# 2012 tail lights intermittent



## Pafsa (Dec 28, 2018)

Hello all,
I have a 2012 eco. Recently, I have noticed that the drivers tail lights don't come on with the rest of the lights when I unlock at night or start the car at night. Light controls are set to auto. I haven't found a sure way to fix this. Brake lights and turn signals work even when tail lights don't. Sometimes switching the light controls on and off multiple times helps, other times, they just start to work again on their own. This involves only the driver side rear lights, both bulbs, the one in the trunk lid and the one on the body. Any Ideas where to start on this? I have looked for fuses and relays, but don't think that is the right hole to go down anyway because this problem comes and goes. Have not checked for any codes yet. Will do that tomorrow.

Last night, They were out when I unlocked. Still out after cycling light switch a few times. Still out after locking and unlocking a few times and still out after starting the engine, so I started driving. After about 30 minutes, I stopped and the lights were on.


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## ChevyGuy (Dec 13, 2014)

I think either you have a flaky fuse, a connector coming part, or a BCM going bad. The last one is expensive, so I'd focus on the prior ones. I'd also follow the wiring in the trunk to make sure it's hasn't been damaged anywhere.


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## plano-doug (Jul 1, 2015)

Pafsa said:


> This involves only the driver side rear lights, both bulbs, the one in the trunk lid and the one on the body.


I've attached a drawing. If I understand correctly, the two problem lights are circled in yellow. What about the left rear side marker light? Is it doing the same thing? It looks like the problem is downstream of the red arrow, so I would expect that side marker to do whatever the two problem lights are doing.

So the left front parking and side marker lights are good?

I would expect the problem to be downstream of the red arrow, but no splices are shown between the front and rear that might cause the two problem lights (E5AA and E5E) to fail without affecting the fronts. 

I could see, if splice X410 is intermittent, then it would affect E5AA and E2LR (side marker) but not E5E (on the trunk lid, ?).

HTH.

Doug

.


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## Pafsa (Dec 28, 2018)

ChevyGuy said:


> The last one is expensive, so I'd focus on the prior ones.


Thanks! I really hope I find a wiring problem instead of a failing BCM.


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## Pafsa (Dec 28, 2018)

plano-doug said:


> So the left front parking and side marker lights are good?


I can't say for sure now that you ask this. The two missing red lights stand out so much that I didn't look any further. I know I did look at the front left headlight and it was on. not 100% sure on the yellow front side marker and the marker in the tail light. I will check that next time this problem arises. In the mean time, I'll be following wire harnesses looking for damage or possible bad connection at j401. Thanks a ton for the wire diagram. I'll report back tomorrow.


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## Blasirl (Mar 31, 2015)

This will explain how to remove the tail light if you want to bench test. Once you clean up the connector, dab some dielectric grease on it to prevent further corrosion.

*HOW-TO: Install Reverse Light Lamps*


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## Pafsa (Dec 28, 2018)

OK, looking at Plano-Doug's diagram, I have confirmed that ALL of the lights (thanks for making me look closer) on that VT/GY line are out when this happens. BUT...Turn signals and brakes still work. I won't have access to a code reader for a couple days. Fuse FD3A could be the problem? Connector J401? Connectors at K9 BCM?


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## plano-doug (Jul 1, 2015)

Pafsa said:


> OK, looking at Plano-Doug's diagram, I have confirmed that ALL of the lights (thanks for making me look closer) on that VT/GY line are out when this happens. BUT...Turn signals and brakes still work. I won't have access to a code reader for a couple days. Fuse FD3A could be the problem? Connector J401? Connectors at K9 BCM?


The brake and turn lights are powered by different circuits. In the case of the dual filament bulbs (such as the tail lights), they share common grounds. So it's entirely possible for the brake lights to work but not the running/parking lights.

As for where the intermittent connection is, since all 5 bulbs are failing together, the problem has to be upstream of the junctions X400, X410 and X900. 

So I would start with the fuse. Pull out F3DA and inspect it. I've seen loose, intermittent filaments in light bulbs before - not sure about fuses - but take a look anyway to be sure the bridge is solid. Also, note how it feels coming out/going in - does it fit tightly, or is it sort of sloppy/loose? You want to be sure the fuse pins are making good contact in the mating sockets. 

