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Heating and Air Conditioning Auto Service Tech Tips
These Tech Tips are provided courtesy of autoclassroom.com, an online training resource for automotive instructors, students, and technicians.
Check the radiator, not the overflow container
If an engine is overheating or if the heater produces heat only once in a while, the coolant level should be checked. Many people check the level of coolant in the overflow container and believe that if the radiator were low, the coolant would be drawn from the overflow container into the radiator. However, if there is a leak such as from a defective water pump, the cooling system will not be airtight and a vacuum will not be formed to draw coolant from the overflow back into the radiator. Therefore, if there is a cooling system problem, always check the level of the coolant at the radiator itself. Always check the coolant when the engine is cold or before the engine is started to avoid getting burned by hot coolant that will gush from the radiator if the pressure cap is removed when the coolant is hot.
The difference between heat and temperature
Temperature is measured in degrees, either Fahrenheit (F) or Celsius (C). The temperature of a lighted match may be the same as the temperature of a bonfire, yet the bonfire produces a much greater amount of heat. Heat is measured in British Thermal Units (BTUs) or Calories (C) and represents a measurement of the heat needed to raise the temperature of a given amount of water.
BTU: The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.
Calorie: The amount of heat required to raise the temperature of one gram of
water one degree Celsius.
A lighted match may be as hot as a bonfire, but it certainly does not have enough heat energy to heat a pound of water. The water has to absorb heat before its temperature increases. Therefore, an air conditioning system must remove heat before the temperature of the air is reduced.
Broken condenser line: Check the engine mounts
Most air conditioning systems use aluminum and flexible rubber lines between the compressor and the condenser. Because the compressor is mounted and driven by the engine and the condenser is mounted to the body, these lines can break if the engine mounts are defective. The rubber hoses attached between the aluminum fittings of the compressor and condenser are designed to absorb normal engine movement. Worn engine mounts would allow the engine to move. Aluminum lines cannot be flexed without crashing and breaking. Therefore, the wise technician will carefully inspect and replace any and all worn engine mounts if a broken aluminum condenser line is discovered to prevent a premature failure of the replacement condenser.
Hot/cold/hot/cold heater diagnosis
A common customer complaint is a lack of heat from the heater, but only while driving there seems to be plenty of heat when the engine is at idle speed and the vehicle is stopped. This is a classic symptom of low coolant level. The lower than normal coolant level in the radiator prevents enough flow to supply the heater core. When the engine speed is reduced, the water pump turns slower and coolant can more easily flow through the heater core, resulting in heat from the heater. As the engine speed increases, the water pump speed also increases. Because there is less than the proper amount of coolant in the system, the water pump will be able to supply coolant through the engine (a path of lower resistance).
The hand test
To check a radiator or condenser for possible clogged or restricted areas, simply touch the outside of the unit with your hand. Any cool spot indicates that the radiator or condenser is clogged in that cool area. |
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