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Discussion Starter #1
So, this story begins with getting pulled over the other morning for running without lights (unbeknownst to me.) My 2007 Impala LT 3.9L's parking/marker lights aren't working. I've changed the bulbs. It's both Left and Right rear marker/parking lights. Both Brake lights, turn signals, and reverse lights are functional. What could be causing this issue. Thanks in advance for any info anyone can provide.
 

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It's both Left and Right rear marker/parking lights. Both Brake lights, turn signals, and reverse lights are functional.
What about the front parking and marker lights? They work?

Do the license plate lamps work?

Edit: Also, in the underhood fuse box, check fuses "RT PARK" and "LT PARK".

I suspect it's not a fuse, but let us know what you find, an we'll go from there.

Doug

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Discussion Starter #3
What about the front parking and marker lights? They work, but the rears do not?

Doug

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Thanks for the reply Doug, just went and checked, both the front and rear are non-operable. (I mistook the inner amber turn signal bulb for the marker bulb). so the marker/parking lights on both front and back are inoperable.
 

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Thanks for the reply Doug, just went and checked, both the front and rear are non-operable. (I mistook the inner amber turn signal bulb for the marker bulb). so the marker/parking lights on both front and back are inoperable.
Check the fuses I indicate in my edited message above.

Doug

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Discussion Starter #5
What about the front parking and marker lights? They work?

Do the license plate lamps work?

Edit: Also, in the underhood fuse box, check fuses "RT PARK" and "LT PARK".

I suspect it's not a fuse, but let us know what you find, an we'll go from there.

Doug

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Checked the fuses, they looked fine, but also tested with new fuses to no avail.
 

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Checked the fuses, they looked fine, but also tested with new fuses to no avail.
Do you have a volts-ohm-meter (or digital volt meter)? My next step would be to start probing around, looking for +12V. I've attached the drawings I found for the parking lights. Check that the fuses are getting power. Verify the bulb sockets at the other end are not. Then start working back.

Or, if you get +12V at the bulb sockets, trace the return wires looking for a broken wire or bad ground.

HTH.
Doug

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Discussion Starter #8
Do you have a volts-ohm-meter (or digital volt meter)? My next step would be to start probing around, looking for +12V. I've attached the drawings I found for the parking lights. Check that the fuses are getting power. Verify the bulb sockets at the other end are not. Then start working back.

Or, if you get +12V at the bulb sockets, trace the return wires looking for a broken wire or bad ground.

HTH.
Doug

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So update. I also replaced the front marker lamp bulbs and turn signal bulbs. When I was doing this I happened to have the RT PARK fuse pulled and the left side parking lamps started working. When I replaced the RT PARK fuse, the left side once again stopped working, and when I once again removed the RT PARK fuse the left side lights started functioning again.
 

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So update. I also replaced the front marker lamp bulbs and turn signal bulbs. When I was doing this I happened to have the RT PARK fuse pulled and the left side parking lamps started working. When I replaced the RT PARK fuse, the left side once again stopped working, and when I once again removed the RT PARK fuse the left side lights started functioning again.
Interesting. And stupefying, too :(

From the drawings, the lamp switch goes to the BCM, and the BCM controls the relay, which feeds all the parking lights. So we can assume that everything up to the relay works. The relay feeds both LT and RT fuses. The LT fuse feeds the LF park and marker lights, and the LR taillights. With the RT fuse pulled, we know these work.

Test: For completeness, remove the LT fuse, with the RT inserted, and see what the right side lights do.

At this point, there does not appear to be a short anywhere pulling the voltage down - else one or both of the fuses would have blown.

My best guess is too much resistance somewhere in or downstream of the relay. If there's enough corrosion somewhere, or a wire with enough strands broken, when both sets of bulbs are connected - ie, when the RT fuse was re-inserted - the increased load pulls more current thru that resistance thereby dropping the voltage to all the bulbs resulting in no (or very dim) lights.

That's about the only plausible explanation I can come up with.

Testing with the LT fuse removed and the RT inserted, I would expect the opposite as before, with the RT side working, but the LT side not. The only diff in the two sides is that, per the drawings, the RT fuse also powers the lamp on the license plate.

Let us know what you find out, and we'll go from there.

Doug

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Discussion Starter #10
Interesting. And stupefying, too :(

From the drawings, the lamp switch goes to the BCM, and the BCM controls the relay, which feeds all the parking lights. So we can assume that everything up to the relay works. The relay feeds both LT and RT fuses. The LT fuse feeds the LF park and marker lights, and the LR taillights. With the RT fuse pulled, we know these work.