The next place I would look is at the BCM connection, both where the power comes in - X2-3 - and where it goes out - X4-6. With the battery disconnected, remove the connector from the BCM and inspect both the pin and the mating socket. Look at the wire on the backside of the connector and make sure it's not damaged. I would avoid probing or prying on the mating surfaces of the pins and sockets - just inspect them. Use a magnifying glass, if necessary.

After that, you would want to look at the backside of the fusebox, and that's a pain. Instead, I suggest, next time the problem occurs, try probing the backside of the BCM connector pins for power. Not sure what all that entails - how hard it is to access - but you want to determine if the voltage it missing at the input, or at the output. If it's missing at the input, then the problem is upstream of the BCM, towards the fuse. If you see 12V at the BCM input (X2-3) but not at the output (X4-6) , then it's the BCM (which I doubt).

If I can find a drawing of the light switch, I'll check to see if there's anything there that might factor in, but at first thought, if the switch was part of the problem, I would expect the issue to occur on both sides of the car and not just the left side.

You mention code reader - are you getting any warning lights? Not sure about the Cruze in particular, but generally speaking, some BCMs monitor the output currents and can alarm if the current is out of range, altho that may not apply to park/running lights. For sure, I've read of many cases where folks have swapped in low-current LEDs for incandescents only to cause hyper-flashing because the BCM didn't see enough output current. 

Lastly, and maybe should be higher on the list, as you are performing the other tasks, inspect the wires in each location looking for kinks and abrasions. Either of those could reveal a break in the wire that is causing the intermittent behavior.

HTH.

[Edit]I finally found J401 on the drawing. Yes, if you can find that split, probably near the BCM, that would be a good place to inspect for a loose/bad connection.

Doug
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## plano-doug (Jul 1, 2015)

<delete>


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## Pafsa (Dec 28, 2018)

Thanks for the detailed response Doug. What I have done today....

I disconnected the battery for a couple hours this morning thinking it couldn't hurt if there was a possible glitch stuck in the bcm. (don't know if it actually works that way but hey!)

Reconnected and all started fine. Won't be able to test automatic lighting until dark of course.

Pulled fuse 3 under the dash. It is good and tight. needed the fuse puller to get it out. Cycled it in and out a few times to scrub the connections.

I'll wait and see now. If it happens again, I'll dig deeper under the center dash console for the bcm. I believe I can get at it from the passenger side footwell. will check connections there and read voltages. 

Thanks for the help. I'll check back tomorrow.


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## ChevyGuy (Dec 13, 2014)

It wouldn't hurt to replace the fuse. It may have developed a hairline crack.


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## Pafsa (Dec 28, 2018)

ChevyGuy said:


> It wouldn't hurt to replace the fuse. It may have developed a hairline crack.


Good Advice. I'll do that.

I should confirm to the group that I am using standard bulbs. no LED


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## Pafsa (Dec 28, 2018)

last night, all seemed to work well. Will keep watching. I have a code reader to use today. I'll let you know what it sees.


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## Pafsa (Dec 28, 2018)

Code reader saw nothing at all. Probably due to the fact that I had the battery disconnected for a few hours just a day before. Since disconnecting the battery and pulling and re-seating the 25A fuse#3 in the dash, The left side lighting failure hasn't happened. It has been good for 5 nights now. I'll call this thread solved unless the issue returns.

I did not replace the fuse as recommended by ChevyGuy, but I did clamp leads on its terminals to tone continuity and then flexed and twisted the fuse quite a bit, trying to drop tone. No hint of any loss of continuity there, so I left that fuse installed.

Thanks to all who helped. I really appreciate it.


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## Pafsa (Dec 28, 2018)

Just a followup.

The light system still works perfectly. Been over two months now and not one instance of lights not working properly. I suspect the connection at the fuse was the culprit. Re-seating the fuse multiple times surely scrubbed the connection clean of oxidation. I would not have found that fuse without the wiring diagram provided by Plano-Doug. Thanks again!


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## Pafsa (Dec 28, 2018)

So I should have kept my big mouth shut. A couple days after my "all is well" post, it started happening again. There is some confusion here though....

The same set of lights was out when I unlocked the car at night, the markers and tail lights on the driver's side. I went to the fuse box under the dash and pulled fuse 3. I went to the back of the car to inspect and found that now the driver side lights were on again, even though I was holding fuse 3 in my hands, AND, the Passenger side lights were now out. I looked again at the wiring diagram posted by Plano-Doug and this doesn't make sense. I put fuse 3 back in place and all the lights worked again. 

