2004 CTS 6 cyl. 2 whl drive Bar light on trunk and everything else BUT the brake lights won't work

Tiny
RIVERATT
  • 2004 CADILLAC CTS
  • 10,200 MILES

Buddy owns a body shop and just checked everything
Bulbs ckecked good
2 boxes under rear seat checked. All relays, fuses, everything checks good.
Bar brake light on trunk lid works
Rear running lights and blinkers work.
Everything works except the R/L brake lights

He says the diagnostic machine indicates that I basically need to take it to the Cadillac Dealer. Something about a module or something and that Cadillac doesn't want me to know what's causing the light failure.
I'm trying to avoid the expenses if possible.
Would anyone know what the problem may be?
Or perhaps this has happened to you and yours was repaired. How?

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Tuesday, January 17th, 2012 AT 12:41 AM

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Tiny
CARADIODOC
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Avoiding expensive repairs went off the table when you bough an expensive car with a lot of "got'chas" built in by one of the three least customer-friendly manufacturers in the world. GM allows internet access to independent shops for only three of their up to 47 computers. They are mandated by the government to allow that access because those three computers are involved with emissions. They tie everything else up for themselves.

First you can try a new brake light switch. The high-mount light is on a different circuit than the other two lights. You should be able to get a clue to by the operation of the cruise control.

Also be aware the Body Computer is the master computer that all the other ones communicate with before they turn on. In an attempt to prevent owners from replacing the high-failure radios, GM built the Body Computer into the radio. When using their scanners on your car, the dealer's mechanics have the option of pressing the "lock" button. That electronically ties every other computer on the car to that radio. You will never notice that was done, and that programming can not be undone. From then on, if that radio ever fails, every other computer on the car must also be replaced and the software installed over their internet connection. Can you think of any legitimate reason to design a car like that?

If the brake light switch doesn't solve the problem, take the car to the dealer and have your wallet ready. If the day ever does come that multiple computers are required, you don't want to have to guess who is to blame for locking them together.

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Tuesday, January 17th, 2012 AT 1:04 AM
Tiny
RIVERATT
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Caradiodoc: I really appreciate the detailed information provided in your reply. Thanks a bunch for that, my friend.
No doubt you answer has much merit. I wasn't really privvy to exactly what my buddies diagnostic machine was doing but he did mention something about a "coded lock" when mentioning taking it to a dealer because they don't want me knowing what's actually wrong. Most likely as a means to charge their high prices. "Got'cha's". That's about right.
What "clue" might I find when operating the cruise control?
Will it not function correctly or function in a manner that is noticeably different?
This may or may not have any bearing on what the problem is or maybe it will shed some light on exactly what the problem is but I've noticed that the sunroof has open up in the tilted position 2 or 3 times on it's own over the past few weeks. The switch seems to work properly in all positions but opening up on it's own certainly isn't normal.
I will read up some more on changing the brake light switch. I gather the 3rd light switch is in the truck so the common light switch is likely mounted up on the pedal somewhere.
I seem to recall having to do a few steps with the ignition switch and gas pedal when installing a new battery and wonder if anything like that is necessary when replacing the brake light switch?
I'm hoping it's as simple as unplugging it and unmounting it and then maybe having to set the length when installed. And I'm hoping the switch corrects the problem (though I'm seriously doubting it will)
Would you know if it's a NO or NC switch? Maybe I can put a meter on it and test for continuity before buying a replacement that I don't need?
Thanks again for you time in sharing your knowledge, sir.

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Tuesday, January 17th, 2012 AT 7:52 AM
Tiny
CARADIODOC
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You got me now. The only reason I know about the switch is a former student calls me to chat a couple of times per week and he likes to share some of his stories. He had a GM car last week with the dead main brake lights and just the high-mount one working. On older cars, after leaving the switch, one wire goes to the high-mount light and one goes to the turn signal switch, then to the two brake lights. That is on cars where the brake light and turn signal light are one and the same. His problem was caused by the brake light switch.

The cruise control is tied in, typically on both sides of the main brake light switch which is normally closed, but held open when the pedal pushes it back. When it closes, voltage is sent to the signal switch and to the Engine Computer or cruise control module for the "cancel" signal. Something in the system, commonly the servo, gets its voltage from the feed side of the brake light switch. That is so the cruise control won't work if the brake light fuse is blown. If that fuse is blown, there would be no way to get that cancel signal when you tap the pedal, and the car would keep on going until they found you in the next county!

