What is Retained Accessory Power? (Explained)

Auto wiring comes with some complexity but with quality information, you can take care of things. Regardless of the model, a car features a retained accessory power (RAP). The electrical part of the vehicle is critical to the proper functioning of all parts, including the engine. What does this auto wiring part mean and do and how does it work? We’ll find out in this detailed post.

What is a RAP and How does it Work?

The RAP system enables specific car functions and accessories to work regardless of whether or not the ignition switch is temporarily turned off. The BCM uses inputs from the passenger compartment door switches to know the operation of the relay when the ignition is in OFF status.  

Every car features an accessory system that’s connected to the circuits. The accessory circuits include power windows, radios, stereos, and others. The retained accessory power system helps these accessories to stay on even when the car ignition is turned off. The accessories can be active for more than 600 seconds after switching the ignition off. In other words, the RAP allows the accessory circuits to remain active until. The radio will be live even when the car door is left open. 

Now, let me walk you through the RAP schematics. The RAP gets its direct power from two parallel sources: the ignition switch and junction block underhood. Depending on your car model, the RAP electrical schematic shows that power comes from the top and ground from the bottom. The power gets its ground from the radio. Essentially, when there’s power and ground, the radio is supposed to be naturally activated. 

How RAP works and is enabled

What enables the RAP? Your car’s body control module (BCM) activates the RAP to keep it active. The BCM is the engine room of the RAP system, whether it is relay, ground, or power. In the RAP power mode, the BCM activates the right and left power windows. It achieves this using the parallel powers the RAP relay supplies.  

Besides, the BCM also enables the windshield washers and wipers to use the RAP relay and adaptive cruise control (ACC) relay. Plus, the module is also responsible for sending a class-2 message to the serial data module. Even though the module works in the RAP power mode, it’s not typically attached to the RAP/ACC relay. 

However, the BCM monitors the module through the class 2 power messages. The radio won’t work in the RAP power mode if the BCM malfunctions or fails to send a class 2 message about the active status of the RAP.

How the RAP gets Deactivated

Although it can be self-activated, the retained accessory power system can become deactivated by a couple of factors. When the RAP is disabled, the following is likely to have occurred:

Car door open

The car door is open: Typically, when your car door is closed, the radio should stay on but should go off once you open the door. The RAP relay is supposed to get power from the ignition switch. Meanwhile, when you see the RAP at rest, it means that the door is closed. 

Low battery voltage range

A low voltage from the battery can cause the retained accessory power to deactivate. Remember, the BCM monitors the amount of acceptable battery voltage that gets to the RAP relay. Once the BCM detects a low battery voltage, the RAP deactivates.    

The BCM doesn’t enable the RAP power if the doors are not closed and the battery voltage is short of an acceptable range.

Switch ON-OFF Transition

The BCM not only monitors the position of the ignition switch; it also controls the amount of voltage from the battery and the status of the passenger compartment door. These functions aim at determining if the RAP power mode needs activation. The BCM activates the RAP power as soon as the ignition switch is in the OFF position.

Elapse time after the ignition turned OFF

If 10 minutes elapse after the ignition switch has turned on, the RAP power mode with disenable. After 10 minutes of enabling the RAP power, the BCM goes on to send a class 2 message to the data network through a serial communication system. This idea is to enable the message to deactivate all BCM-powered modules. 

Retained Accessory Power (RAP) Relay and ACC

With its low-side drive, the BCM controls the RAP and Accessory Power (ACC) relay coil circuits. The power output from the RAP relay produces battery voltage to the wiper windows and windshield wipers. When you select the ACC and Run power modes, the power windows and windshield wipers receive voltage from the RAP/ACC relay.

As a general rule of thumb, if we have the power and the ground from the BCM, the radio should work. The BCM always looks for different signals, the top of which is whether the front or rear door is closed or open. The BCM provides the ground through the black wire to the control relay which in turn energizes and then enables the RAP. The BCM provides the ground and power to the RAP relay.

Retained Accessory Power Malfunction: How to Diagnose

To check the Radio retained Accessory Power, do the following:

  • Close all passenger compartment doors
  • Open and close the driver’s door
  • While doing the above, observe the courtesy lamps 
  • The lamp should come on when the door is ajar and close 30 seconds after the door is closed
  • If the courtesy lam fails to respond accordingly, then there is a problem.
  • However, you may need to replace and reinstall a new radio if the courtesy lamp is working properly. The radio may be the RAP problem. 

Faulty Relay Retained Accessory Power: How to Diagnose

  • Turn the switch ignition on
  • Open and close each vehicle door intermittently. 
  • While doing the above, observe the results of the scan tool on the right rear door ajar switch, left rear door ajar switch, drive door ajar switch, and passenger compartment door ajar switch.
  • Switch the scan tool off on the RAP/ACC power relay command
  • Try to operate any RAP relay within 5 seconds 
  • Turn on the Ignition 
  • Close all doors
  • Observe and take the scan tool readings 
  • Turn off the Ignition 
  • Remove the ignition key
  • Observe that the scan tool parameter is active for at least 10 minutes before turning inactive.

Final Thoughts

Now you’re informed about what retained accessory power (RAP) is. You shouldn’t have issues identifying or fixing the system when it malfunctions. Like all vehicle components, the RAP is susceptible to damage. However, it plays a critical role in monitoring the operation of the radio and other essential accessories in your car.    

  • James Smith

    I'm James Smith, a car enthusiast and a content writer for Daily Car Tips. I have a passion for all things cars, from classic muscle cars to the latest electric vehicles. When I'm not writing about cars, you can usually find me tinkering with my car or checking out the latest automotive trends.