Ok, everyone, I'm not on drugs here.... but I think my Bonneville may be...

A few weeks ago, a friend pointed out that my back-up lights were on any time that my headlights were on, either manually or automatically. The problem disappeared for a bit, and I shrugged it off until it happened again a few days ago.

I couldn't get the back-up lights to go out, though when I put the car in reverse, the lights got brighter. I assumed from that, that a bulb must be out and grounded across, causing a backfeed through the back-up lights, and I started investigating. While I was messing with the lefthand taillight assy, the back-up lights went out, but so did my lefthand taillights. I now have no lefthand taillights or front marker light on the lefthand side, though all the turn signals, brake lights, back-up lights, etc. work just fine on the left and right.

Using my multi-tester, I checked wires to the taillight bulbs, and while I have brake light power, I'm getting nothing to the taillight wires. My assumption is that I have no power to the front either, though I haven't delved into that end of the car.

Does anyone have any insight on what might be causing this? What fuse are the tail and marker lamps on, and would the righthand and lefthand lights be on separate fuses? Could it be a relay that'* 'half' bad letting some things function, but causing my park lamps to not work? Would anyone have any wiring information in form of a diagram or such, that might help me in tracing my problem?

If anyone could offer a suggestion, I would be most appreciative! Please reply here, or e-mail me directly to make sure I see your reply - ben@nebulasgroup.com

Thanks in advance to all!

Ben
2000 Bonneville SE

Ben,
while you have your left turn signal on, tap on the relay with the butt of a srew driver. Do the lights flicker or turn on? If so bad relay. If not check power at the relay. It still may be the relay, if the contacts had stuck closed at first (lights always on) and are now open and wont close they may have burnt off? Im also thinking you might have taken out a fuse while you were 'wiggling' wires at the tail lights, so check that first. There is a fuse/relay legend in your car manual and on the inside of the 2 fuse/relay box covers.

Here are the fuse designations in the rear fuse block:
Fuse 10 (10A) Right Tail/Stoplamp, Sidemarker Lamp
Fuse 13 (10A) Left Tail/Stoplamp, Sidemarker Lamp
Fuse 20 (10A) Park/Reverse Relays (Backup Lamps)

When selecting reverse on the gear selector, the Powertrain Control Module (PCM) issues a signal (serial link) to the Rear Integration Module (RIM), which in turn closes in the ground path for the reverse relay to toggle. The reverse relay (45) delivers 12+ to the back up lamps. There should not be any association with tail lamps, or side markers in the above mentioned scenario. You may want to check the wiring harnesses in the trunk area for crushed leads or something like that. It appears that inadvertant potential is being delivered to the back up lamps from another source.

Thanks both of you for your replies! ...and in fact, my car has sobered up!

After further investigation, I managed to get the problem solved. After checking my taillight bulbs with a continuity tester, I found that one indeed had a broken filament. I changed the bulb and continued my search. Finally, I managed to run across the rear fuse block (ok,ok... I bought my car recently and hadn't had the back seat out to know there was another block there. My manual only had a diagram of the fuse block under the hood - so my bad!) That confirmed my suspicion that left and right lamps were on separate fuses. Indeed, the left park lamp fuse had blown - replaced it and now all works.

My bet is my original suspicion that my taillight bulb burned out but the filament didn't break entirely, instead "slightly" grounding itself out - causing a backfeed through my back-up lights and making them come on. I had seen similar instances of this before in more industrial applications, but wasn't sure if new cars would do it, too. I guess when I started giving some love taps to my taillight assy, the filament got just a bit more ground - enough to blow my fuse.

Thanks again for the help! I appreciate knowing this forum is here with people who are so willing and able to help!

Yeah OK, good troubleshooting. Backfeeding the back up lamps (reverse relay dropped out) basically places the back up lamps in sereis with one another and what ever power source ie. tail lamps in your case.