III-77
B. Warm up engine – Next, let the engine warm up to normal operating
temperature and set both fan and temperature on high. Feel both heater sup-
ply and return hoses. If there is a noticeable difference in their temperature,
it indicates a low flow of coolant through the heater core (a partially closed
or blocked heater control valve). This could result in poor heating perfor-
mance during cold weather conditions.
2. Inspect Other Functions
There are some things you can·t see or feel when you inspect the thermostat,
heater core, radiator pressure cap, electrical switch and control valve functions.
Some of these can be checked with the pressure and thermostat testers as de-
scribed in Chapter 6. A hand pump pressure tester can also be used to check for
coolant leaks. This is done by using the pump to raise the pressure inside the
system above normal operating pressure to force small suspected leaks to show
up.
Heater/cooling electrical and valve component inspection is the same as air
conditioner inspection. The controls are operated to see if they function correctly
to maintain or vary cab temperature and air flow.
Preventive Maintenance Worksheet
Please feel free to modify or copy the worksheet in Figure 7-9. Actual vehicle use, mile-
age, operating conditions and maintenance budget may influence service frequency.
Inspection & Maintenance
Figure 7-8
Heater/cooling system
potential problem areas and
checks are indicated in this
drawing.
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