It happened again the next morning when I unlocked the car. I pulled fuse 3 again and same thing happened. Passenger side went out and driver side came on. Put fuse 3 back in and all worked fine. Engine running, I decided to pull fuse 24 (should be passenger side tail lights and markers). This made all the lights go out and the car lost its mind on me. Bells starting going off, the battery light came on, the power steering light came on and the info screen started complaining about a power steering system failure. I turned off the car and reinstalled fuse 24. Started the car and it seems to be ok now. It has been a week with no more troubles.

Any thoughts? Could there be a different wiring diagram for the 2012 eco model? Why does fuse 3 in my car seem to control the passenger side tail lights and markers? What fuse actually controls the Driver's side? I'm not going to "trial and error" pull any fuses to figure this out. Not after what happened with fuse 24, which according the the diagram posted above should have been the passenger side tail lights and markers.


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## plano-doug (Jul 1, 2015)

Pafsa said:


> Any thoughts? Could there be a different wiring diagram for the 2012 eco model? Why does fuse 3 in my car seem to control the passenger side tail lights and markers? What fuse actually controls the Driver's side? I'm not going to "trial and error" pull any fuses to figure this out. Not after what happened with fuse 24, which according the the diagram posted above should have been the passenger side tail lights and markers.


It's hard to tell the actual fuse distribution inside the BCM. In the case of F24, it may be that, besides lights, it also provides power to part of the BCM, which may explain the weird behavior.

One thought I have on this is to check the dual filament bulbs starting in the rear. I've seen before, where there was a bad ground for the bulb, that current flowed into one filament and out the other. If the parking lights are on, that might also cause a brake/turn light to come on. This implies some illogical current routing, but, without a detailed drawing of what's in the BCM, it's hard to prove one way or the other. But I have seen where, when the source end of the wire to the bulb was not powered, it was grounded, which can provide the necessary path to ground to light the off-side filament in the bulb.

I would pull the dual filament bulbs out and ohm them out - make sure they have continuity to the ground terminal from both power terminals. Inspect the bulb socket and make sure there's no corrosion in it, especially on the ground terminal. Ohm that terminal to ground to be sure you have continuity.

I'm not sure which of the bulbs are dual filament, but at least one on each side of the tail board, plus each of the rear side markers, I think.

I know it's a long shot, but I don't have any other explanations.

HTH.

Doug


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## Pafsa (Dec 28, 2018)

Thanks Doug,
Am I reading the diagram correctly? F3DA is Fuse 3 in the under dash fuse block? F24DA is Fuse 24? etc...
Ill check the bulbs and the socket.


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## plano-doug (Jul 1, 2015)

Pafsa said:


> Thanks Doug,
> Am I reading the diagram correctly? F3DA is Fuse 3 in the under dash fuse block? F24DA is Fuse 24? etc...


Yes, that's the way interpret them.

A couple others:

UD is battery fuse box.​UA is underhood fuse box.​
Doug

.


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## ChevyGuy (Dec 13, 2014)

plano-doug said:


> It's hard to tell the actual fuse distribution inside the BCM. In the case of F24, it may be that, besides lights, it also provides power to part of the BCM, which may explain the weird behavior.


Agreed. One would have to look at all the wiring diagrams to see what all that fuse powers.



plano-doug said:


> One thought I have on this is to check the dual filament bulbs starting in the rear. I've seen before, where there was a bad ground for the bulb, that current flowed into one filament and out the other. If the parking lights are on, that might also cause a brake/turn light to come on. This implies some illogical current routing, but, without a detailed drawing of what's in the BCM, it's hard to prove one way or the other. But I have seen where, when the source end of the wire to the bulb was not powered, it was grounded, which can provide the necessary path to ground to light the off-side filament in the bulb.


Usually what happens is other filaments in parallel provide the ground. For example, if the ground for all side lights and back tail light were to open, the current intended for them would flow though the rear tail/brake light, to the open ground and then out the brake light filament, where it would find ground - perhaps via the other brake light.


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## Pafsa (Dec 28, 2018)

.....speaking of wiring diagrams, are those available somewhere a non-GM tech could access them?


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## Pafsa (Dec 28, 2018)

......and by "non-GM" tech, I mean, copier and printer tech.


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## ChevyGuy (Dec 13, 2014)

Not publicly. You'd have to either have a manual ($$$$) or subscribe to a service like AllDataDIY ($)


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