Hood Sensor/Switch (For Alarm): Description and Operation
The hood ajar switches are interchangeable physically, but not functionally. In each case, the hood ajar switch is a normally closed switch that is held
open as the spring-loaded switch plunger is depressed by the inner hood panel reinforcement when the hood panel is closed and latched. When the hood
is opened, the spring-loaded switch plunger extends from the switch body and the switch contacts are closed.
In the underhood security application (export vehicles only - dark brown retainer ring), the switch is connected in series between ground and the hood
ajar switch sense input of the Front Control Module (FCM). The FCM uses an internal resistor pull up to monitor the state of the hood ajar switch
In the remote starter system application (domestic vehicles only - white retainer ring) the switch has a 1 kilohm diagnostic resistor connected in parallel
to the switch contacts between the two switch terminals. The switch is connected in series between an output of the FCM and the hood ajar switch sense
input of the FCM. The FCM continually monitors this circuit and will store a Diagnostic Trouble Code (DTC) for any fault that it detects.
The components of the switch self-adjustment feature include an integral stop on the shaft of the plunger and a ribbed, ratcheting sleeve-like retainer ring
at the top of the switch body from which the plunger extends. With the switch mounting collar secured in its mounting bracket, the plunger is depressed
by the hood inner reinforcement as the hood is closed. As the plunger is depressed, the plunger stop contacts the top of the retainer ring and it is driven
downward, ratcheting through the switch mounting collar until the hood is fully closed and latched. The ribs on the retainer ring are engaged within the
mounting collar to maintain this adjusted position.
The hood ajar switch as well as the hard wired circuits between the switch and the FCM may be diagnosed using conventional diagnostic tools and
procedures. Refer to the appropriate wiring information. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the
electronic controls or communication between modules and other devices related to hood ajar switch operation that provide some features of the export
vehicle theft or domestic remote starter systems. The most reliable, efficient, and accurate means to diagnose the electronic controls and communication
related to hood ajar switch operation requires the use of a diagnostic scan tool. Refer to the appropriate diagnostic information.