resistance and cause faulty re­
action.
Testing Switch Contacts
gine manifold in order to make this test. Before
testing unit, be sure that entire unit is pre-heated
to approximately normal engine temperature.
in hydraulic oil, heated to a temperature lj f F. to
20°F., higher than unit point contact setting listed
in "Specifications." Agitate oil thoroughly (and
gently tap unit). If points contact readily, correct
working of instrument is indicated.
outlet manifold on all gasoline engines. The ther­
mostat on diesel engines is installed in top of the
engine front cover at the outlet to the radiator.
controlled by a thermostatic element. Restriction
valve cracks or just starts to open at predetermin­
ed temperature and continues to open as engine
coolant temperature increases. Refer to "Coolant
Circulation" earlier in this section for operation
of engine thermostats.
ENGINE COOLING SYSTEM
E N G IN E O VERHEAT SW ITCH
(WIRE EN D RETAINED BY SCREW )
E N G IN E TEMPERATURE
G A U G E S E N D IN G UNIT
(WIRE EN D RETAINED BY NUT)
Figure 3 — E ngine Units for Overheat Switch
7. When installing engine unit, do not use
NOTE: Switch unit must be removed from en­
Insert unit tube and lower half of body threads
ENGINE THERMOSTATS
Thermostats are installed in engine water
Thermostats consist of a restriction valve
Sec. 6K
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