Some Chrysler models use a third switch inside the brake light switch for the cruise control system. That one is normally open but is held closed when the pedal is released. Current flows through it to power the solenoids in the servo. That is like a backup to insure it cuts out when the brake pedal is tapped. Pitted contacts can prevent that system from working although that isn't real common. To set the system, the brake light circuit still has to be working, but as I recall, the cruise will still work if the brake light switch contacts are open.

I can't remember what I was told last week but the cruise control did provide the clue. I would assume it either didn't set at all or it didn't cancel when tapping the pedal. I find it hard to believe it would work and not cancel. Designing the system that way could leave them open to lawsuits. People will notice a non-working cruise control and investigate further, but most drivers will not notice non-working brake lights.

First look at how many wires go to the switch. On the off chance there's only two, that switch has to be good if the high-mount light is working. Then suspect the turn signal, (multifunction), switch. If your rear signal light bulbs are different than the brake lights, all three brake lights should be wired together. In that case, look for a corroded splice, cut wire, or rusted ground wire. Your car is too new to be expecting corrosion already. What I would expect to find would be where someone used a ScotchLok connector to splice in a wire for a trailer harness. Those connectors don't seal out moisture.

If your brake light switch has four or six wires, you're going to just have to check with the voltmeter to see if voltage switches state when you press the pedal. On older cars, the orange wire was the 12 volt feed and the white wire was the switched 12 volts that went to the signal switch.

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Tuesday, January 17th, 2012 AT 4:50 PM
Tiny
RIVERATT
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And thanks again. I'm now out of town and won't be able to check things but the cruise control does work. I'll know tomorrow if it seems a bit more touchy or seems out of the norm. I haven't actually looked for an added "tie in" wire for towing brake lights but the crimped, pinched, cut wire theory certainly hit home. The rear bumper has been recently pulled for a minor nick/crack repair. By my buddy that owns the shop and didn't hesitate to put an hours labor into testing what his tech could with the machine. Perhaps a wire has been pinched pulling and replacing the rear bumper. The brake lights failed within 30 days of that work having been done. I'd welcome your opinion on that matter.

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Wednesday, January 18th, 2012 AT 12:42 AM
Tiny
CARADIODOC
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I'm not an expert at bodywork, but I've already caused problems on my own cars. The place to start looking is where work was recently done.

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Wednesday, January 18th, 2012 AT 7:27 AM
Tiny
FACTORYJACK
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Look at the schematic I have included. If the high mount works, it verifies that the switch, relay, and at least one ground are good. You say the bulbs 'checked' good. Was this check done by just looking at the filaments? Often times it is not the filament that is open. Before you spend much more time pulling your hair out, try some known good bulbs. You may be surprised. If bulbs still don't correct the concern, then look for power getting to the sockets, as well as ground. If you have power, ground, and known good bulbs still provide no stoplamps, you have faulty sockets. Two sockets to be failed, would be rare. Two bulbs failed is more probable. The only module in play is the ABS module(EBCM). If there was a problem there, causing no BAS relay activity, you would have no brake lights at all, and more than likely no cruise. I'm betting you have two bad bulbs.

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Wednesday, January 18th, 2012 AT 4:04 PM
Tiny
RIVERATT
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Thanks for the schematic.
Bulbs checked out = new bulbs were installed.

A new switch has been ordered and we'll see if that corrects the problem.
The cruise control does work at the moment. Whether or not it's "touchy", I don't know. The wife says it feels the same to her but I'm not able to check that at the moment.
I will be forwarding the attached schematic to the Body shop. It may help when they change the switch.
Thanks to all who have posted their knowledge to help. That's really awesome.
Switch shouyld be installed by Friday but I will keep this thread informed as it may prove to be as helpful to someone else as it has been to me. And when it's all wrapped up I will be makin g a donation. This truly is a great site with an even greater cause. Thanks again guys.

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Thursday, January 19th, 2012 AT 6:50 PM
Tiny
CADIEMAN
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Check the brake switch and check at the sockets for power with the pedal pushed with a test light. If voltage goes through the brake switch but not to the sockets. Buy a turn signal switch and plug it in without installing it. The rear lights will start working. Then u will need to install the switch. Use a mobil steering colume repair co. Cheeper. Ty for using 2carpros

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Thursday, January 19th, 2012 AT 8:11 PM
Tiny
RIVERATT
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So with the pedal pushed in (or the switch in the position it would be with the pedal pushed) if power is going through the switch but not making the circuit through the bulbs, the blinker switch now comes into play? Which side of the blinker switch: hot or ground? I take it hot travels through it on it's way to the relay? And where might the harness/plug in the blinker switch be located? In looking at the schematic, I'm curious about this "EBCM" module. It's location in the vehicle and a means of testing it.
Thanks for sharing. As for using 2carpros, ikt's my pleasure. And thank you and the many like you that share your cost free knowledge.

