Engine Cooling Fan GMC Cab & Chassis S15

DESCRIPTION

Basic liquid cooling systems consist of a radiator, water pump, thermostat, cooling fan, pressure cap, heater, and various connecting hoses and cooling passages in the block and cylinder head. Many vehicles use a clutching fan, which may incorporate thermostatic control, or a flexible blade fan, or both, to reduce noise and power requirements at high engine speeds.

Some vehicles use a thermoswitch (or thermoswitches) usually located in the radiator tank to activate electric cooling fan(s). Many vehicles with transversely mounted engines use this system. For a better exhaust emission control system, some vehicles use a thermostatic vacuum switch to advance ignition timing in case of overheating.

As most new models require use of permanent, year round (ethylene glycol) type anti-freeze, coolant recovery systems are being used more commonly to prevent coolant loss. Requirements for antifreeze have changed due to materials used in engine manufacture.

DRAINING

Remove radiator cap and open heater control valve to maximum heat position. Open drain petcocks or remove plugs in bottom of radiator and in engine block. In-line engines usually have one plug or petcock, while V-engines usually have one for each bank of cylinders.

CLEANING

A good cleaning compound will remove most rust and scale. Follow manufacturer's instructions in use of cleaner. If considerable rust and scale have to be removed, flushing will be necessary. Clean radiator cooling fins by blowing out with compressed air from back to front of radiator.

Note. Some models use plastic or aluminum components in radiators. Ensure proper cleaning solution is used.

FLUSHING

1. Back flushing is a very effective procedure for removing rust and scale from cooling system. Radiator, engine and heater core should be flushed separately for best results.
2. To flush radiator, connect flushing gun to water outlet of radiator and disconnect water inlet hose. Use hose, connected to radiator inlet, to prevent flooding engine. Use air in short bursts only, as clogged radiator could be easily damaged. Continue flushing until water runs clear.
3. To flush engine, remove thermostat and reinstall housing. Connect flushing gun to water outlet of engine. Disconnect heater hoses from engine. Flush using short air bursts until water runs clean. Flush heater core as described for radiator. Ensure heater control valve is set to maximum heat position before flushing heater.

REFILLING

Engine should be running while refilling cooling system to prevent air from being trapped in engine block. After system is full, continue running engine until thermostat is open. Recheck coolant level. Do not overfill system. Refer to COOLANT SPECIFICATIONS.

Scheme 1

1. Visually inspect thermostat for corrosion and proper sealing. If satisfactory, suspend thermostat and thermometer in container of water. Do not allow either thermostat or thermometer to touch bottom of container, as concentration of heat could cause an incorrect reading.
2. Heat water until thermostat just begins to open. Note temperature at which thermostat begins to open. Compare to specification in table. See COOLANT SPECIFICATIONS. Replace thermostat if it does not open. (Scheme 1): Testing Thermostat NOTE: Thermostat should open as water is heated. NOTE: Pressure tester is used to test both radiator cap and complete cooling system. Test as follows, or follow tool manufacturer's instructions.

Pressure Testing Radiator Cap

Visually inspect radiator cap. Dip cap in water and connect to tester. Pump tester to bring pressure to upper limit of cap specification. Refer to COOLANT SPECIFICATIONS. If cap fails to hold pressure within specified range, replace cap.

Testing Radiator Pressure Cap Attach cap to tester and apply correct pressure. Scheme 2

Scheme 3

1. With engine stopped, wipe radiator filler neck seat clean. Fill radiator to correct level. Attach cooling system tester to radiator and pump until pressure is at upper level of radiator rating. If pressure drops, inspect for external leaks. If no external leaks are evident, start engine and run with tester attached. NOTE: Pressure may build up quickly. Release any pressure above limit of pressure cap specifications or cooling system damage may result.
2. Extremely rapid build-up of pressure may indicate combustion leak into cooling system. Check for water at tailpipe. Water could indicate faulty head gasket, cracked block or cylinder head near exhaust ports. Remove oil dipstick and check for water droplets in oil. If water is evident, serious internal leak is indicated. (Scheme 3): Pressure Testing Cooling System

Anti-Freeze Concentration

Test anti-freeze concentration. Tester should have temperature compensating feature. Failure to take temperature into consideration could cause an error as large as 30°F (18°C) in freeze or overheating protection. Follow manufacturer's instructions for correct use of tester.

