Need help on diagnosing light problem. Reverse lights do not come on when using keyfob, under hood fuse and passenger floor fuse good. Put in new bulbs even though the old ones looked good. I see on a wiring diagram that the BCM has a back up relay in it. Should I look there next?

 

Please....for the love of all that is holy in this vast world, and beyond.....give a year and transmission when asking these kinds of questions. I will go with 2007 A6 info....on your own if I guessed wrong:

Backup Lamps Malfunction (Automatic Transmission)

Circuit/System Description
The underhood fuse block supplies battery voltage through the STOP/B/U/ LPS fuse to the coil and switch terminals of the BCK/UP LAMP Relay located in the body control module (BCM). When the transmission is placed in the REVERSE position, the transmission control module (TCM) sends a class 2 message to the BCM. The message indicates that the gear selector is in the REVERSE position. The BCM grounds the back up lamp relay control circuit of the reverse lamp relay . This energizes the reverse lamp relay, closing the switch side contacts and provides battery voltage to the REVERSE LAMPS fuse through to the backup lamps. The backup lamps are permanently grounded at G302. With both power and ground the backup lamps illuminate. Once the driver moves the gear selector out of the REVERSE position, a message is sent by the TCM via class 2 serial data requesting the BCM to remove ground from the back up lamp relay control circuit. This opens the REVERSE LAMP Relay switch contacts turning OFF the backup lamps.

Circuit/System Testing
Ignition OFF, disconnect the backup lamp relay.
Ignition ON, verify that a test lamp does not illuminate between the relay controlled output circuit and ground.
⇒ If the test lamp illuminates, test the relay controlled output circuit for a short to voltage.

Verify that a test lamp illuminates between the relay coil B+ circuit and ground.
⇒ If the test lamp does not illuminate, test the relay coil B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the relay controlled output circuit for a short to ground. If the circuit tests normal, test or replace the reverse lamp relay.

Verify that a test lamp illuminates between the relay switch B+ circuit and ground.
⇒ If the test lamp does not illuminate, test the relay switch B+ circuit for an open/high resistance.

Disconnect the harness connector at the inoperative backup lamp.
Test for less than 1.0 ohm of resistance between the inoperative backup lamp ground circuit terminal 1 and ground.
⇒ If greater than the specified range, test the ground circuit for an open/high resistance.

Connect the harness connector at the inoperative backup lamp.
Connect a 10A fused jumper wire between the relay switch B+ circuit and the relay controlled output circuit. Verify the backup lamps are activated.
⇒ If the backup lamps do not activate, test the relay controlled output circuit for an open/high resistance. If the circuit tests normal, test or replace the inoperative backup lamp.

Connect a test lamp between the relay coil B+ circuit and the relay coil control circuit.
Command the backup lamps ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
⇒ If the test lamp is always ON, test the relay coil control circuit for a short to ground. If the circuit tests normal, replace the BCM.

⇒ If the test lamp is always OFF, test the relay coil control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

If all circuits test normal, test or replace the reverse lamp relay.

Relay on BCM. If same part number as another relay, can swap to see if it fixes it before buying another one:

 

By any chance did you install LED bulbs? If so, flip them 180°.

 

My 2006 base vert had that same problem....turned out to be a bad ground.

 

The reverse lights do not come on with the fob, but do they come on when you shift into reverse?

 

Yes, please include all info about your car as the electrical systems in these car can vary by model year…do you have a 12 volt test light and know how to use it ??

 

Sorry guys, I have googled this light issue for 2 days and forgot to include my specs. I’ve since then joined your forum for correct info.

My car is a 2011 with auto trans. and is a convertible for future reference.
I do have a 12V test light and a decent Fluke meter.
Thank you C6ToGo for the good info and wiring diagrams.

 

Do you know how a relay works and how to test it ??...unfortunately the back up relay is on the bottom of the BCM so you will have to unbolt it to flip it over and check it...that is what the dashed lines on that relay and the run/crank relay represent in the schematic...and it's a good idea to include your car info in your profile that way down the road we don't have to pull this information from you if you have another concern.

 

E brake on, push on bottom of starter button for 5 seconds to put car into run mode, motor off, then foot on brake pedal to put trans in reverse.

Now start with checking back up light bulbs sockets to see if you have power coming in, and good ground to ground on the other pin leg. If yes, and LRD builbs in play, reverse them in the socket to see if that solves the problem.

If bad ground, then chase the ground wire to frame ground. If no power on the positive side of the sockets, remove the inner passenger side tail light and check the rear tail light connector for corrosion or not plugged in all the way. It in and slightly down in the rear bumper void.


If still not power there for reverse lights and fuse is good in engine fuse box to send power to BCM for the back up lights, then time to trace the power line back up to the BCM, and even pull the BCM to check the back up relay on the bottom side of the unit.

if still a problem, then time to put tech 2 on car, to make sure that trans is showing trans in reverse to start, so BCM can activate the back up lights in the first place.