Garland MCO-GD-10M Service Manual

Contents

Garland MCO-GD-10M Service Manual Manual pdf 15 page image

15CHECK GROUNDINGA common ground is required for the pilot burner, the ignitor-sensor, the GND terminal of the S86, and the mainburner. The main burner generally serves as the common ground. If the ground is poor or erratic, safety shutdownsmay occur occasionally even though operation is normal at the time of the checkout. Therefore, if nuisanceshutdowns have been reported, be sure to check the grounding.NOTE: If the ground circuit path is incomplete, the S86H system control will allow one trial-for-ignitionbefore going into safety lockout.Electrical grounding connections at the pilot burner, ignitor/sensor and S86 must be clean and tight. If lead wire isdamaged or deteriorated, use only No. 14 of 18 gauge, moisture-resistant, thermoplastic insulated wire with 105°C.(221°F) minimum rating as replacement. Excessive temperature at the ceramic flame rod insulator can also permitelectrical leakage to ground. Examine the flame rod and mounting bracket, and correct if bent out of position.Replace ignitor/sensor of insulator is cracked.CHECK SPARK IGNITION CIRCUITThe electronic module and step-up transformer in the S86 provides spark ignition at 15,000 Volts (open circuit). Thiscircuit can be checked at the S86 module as follows:1. Turn off the manual gas cock to prevent the flow of gas.2. Disconnect the ignition cable at the S86 stud terminal to isolate the circuit from the pilot burner/ignitor/sensor, andprepare a short jumper lead using heavily insulated wire, such as ignition cable.3. Energize the S86. Touch one end of the jumper firmly to the S86 ground terminal (GND). Do not disconnect theexisting ground lead. Move the free end slowly toward the stud terminal to establish a spark and then pull thelead wire slowly away from the stud. Note the length of the gap at which arcing stops.CAUTION: Do not touch either stripped end of jumper or stud terminal. This is a very high voltage circuitand electrical shock can result. Perform the test immediately upon energizing the system -before the S86H goes into safety lockout and interrupts the spark circuit.4. An arc length of 1/8" (3.2 mm) or more indicates satisfactory voltage output. Replace the S86 if no arc can beestablished or the maximum gap is less than 1/8" (3.2 mm), and the fuse and power to the S86 input terminalwas okay.CONTROL MODULE FLAME SENSOR CIRCUITThe control module provides AC power to the ignitor/sensor which the pilot burner flame rectifies to direct current. Ifthe flame signal back to the control module is not at least 1.0 μA DC, the system will lockout. The output of the flamesensing circuit cannot be checked directly, so check the flame sensing circuit indirectly by checking the flame sensingcurrent from the ignitor/sensor to the control module as follows:1. Connect a meter (DC micrometer scale) in series with the flame signal ground wire (Burner Ground Terminal).Disconnect the ground wire at the control module. Connect the red (positive) lead of the meter to the free end ofthe ground wire. Connect the black (negative) meter lead to the quick-connect ground terminal on the controlmodule.2. Restart the system and read the meter. The flame sensor current must be at least 1.0 μA, and the reading mustbe steady. If the reading is below the value designated or the reading is unsteady, check the pilot flame andelectrical connections as described above. Also, replace the ignitor/sensor if the ceramic insulator is cracked.EXAMPLES OF UNSATISFACTORY PILOT FLAMES