PULLEY-FAN DRIVE 4P1679 - Caterpillar

Principle Of Operation (See Schematic, Page 22)
(1) The refrigerant sensor tip has a filament with an output collector installed near it. When the filament is heated, a small current will flow to the output collector lead. The filament has a special cover material which causes the current flow to increase when it is in contact with a refrigerant gas. This increased current causes a voltage increase to a level detector. When the voltage goes beyond the point at which it was set, the level detector output activates the indicator light and horn through a transistor amplifier. Power supply for the circuits is from two 1.5 volt D-size alkaline batteries connected in series to make 3 volts. Since this voltage is not high enough to activate the level detector circuit, a switching type power supply is used to increase the voltage to positive (+) and negative (-) 9 volts.Circuit Description (See Schematic, Page 22)
(1) Power from the two 1.5 volt batteries flows through switch S1 before it goes to the circuits. Power to the filament in the sensor tip flows from S1 to the battery control R10 which adjusts the current that flows through this circuit. A change in the filament current changes the detector sensitivity. Wires, in a location inside the flexible arm, make a connection from the sensor to the printed circuit board. The filament connections have an identification of E3 and E4 on the circuit board. The output signal on E2 flows through capacitor C1 to the level detector input. The negative 9 volt supply is connected to the sensor output lead through R8 and CR5. The filament is connected to the positive 9 volt supply line. This gives the voltage level across the sensor that is necessary for correct operation. Capacitor C8 is a filter for this voltage.(2) The level detector uses an integrated circuit operational amplifier A1. It needs a +12 and -9 volt supply. A reference voltage, adjusted by the BAL (balance) control R9, is sent to the + input. When the voltage on the - input goes beyond this reference voltage, the output of A1 will go positive. Capacitors C6 and C7 are filters that remove transient noise pulses from the input signal, so they do not trigger (activate) the level detector. Resistors R1 and R3 hold the - input to an even voltage level when no input signal is present. R2 and R5 control the maximum range of BAL (balance) control adjustment.(3) The PNP output transistors Q1 and Q2 are driven through R7 and zener diode CR1 by the level detector output. R6 holds Q2 off when no signal is present. Transistor Q2 gives base drive for Q1 in a darlington arrangement for increased power amplification. The output indicators, lamp L1 and horn H1, are driven by transistor Q1 when it is conducting (activated). When the horn and light come on, a large amount of current is taken from the batteries, which will cause their voltage to drop (decrease). This decreased