Component Functional Description - Hood Latch/Ajar Switch 1

Component Functional Operation:  The Hood Ajar Switch 1 is a hard-wired input to the Body Control Module (BCM). For most vehicles it is integrated as part of the Hood Latch assembly. If it is incorporated into the latch assembly there will be an electrical connector on the latch assembly. If the latch does not have an electrical connector, there should be a 2nd plunger switch under the hood. The Hood Ajar Switch 1 receives a Battery voltage signal from the BCM through an internal pull up resistor. When the ajar switch is open the signal passes through both internal resistors and the resistance through the switch should be approximately 1000 Ohms. When the switch is closed the signal passes through only the RSC resistor and the resistance through the switch should be approximately 500 Ohms.

Technical Data: 

Parameter	Nominal
RSC	499 ohms	Courtesy of CHRYSLER GROUP, LLC
Roc	499 ohms
Rpu	1100 ohms
Signal reference voltage	Battery/system voltage
Protection	Short to Ground/Short to Battery
Detection	Short to Ground/Open Circuit/Short to Battery

The BCM monitors the Hood Ajar Switch 1 signal circuit and converts the voltage signal into counts to broadcast over the CAN Bus network. 

• Electronic Control Modules read an analog signal voltage and converts it to digital counts. Each count is an equal percentage of the signal reference voltage. This can be seen when viewing some sensor or switch voltages and converted values on the scan tool. The values will change in blocks (Ex. 0.08v, 0.12v, 0.16v or 8 Pa, 16 Pa, 24 Pa etc.). The signal voltage could be anywhere from 5.0 volts to a Battery voltage reference.
• The resolution of the signal (number of counts) is determined by the required precision of the input by the ECU. Typically, sensor signals are high resolution. This allows the module to read small changes over a wide range in a variable resistance sensor signal. These are typically used on a temperature, pressure, position or MUX type of sensor.
• Switches on the other hand are usually of much lower resolution than sensors. This is because an ECU is generally looking for a switch to be in only a few positions that use fixed resistors. The lower resolution also helps the ECU to overcome some resistance in a circuit and still read the correct state. This also lowers the sensitivity to setting DTCs.

The Hood Ajar Switch 1 signal voltage is typically divided into 15 counts. The table below shows the relation between for the Hood Ajar Switch 1 states. The voltage ranges given are based on a nominal reference voltage of 13.5 volts. The counts and percentage of Battery voltage will remain constant, but the signal voltage will vary as actual Battery voltage changes.

• Using the following method to convert the actual Hood Ajar Switch 1 signal voltage to a percentage of Battery voltage. (Hood Ajar Switch 1 signal voltage / Battery voltage value = % of Battery voltage).
• Examples: (4.7v signal / 14.2v Battery = 33%) 5 counts. (6.7v signal / 12.6v Battery = 53%) 8 counts