Test: For completeness, remove the LT fuse, with the RT inserted, and see what the right side lights do.

At this point, there does not appear to be a short anywhere pulling the voltage down - else one or both of the fuses would have blown.

My best guess is too much resistance somewhere in or downstream of the relay. If there's enough corrosion somewhere, or a wire with enough strands broken, when both sets of bulbs are connected - ie, when the RT fuse was re-inserted - the increased load pulls more current thru that resistance thereby dropping the voltage to all the bulbs resulting in no (or very dim) lights.

That's about the only plausible explanation I can come up with.

Testing with the LT fuse removed and the RT inserted, I would expect the opposite as before, with the RT side working, but the LT side not. The only diff in the two sides is that, per the drawings, the RT fuse also powers the lamp on the license plate.

Let us know what you find out, and we'll go from there.

Doug

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I meant to say in my last post that I had actually done that, pull the LT PARK fuse and leave the RT Park fuse installed and no, the right lights do not operate. I am not very skilled at reading electrical drawings so I'm not very familiar with all the different symbols. However, don't both circuits share a common ground, and therefore it be a fault in the ground somewhere along the line?
 

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I meant to say in my last post that I had actually done that, pull the LT PARK fuse and leave the RT Park fuse installed and no, the right lights do not operate. ..... However, don't both circuits share a common ground, and therefore it be a fault in the ground somewhere along the line?
Your half way right. Both sets of rear lights share a common (G302) ground. The front lights use 2 different grounds (G100, G101). Bad grounds will drive you crazy with weird symptoms that defy all normal logic. If your problem was just the rear, ground (G302) could cause the symptoms you're having. You may have more than one bad ground, check them ALL !! Follow the BLACK wire from the bulb socket. Make sure you have clean metal under the ground terminal. Good luck lets us know what you find.
 

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I meant to say in my last post that I had actually done that, pull the LT PARK fuse and leave the RT Park fuse installed and no, the right lights do not operate. I am not very skilled at reading electrical drawings so I'm not very familiar with all the different symbols. However, don't both circuits share a common ground, and therefore it be a fault in the ground somewhere along the line?
At this point, I'm getting stumped, but still have some stuff to try. As a55bruce noted, a bad ground can cause some weird problems. But first, I'm thinking you should focus on the bulbs on the RT side.

We know the left side works, so remove the LT fuse, to take it out of the picture. Then (with the RT fuse in place) start removing the right side bulbs, one at a time, without replacing them, and turn the lights on after each step, to see if anything lights up.

I would like to swap the relay, but it appears it's buried in the underhood fuse box, and soldered in place. So removing it is non-trivial. The reason I'm interested is that, because the left side works with the RT fuse removed, I suspect some resistive corrosion upstream of the RT fuse, which points back toward the relay. But, given the challenge, this is saved as a last resort.

A couple other tests to help rule things out: If the RF turn signal works, the GND for the RF parking lamp is good. If the RR brake light/turn signal works, the GND for the RR parking (running) lamp is good.

One interesting side note: This relay controls all the park/running lamps. But all the brake/turn lamps are controlled by the BCM.

Let us know what happens when you remove the RT bulbs.

Doug

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Discussion Starter #13
At this point, I'm getting stumped, but still have some stuff to try. As a55bruce noted, a bad ground can cause some weird problems. But first, I'm thinking you should focus on the bulbs on the RT side.

We know the left side works, so remove the LT fuse, to take it out of the picture. Then (with the RT fuse in place) start removing the right side bulbs, one at a time, without replacing them, and turn the lights on after each step, to see if anything lights up.

I would like to swap the relay, but it appears it's buried in the underhood fuse box, and soldered in place. So removing it is non-trivial. The reason I'm interested is that, because the left side works with the RT fuse removed, I suspect some resistive corrosion upstream of the RT fuse, which points back toward the relay. But, given the challenge, this is saved as a last resort.

A couple other tests to help rule things out: If the RF turn signal works, the GND for the RF parking lamp is good. If the RR brake light/turn signal works, the GND for the RR parking (running) lamp is good.

One interesting side note: This relay controls all the park/running lamps. But all the brake/turn lamps are controlled by the BCM.

Let us know what happens when you remove the RT bulbs.

