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HomeTips & GuidesRetained Accessory Power (RAP): How It Works

Retained Accessory Power (RAP): How It Works

About Retained Accessory Power (RAP), A car’s ignition switch controls the power on/off to most vehicle functions. So, when you turn it off, most of your car’s electrical system goes off.

But the Retained Accessory Power (RAP) system keeps the power on to enable you to close the windows and complete other tasks before you get out of the vehicle.

Let’s analyze the function of this system, including how to deactivate it and diagnose a faulty system. Read on to learn more!

Table of Contents

What Is RAP (Retained Accessory Power)?

As the name suggests, retained accessory power is a system that keeps accessory circuits electrified when you turn off the ignition. The keyword is “retained.”

Vehicles have several accessory circuits, such as head units, sunroofs, and power windows. The RAP system keeps these accessories powered for a preset period after switching off the ignition to give you time to close or shut everything before exiting.

This feature is more typical for power window switches because it lets you roll up the windows without putting the key back into the ignition. Most people forget to roll up the windows before switching off, making the feature convenient.

How Retained Power Accessory Works

The ignition switch cuts off power to these accessory circuits. So, where does the electricity come from?

Modern vehicles feature Body Control Modules (BCMs) that activate the RAP system using a relay or serial data connection.

The BCM first monitors the ignition switch position, battery condition, and the door’s open switch status to determine if it should initiate the RAP system. It then regulates this system using serial data circuits and relay control.

A body control module

Some accessory circuits receive the RAP serial data message from the BCM via serial data circuits. Such RAP-message-controlled power gets deactivated as required by their module’s mode of operation.

On the other hand, other subsystems get controlled directly by the BCM via a relay.

Relay Controlled RAP

BCMs power the following components using the RAP relay coil.

  • Power window switches
  • Sunroof switch
  • Traffic data receiver
  • Seat heating control module
  • Console accessory power outlet
  • Sunroof control module
  • Cigarette lighter receptacle
  • Transmission shift-lever position indicator
  • Mobile telephone control module
  • Mobile device wireless charger module
Sunroof control buttons

Sunroof control buttons

The control module keeps this relay on in all power modes except crank and off-awake. When the ignition is off, it keeps the relay powered for about 10 minutes after removing the key, provided all doors remain shut.

Here’s a summary of the conditions the BCM checks to cut the relay’s power.

  • It receives input from any door ajar open switch indicating the door is open after removing the key from the ignition switch.
  • The battery capacity decreases past the prescribed limit.
  • Its internal timer for the RAP module reaches the 10-minute time limit.

Serial Data Controlled RAP

The systems controlled using this avenue are the radio and vehicle communication interface module.

With the radio, the conditions for power cuts are similar to the relay’s above, but with one exception. Only the driver’s door switch will turn it off when open.

A luxury vehicle with a double-din head unit displaying images from the 360° cameras

A luxury vehicle with a double-din head unit displaying images from the 360° cameras

With the vehicle communication interface module, the BCM follows the radio conditions above to turn off the RAP, but with one exception.

If you have an active call and turn off the ignition, this module will continue being in RAP mode and keep the radio on until the call ends.

How To Deactivate RAP

You might already know the conditions for deactivating RAP from the ones described earlier, but let’s look at them in detail.

Open Car Door Switch

The BCM monitors the car door switches and checks for an open state. If the key is out of the ignition and the RAP timer has not elapsed, the RAP system will go off if you open the door. It is all doors in some cases and only the driver’s door in others.

An open car door

An open car door

Low Battery Power

BCMs also monitor the battery voltage level to ensure it is within the acceptable range to power the RAP relay. If it detects a decrease in battery voltage, the BCM will deactivate or not power the RAP relay. 

Elapsed Timer

Body control modules feature internal timers for the RAP system with a setting of 10 minutes or 600 seconds after turning off the ignition and removing the key. Past this time, the BCM cuts off power to all the modules powered by the RAP system.

Diagnosing a Faulty Serial-Data Controlled RAP Function

If you recall, the serial-data-controlled RAP powers the radio. So, we’ll use this accessory device to diagnose the RAP system using the following steps.

  • Turn on the ignition.
  • Close all the passenger doors to close the ajar switches.
  • Turn off the ignition and remove the key.
  • Open the driver’s door, then close it while observing the courtesy lamps.
  • The courtesy lamps should turn on when you open the door and close 30 seconds after you close it.
A vehicle’s courtesy lamps

A vehicle’s courtesy lamps

  • If the radio does not turn off, you might have a defective head unit (with a RAP issue).
  • But the radio is okay if the lamps don’t turn off after 30 seconds. Look at other parts of the system or the ajar switch. 

Diagnosing a faulty power relay circuit is easier because you can use a scan tool to check the condition of the solenoid circuit.

A mechanic testing a vehicle’s relay circuit

A mechanic testing a vehicle’s relay circuit

Wrap Up

The retained accessory power system is a BCM-powered feature that keeps the power running to certain accessory circuits when you switch off the ignition. 

The system gets its power connection via a relay or serial communication, and both have specific deactivation conditions for deactivation, which we’ve looked at above. 

That marks the end of this article. Check out this power window wiring article to learn more about this feature, and comment below if you have questions, sentiments, or thoughts to share. Cheers!

Hi I am Christa, sales manager of Cloom.

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And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

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