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Thursday, January 19th, 2012 AT 11:36 PM
Tiny
FACTORYJACK
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So, which switch are you working with? There is not a switch in the circuit. There is a brake pedal position sensor. After this sensor has been replaced, it needs to be calibrated after replacement. You need to see if the data for the EBCM is showing a brake pedal applied. Which, by the way, was confirmed before you replaced the switch, as indicated by the high mount illuminating. If the high mount is illuminating, the only faults you have to consider, are wiring, connections, and sockets. The schematic shows wire colors, if there is not power present on the appropriate circuits at the sockets, then start tracing the circuits back to find the loss.

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Friday, January 20th, 2012 AT 12:12 AM
Tiny
CADIEMAN
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The turn signal harness is mounted to the side of the steering or the wires come out the steering column and they run right to a wiring block. Take the block apart. The turn signal locks on one side of the connecter and the wiper switch on the other side. Lock the 2 connectors to the harness then screw it into the body harness. The brakes should work then u need to have the switch installed. As I said.

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Friday, January 20th, 2012 AT 1:26 AM
Tiny
RIVERATT
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Cadieman: Thanks again. I'm hoping that replacing the brake switch will solve the problem. Replacing the blinker switch sounds like a PIA. So as not to be confused, the brake switch on the pedal needs only be unplugged, unmounted with the new switch going in the same way."Nothing further" required?

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Friday, January 20th, 2012 AT 2:54 AM
Tiny
FACTORYJACK
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I don't know what schematic you all are looking at, but you are getting blown way off course. The turn signal switch does not even come into the picture, because it has separate turn signals, the little amber squares in the bottom of the tail lamp assemblies. Replacing the brake pedal position sensor will not cure the condition, because the functionality is confirmed by the high mount illuminating, right? The white wire that leaves the BAS relay, goes to a junction within the rear fuse block, and then goes to three places, the right/left brake lamps, AND the high mount. It is all the same circuit. If the high mount lights, you have problems at one or more of the following; connector C410 AND C411 on the power side, C400 AND C411 on the ground side, grounds G402 AND G401, terminals S24 AND S25 at the joint connector, BOTH tail/brake sockets, or the appropriate wiring in between.

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Friday, January 20th, 2012 AT 3:09 AM
Tiny
RIVERATT
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The schematic being referred to is the attachment you enclosed in your reply (3 replies up). This is becomming frustrating due to the fact that I know absolutely nothing about the problem I'm having with these lights. Nor how to go about correcting it as cost-effective as possible.
If your 2004 CTS slid around a corner and off the road into a small gulley of a ditch causing minimal body damage and a stripped out plug in the radiator bottom (losing most of the ATF) only and a shop had replaced both headlight assemblies (complete)in addtion to removing/replacing the rear bumper and racking the vehicle to clean the undercaridge, where would you likely start regarding this brake light issue? All lights as well as everything else was checked and confirmed functioning when it left the shop. Somewhere between then and now the brake lights stopped working. Thanks for the help. It's greatly appreciated.

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Friday, January 20th, 2012 AT 5:47 AM
Tiny
RIVERATT
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I sincerely appreciate all the responses and help I've been given regading these brake lights. I don't mean to be misunderstood here but it's difficuly, if not impossible to dertermine who I should be listening to for a sollution because I know nothing about how and why a CTS does what it does. Someone could tell me that the horn is mounted somewhere in the trunk and I wouldn't really know the difference. (Well, actually I would but you know what I'm saying.) So it's been your experience in the past that when the brake lights on a CTS quit working it's not likely the problem with be found in the brake light switch? Thanks again

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Friday, January 20th, 2012 AT 6:12 AM
Tiny
RIVERATT
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So is there more than 1 relay in the circuit that controls the brake lights or are there 2(or more) seperate relays: one for the high light and one for the brake lights. I ask this because while the rear seat was pulled and the shop was checking things over it was noted that a relay in one of the rear boxes was heard "clicking" when pushing on the brake pedal.

