Accessory Delay Relay Coil Circuit Failure

OBD II fault code B1302 is almost universally defined as “Accessory Delay Relay Coil Circuit Failure”, and is set when the PCM (Powertrain Control Module), and/or the GEM (Generic Electronic Module) detects a failure of one or more circuits that control delayed access to system power by one or more accessories.

In practical terms, most vehicles are equipped with accessories such as sunroofs, power windows, courtesy lights, dome lights, and others that are able to access vehicle system power for a set period of time after the ignition key is removed from the ignition lock.

This feature allows drivers to close windows and sunroofs while the vehicle is switched off, but delayed power also allows for headlights and courtesy lights to remain switched on for set periods of time to allow drivers and passengers to find their way should they arrive at a destination in the dark.

Moreover, on some applications, most notably VW products, the electrically operated radiator cooling fan is fed with delayed power to combat the effects of heat soak, which raises the coolant temperature significantly when a hot engine is switched off.

Although design specifics vary somewhat between applications, the general operating principles of delayed power to accessories involve a controller known as the GEM (Generic Electronic Module) or sometimes the SJB (Smart Junction Box) that controls one or more relays based on the position of one more accessory control switches. Note that the GEM itself does not supply power to accessories; it only supplies power to relays that ultimately feed power to the accessory involved.

In some cases, one relay can control more than one accessory circuit, in which case all accessory control switches and/or sensors on affected circuits is monitored. All inputs are communicated to other affected controllers via the CAN (Controller Area Network) and based on switch/sensor inputs and information received from other controllers, the GEM activates the relevant relay to feed power to the accessory circuit(s) controlled by that relay.

For instance, based on inputs from the headlight switch and other sensors, such as an ambient light meter, the GEM will provide power to the headlight control circuit to keep the headlights on for a set period of time after the key is removed from the ignition lock. NOTE: GEM controllers are sensitive to input voltages, which is why they are protected by Field Effect Transistors. Therefore, great care must be taken when testing input circuits; do not use test lights inside fuse boxes.

Use a proper circuit tester like the example shown below to prevent accidental short circuits and to remove all the guesswork that goes with using test lights. Note that the tester shown below plugs into the fuse holder, thus giving accurate results and readings.