Radiator cap consists of pressure valve and vacuum valve. Cap has several different functions: preventing coolant loss when vehicle is in motion, preventing impurities from entering cooling system to minimize corrosion, allowing atmospheric pressure to eliminate vacuum that occurs in system during engine cooling period, and raising coolant boiling point approximately 3°F (1.6°C) per psi increase by maintaining constant cooling system pressure. See COOLANT SPECIFICATIONS.

Radiator Cap Operation. Scheme 4

Note. Cap should maintain constant pressure in cooling system.

Coolant recovery systems differ from normal cooling systems in that an overflow bottle is connected to radiator overflow hose. Overflow bottle is transparent or translucent to permit checking of coolant level without removing radiator cap. No adjustment or test is required beyond keeping vent hole or hose clean and checking pressure relief of radiator cap.

OPERATION

As coolant temperature rises and pressure in system exceeds pressure relief valve of radiator cap, excess coolant flows into overflow bottle. As engine cools and coolant contracts, vacuum is formed in system, drawing coolant, stored in overflow bottle, back into radiator. In properly maintained cooling system, only coolant loss will be through evaporation.

Coolant Recovery System. Scheme 5

Note. System should minimize loss of coolant.

This unit consists of thermostatically controlled fluid fan drive (torque control clutch). Thermal control drive is silicone filled coupling connecting fan to fan pulley. Drive is operated by control valve. Control valve is governed by temperature sensitive bi-metallic coil or strip. Control valve maintains flow of silicone through drive.

During periods of operation when radiator discharge air temperature is low, fan drive limits fan speed. High radiator discharge air temperature causes bi-metallic to allow greater flow of silicone to enter drive. This increases drag between driven member and driving member, resulting in higher fan speed and increased cooling.

Scheme 6

1. When engine overheating or insufficient cooling by air conditioning occur, fan drive and thermostatic control must be tested. Start with cool engine to ensure complete fan drive disengagement. Cover radiator grille sufficiently to induce high engine temperature.
2. Start engine. Operate at 2000 RPM and turn on air conditioning, if equipped. Fan noise will increase when fan drive engages. It will take 5-10 minutes for fan to become engaged. While operating engine under these conditions, observe temperature light to prevent overheating. If hot light comes on, remove cover from radiator grille.
3. As soon as drive engages, remove radiator grille cover and turn air conditioning off to assist in engine cooling. After several minutes fan drive should disengage. This can be determined by reduction in fan speed and noise. If fan drive fails to function as described, it should be replaced.
4. Fan drive should also be replaced if shaft seal is leaking fluid. If noise or roughness can be felt while turning by hand, replace drive. Frozen unit must be replaced. Blade tip rock of more than 1/4" front-to-rear indicates worn drive unit which should be replaced. (Scheme 6): Thermal Control Fan Drive Unit NOTE: Rotation speed will vary with engine temperature.

This unit is same as thermostatically controlled fan drive except it is not controlled by bimetallic (temperature sensitive) coil. Fan drive allows fan to be driven in normal manner at low speeds while higher engine speed limits rotational speed of fan to predetermined rate. Silicone in drive housing provides more positive drive at lower speeds and allows greater slippage between driven member and driving member at higher engine speeds.

TESTING

In case of engine overheating during low speed or idle operation, increase engine speed to approximately 1000 RPM in Neutral. If condition is not corrected by increasing engine speed, replace fan drive unit with unit that is known to be operating properly and test by operating vehicle under same conditions. Replace unit assembly if trouble is corrected with test unit. All units are non-adjustable. Replace unit if damaged or operating improperly.

This unit is fixed blade assembly designed to flex blades as engine RPM increases. As RPM increases, blade pitch decreases, saving power and decreasing noise level. No adjustment or test is required beyond keeping fan belt adjusted to proper tension and ensuring that unit is not damaged.

Flex-Blade Fan. Scheme 7

Note. Blades should flex as engine RPM increases.

ELECTRIC COOLING FAN

Note. On models equipped with 7.4L V8 engine and A/C, an optional electric cooling fan is available. The optional fan provides additional cooling at low speed operation, extended idle, stop-and-go conditions, and when A/C is running. No additional information or specifications were available from manufacturer.