TUBE AS 2W6458 - Caterpillar

System Operation Description:Use this procedure to troubleshoot the electrical system if a problem is suspected with the air shutoff system or if any one of the diagnostic codes in Table 1 is active or easily repeated.
Table 1
Diagnostic Codes Table
Description Conditions which Generate this Code System Response
446-5 Air Shutoff Relay current below normal The Electronic Control Module (ECM) detects an open circuit and/or a short to the +Battery in the circuit for the air shutoff switch ("ASOS"). The ECM can only detect this condition when the "ASOS" is not energized. The code is logged.
446-6 Air Shutoff Relay short The ECM detects a short circuit to ground in the "ASOS" circuit. The ECM can only detect this condition when the "ASOS" is energized. The ECM has the ability to stop the engine by shutting off the inlet air supply in an emergency situation. The ECM activates the output driver for the emergency shutdown system. The "ASOS" energizes the two air shutoff solenoids. The solenoids trip the air shutoff valves.There are two conditions which cause the engine control to activate the output driver for the air shutoff switch. The first condition occurs when the operator activates the customer's emergency stop. When the customer's emergency stop button is activated, the emergency stop inputs U and T to the ECM from the customer connector go high. The ECM will recognize this change. The ECM will energize the output drive for the emergency shutdown system for one second through the diode assembly.The second condition occurs for an engine overspeed. If an engine overspeed occurs, the ECM will activate the "ASOS" for the emergency shutdown system through the diode assembly.Relay "R3A" and relay "R3" are energized at the same time as the "ASOS" through terminal 3 of the diode assembly. Terminal 3 of the diode assembly will energize relay "R3A" and relay "R3" for one second. The contacts of relay "R3" keeps relay "R3A" and relay "R3" energized until power is cycled. The contact of relay "R3A" keeps the coil of relay "R2A" energized. When relay "R2A" is energized, power is removed from the ECM. The ECM will power down. The injection signals to the injector solenoids will stop.The contact of relay "R3A" also supplies power to terminal 3 of the connector for a timer and to the overspeed indicator on the instrument panel. When power is applied to terminal 3 of the connector for the timer, the timer will activate. The timer outputs 24 VDC from terminal 6 of the connector for the timer for 2.5 seconds. The 24 VDC from the timer is applied to the coil of the "ASOS" and to terminal 2 of the diode assembly. The "ASOS" energizes the two air shutoff solenoids. The solenoids trip the air shutoff valves. The 2.5 second time limit for activation of the "ASOS" prevents damage to the air shutoff solenoids. A normally closed switch at the air shutoff valves will open when the air shutoff solenoid activates the air shutoff valve. The switch