Doug

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Thanks for the excellent help you and others have offered. Yes both the RF turn signal, and RR Brake and Turn Signal do work, but at this point it's getting a bit too much for my comfort zone and expertise, so I think I'm going to write up a detailed account of the issue along with what troubleshooting I've done and what I have discovered and take it to a shop in the morning. Hoping they have the expertise to either rule out the relay or replace it if bad. (if that's even possible.) because, as you stated that's seeming more and more likely the cause.

Thanks again for your time and assistance. I'll certainly update the thread with what I find out.
 

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At this point, I'm getting stumped, but still have some stuff to try. As a55bruce noted, a bad ground can cause some weird problems....
A couple other tests to help rule things out: If the RF turn signal works, the GND for the RF parking lamp is good. If the RR brake light/turn signal works, the GND for the RR parking (running) lamp is good.
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With a bad ground the turn signal or stop lamps might appear to be working, but if the marker/tail lamps are looked at closely the filament will be glowing slightly (easier to see at night) indicating current is flowing backwards thru all the marker lamps that share the same ground.
Learned this one the hard way.
 

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Discussion Starter #15
With a bad ground the turn signal or stop lamps might appear to be working, but if the marker/tail lamps are looked at closely the filament will be glowing slightly (easier to see at night) indicating current is flowing backwards thru all the marker lamps that share the same ground.
Learned this one the hard way.
Well technically, when I replaced the bulbs, I replaced them all with their LED counterparts (all high quality Sylvania bulbs). So would that still be the case since LED bulbs don't have filaments?

Also, I tested the contacts on the plugs that plug into each taillight. When the RT PARK fuse is pulled and the LT PARK fuse remains installed, the left side is energized, and as expected the right size is not.
When both LT PARK and RT PARK fuses are installed, there is no power to either side.
When the LT PARK fuse is removed and the RT PARK fuse remains installed, it results in the same outcome, neither the left or the right taillight cables are energized.
 

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Well technically, when I replaced the bulbs, I replaced them all with their LED counterparts (all high quality Sylvania bulbs). So would that still be the case since LED bulbs don't have filaments?

Also, I tested the contacts on the plugs that plug into each taillight. When the RT PARK fuse is pulled and the LT PARK fuse remains installed, the left side is energized, and as expected the right size is not.
When both LT PARK and RT PARK fuses are installed, there is no power to either side.
When the LT PARK fuse is removed and the RT PARK fuse remains installed, it results in the same outcome, neither the left or the right taillight cables are energized.


Did the problem start when you replaced the bulbs with LED bulbs? If so, install standard bulbs and see if that corrects the problem.

When you post a request for help, you should include all relevant information in your post.
 

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Discussion Starter #17
Did the problem start when you replaced the bulbs with LED bulbs? If so, install standard bulbs and see if that corrects the problem.

When you post a request for help, you should include all relevant information in your post.
No, the problem started before I replaced the bulbs, which prompted me to replace the bulbs with LED bulbs. I did include that information in my first post, but thank you for your input.
 

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No, the problem started before I replaced the bulbs, which prompted me to replace the bulbs with LED bulbs. I did include that information in my first post, but thank you for your input.
Replacing the bulbs with LED's is a critical piece of missing info.
................So would that still be the case since LED bulbs don't have filaments?
LED bulbs would not "normally" provide a backfeed path. But, did you also change the stop/turn, plate, and backup lamps ?
....Also, I tested the contacts on the plugs that plug into each taillight. When the RT PARK fuse is pulled and the LT PARK fuse remains installed, the left side is energized, and as expected the right size is not.
When both LT PARK and RT PARK fuses are installed, there is no power to either side.
When the LT PARK fuse is removed and the RT PARK fuse remains installed, it results in the same outcome, neither the left or the right taillight cables are energized.
How are you testing for power? Meter or test light? You should be connecting your meter/test light negative probe to a good ground on the car not the black wire at the light connector.

For a quick test to see if this is a ground problem. Remove a backup lamp, jumper the black wire terminal to a good chassis ground. This should provide a ground for the rear lights, if this gets the rear lights to work, you'll have to find/clean the grounding terminal (G302), or find and fix the broken ground wire.
 

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Discussion Starter #19
UPDATE - So after taking it to the shop, they were able to get both sides tow function by providing power directly to the LT PARK and RT PARK fuses, bypassing the relay. which they identified as the culprit. Since the Parking/Marker lamp relay is soldered component on the PCB of the fuse box, they had to replace the entire fuse box. ($110.00 part -used. No new or refurbished units could be found.)

But with that said, it cost an arm and a leg, but the problem has been resolved. Thanks for everyone's help and advice.
 
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