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Friday, January 20th, 2012 AT 6:23 AM
Tiny
FACTORYJACK
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With the circumstance as you describe, you have to have two problems, for it to affect both sides. That would be what makes it confusing, I guess. Do you have a 12v test light? With a few simple checks with a test light, you can determine what you do not have at both sides. Removal of the bumper fascia should not have caused a concern, but then again, anything is possible. One thing is for certain, the fact that the high mount is working, confirms a few things. That the brake pedal sensor is triggering the EBCM to close the BAS relay and energize the circuit, so you can rule out the BPP sensor, EBCM, and BAS relay as being possibles.
With a test light I would connect one end to ground, and probe the white wire(circuit 17) at one side in the rear, and the other side, with the brake pedal applied and the key ON. This will tell you if you have a signal at the rear lamps. If you do, then check the grounds G402, and G401. Both of those grounds are in the trunk, rear of the wheel wells. Unless the specific wire for the each lamp was removed from the grounding points, to have a problem with ground is going to cause problems with many other systems. Both G401 and G402 are ground points for several systems.
If you don't have a test lamp lit at each rear lamp(on the white wire), then trace back to the next connection, which would probably be at the rear fuse block, or that joint connector. If there is power on the appropriate circuits there, you have a wiring problem between the fuse block, and the rear lamps, on both sides. If you have power and ground at the rear connectors C411, and C410, then you either have bad sockets, or bad bulbs. Let's go back to the symptom, you DO have a high mount when the pedal is depressed, you just are lacking the right and left brake light, correct? To answer your question that you asked while I was composing this reply, you only have the one relay, labeled BAS. You should be able to locate it with the legend under the fuse box cover.

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Friday, January 20th, 2012 AT 6:38 AM
Tiny
RIVERATT
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Yes. The high light works, the R/L lights do not. According to the shop the relay in the rear box can be heard when the pedal is pushed. I guess that doesn't necessarily mean it's working but it seem to be respoding to the pedal.I'll call tomorrow and ask how that particular relay was checked and deemed good. Thank you for the time involved in your replies and I will be back when I know more.

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Friday, January 20th, 2012 AT 6:58 AM
Tiny
CARADIODOC
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Here's what we know. Voltage is present all the way to the end of the red wire because the high-mount light works. The dark blue ground circuit is okay too, again, because that light works. That means either voltage is not getting to the rear lights on the yellow AND purple wires, or there's a break in BOTH light blue wires.

The two dead lights have nothing in common other than that joint connector. The right feed and ground wires both go through connector C411. The left wires go through connectors C400 and C410. Both sides use different grounds. As near as I can see, there are four broken wires or some connectors are unplugged.

Those wire colors are my addition for explanation purposes. In the sockets, the 12 volt feed wires are white. Check those for 12 volts while a helper is pressing the brake pedal.

Also, maybe one of you other guys would know where those grounds are. They only show one high-mount bulb, but I'm guessing that is a strip of LEDs. With a bad ground, current could try to go through the broken grounds of the two rear lights, back-feed through the tail light filaments, through the front bulbs and through their grounds. There could be enough current to light those LEDS. The clue would be the high-mount light would stop working when the head light / tail lights are on.

If those grounds and lights are built into the rear bumper, perhaps there is another single ground strap between the bumper and car body. Just a thought, because I'm trying to find anything the two sides have in common.

If you DO find full 12 volts on either of the white wires at the sockets, check for voltage on the black wires, but those measurements must be taken with the bulbs in the sockets. If there's voltage on them, there's a break in the ground wires.

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Friday, January 20th, 2012 AT 6:59 AM
Tiny
CARADIODOC
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Sorry for getting out of step. It takes me a while to copy the drawing and add my sad lines to it. Factoryjack is telling you to do the same things I did, and he also thinks it sounds like there's multiple problems. We might be getting derailed with that logic though so the best thing is to pick one side and concentrate on that. When we find that problem, there's a 50 / 50 chance the problem for the other side will be staring you in the face.

I also know it can get confusing when multiple people are in the conversation at the same time. You can start anywhere between where you have voltage and where it's missing. Two people telling you different places to start doesn't mean one is wrong. It's like which direction do you look first when crossing the street? Doesn't really matter; you'll get the same result.

I wouldn't worry about that relay. We know it's working because the high-mount light is working. Think of your horn comment. If your horn sounds sick because the high note is dead and just the low note is working, you know there's no need to test the horn button on the steering wheel.

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Friday, January 20th, 2012 AT 7:14 AM

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