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Aircraft Maintenance TroubleShoot > Heater Troubleshooting South Wind 8472 Troubleshooting principlesThis heater must have five supplies fed to it for operation. The first of these is fuel. The fuel pump on the combustion air blower will supply fuel to the heater under the proper pressure. The second supply required is combustion air. On the ground, the combustion air blower will furnish the proper amount of air for combustion. In flight, particularly at altitude, the blower is apt to be marginal and for this reason it's recommended that the static pressure in combustion air pickup be about 4.0 to 6.0 inches of water above the static pressure in the exhaust. This is a function of the installation and must be considered at the time the heater is designed into the aircraft. Next, the heater requires a source of ignition. The ignition system on the heater will fulfill this requirement. It's designed to operate all of the time that the heater is turned on, whether the heater is burning or not. Fourth, the heater requires air to keep it cool (ventilating air). This is the air used to heat the cabin. On the ground the ventilating air blower will supply as little as half the air required for the heater to operate at full output. In flight, when this blower is either supplemented or supplanted by ram air, or air from the pressurization system, there will probably be far more than the minimum requirement flowing through the heater. The final requirement of the heater is a source of power for the control circuit, and to drive the accessories listed above. While this is listed last, it's usually one of the first things to check when troubleshooting a heater that is not functioning properly.The first thing to look for in troubleshooting, is to see if both motors run. If neither one does, the trouble is probably in the wiring up to the heater. If the ventilating air blower operates but the combustion blower doesn't, check the overheat switch, it may be tripped. If the overheat switch isn't tripped, check with a test light to find the open circuit in the wiring to the combustion blower motor. If there is no open circuit and the blower doesn't run, the blower motor itself may be defective. If both blowers run but the heater doesn't ignite, the next step would be to check the ignition system. First check with a test light to see if there is power being supplied to terminal 10. If there is not power and the combustion air blower is running, the trouble may be in the airflow switch. If there is power at this point, the next step is to check the spark plug. Remove the ignition cover, pull the high tension lead from the spark plug, and check to see if the ignition system is supplying high voltage to this point. If the voltage is adequate, remove the spark plug and check. If there is no evidence of spark to the plug, the trouble may be either the ignition coil or in the capacitor system. Substitute a new coil first, then a new ignition unit. If the ignition system checks out satisfactorily, the next step is to check the fuel system. For ease in checking, loosen the fuel fitting on the heater to see if there is fuel under pressure at that point. If so, check with a test light to see if there is power at terminal No. 7 on the terminal strip. If power is indicated, the trouble may be in the fuel solenoid, or the fuel nozzle, and the heater will have to be disassembled to correct the problem. If this terminal is dead, check terminal No.8. Power at this point but not at No. 7 would indicate the trouble was in the thermostat, or the thermostat wiring. If there is no indication of power at No. 8, check back, with the test light, through the airflow switch, using the applicable wiring diagram as a guide. Power at the airflow switch, but not at terminal 8, would indicate a defective airflow switch. Replace the airflow switch if there is power at terminal 6, but none at terminal 8 or 10. Troubleshooting a heater which burns but doesn't have sufficient output is the most difficult type of troubleshooting and requires the use of a thermometer which can be put into the ventilating air duct at a point about 12 to 18 inches downstream from the heater, preferably around a bend in the hot air ducting. With the heater in operation and the thermostat set to the maximum heat position, the thermometer should read 180 to 190 degrees F. If the thermometer doesn't register this, the test light should be used to determine if the thermostat is properly cycling the heater fuel supply on and off. If the thermostat is cycling, but at a lower temperature, it should be recalibrated. When the heater is shutdown, there is no ram air available to cool the heater after shutdown; therefore, it's important to reset the thermometer to minimum setting a minute or two before turning the heater off. If will be noted that this heater doesn't have an automatic "purge" or cooling period after shutdown. Testing for low heat output when the thermostat is not cycling the fuel on and off requires the use of a pressure gage tapped into the fuel line between the fuel pump and the heater. The fuel pressure at this point in most installations should be 19 to 24 psig. If the fuel pressure is less than is required, the fuel pump or the fuel system up to the pump may be defective. many times the apparent fuel pump problems can be traced to a very small air leak into the system on the suction side of the pump. A hot spot in the fuel line up to the pump can cause vapor bubbles in the line. Since the fuel pump is small, it won't pump air or vapor, and sufficient fuel for the heater at the same time. In some installations a powerful fuel solenoid valve in the fuel line up to the pump has been found to generate enough heat to vaporize the fuel in the line and cause what appears to be a defective fuel pump. If the fuel pressure to the heater is adequate and the heater
still doesn't cycle on the thermostat, the trouble probably is in the fuel
nozzle in the heater.
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