I got the FAST FLASH with a left turn signal this morning, so I took a look and both left rears were inoperative while the left front was flashing fast. I pulled the lamp assemblies out of the left rear, put the FLUKE on the wires and no voltage when the brake or the left turn signal was activated.

Looking at the wiring diagram in my Shop Manual, it appears that the BCM has a separate output for the left rear turn and brake filaments.

This concerns me, since there is either an open circuit somewhere between the left rear of the car and the BCM, or the BCM is sick.

Any of you have a similar experience, or lacking the experience, any ideas before I head off to the dealer for diagnosis and repair ? ? ? ? ?

Thanx, in advance

 

check bulbs and fuse?

 

I put LED bulbs in the rear a few years ago, and put ballast resistors in parallel with the LED's to pull enough amps to fool the BCM into NOT thinking there was a burned out bulb and pulsing the turn signals at the high frequency. This morning I did take the car to the dealer, and they said my BCM checked out OK. But the dealer wanted over $500 just to cut out my resistors, splice the wires, and put in stock bulbs. I declined and took my C6 back to my office/laboratory.

At the laboratory I cut out the resistors, installed new 3057 bulbs, and STILL had no left rear turn signal, left rear brake, or left rear 4-way flasher. But then, after starting and stopping the engine several times, and while driving to meet a client, I noticed that when I signaled for a left turn, the blink rate was NORMAL !!

Brake lights all worked, 4-way flashers too !! The only difference between left and right had been that I had 2.5 ohms of resistance on the left rear, and 3.0 ohms on the right rear. I think the BCM finally got tired of seeing too much current go to the left rear, and shut it down, thinking there must be a developing short that could burn up the car.

Now after querying the circuit a few times, it seems that the BCM detected that all was copacetic and it could resume sending current to the left rear. That would be one VERY intelligent BCM !!! Next step, this weekend, reinstall the LED bulbs in the left, but with 3.0 ohms instead of 2.5 ohms. Wish me LUCK .

 

I did it right this time, making heat sinks out of aluminum plate, fastening the correct 6.8 ohm resistors ( one for each brake-turn LED lamp, 3.4 ohms per side ) to the heat sink, and fastening the heat sink to the forward wall of the space between the tail-lights and the trunk, using both screws and 3M 5200 marine adhesive.

6.8 ohms is exactly the right resistance to draw the same current (2.1 amps) as the brake-turn filament of the stock 3057 bulb. After the install, I ran the left turn signal for 10 minutes straight, no problems at all. The heat sink got warm to the touch (which is about 120 F on a 70 F day), but due to some carefully placed isolation slots in the aluminum, the temperature at the attachment screws barely budged.

So all is well, and I have LED tail-lights on BOTH right and left again.