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Old 08-18-2016, 12:33 PM   #1
pitchwite
1st Gear Newbie
 
Join Date: Apr 2010
Location: Toronto
Posts: 14
Puzzling Brake Light Problem (not switch, fuse, wiring, contacts, CM)



1999 E36 M3 Coupe.

So, 1000 points to anyone who can lead me to the answer. I'll even take you out for beers if you're in the GTA.

Here's what I know and what's been done.

-Brake Light Circuit code shows up on console
-Brake lights DON'T work
-All other rear tail light bulbs DO work
-Brake light switch has been replaced and even the original was tested and works.
-Rear Taillight assemblies have been replaced to LED taillight with different bulbs and still same problem
-CM Module has been inspected and has no blown capacitors, shunts, resistors or anything else.
-Trunk wiring looks good. There was one wire that was exposing the copper, but it's been taped and it is not the problem.
-Fuse 46 (and all other fuses) are fine and haven't been blown. I checked their consistency with a multimeter.
-Fuse 46 has a constant 12V whether or not the brake is depressed or not.
-Brake light circuit error comes on as soon as the two contacts in the brake light switch cause the circuit to close (I used my hand to test the brake switch as well as checking them for a 12V output of the leads when it was released).
-I've opened the fuse box and looked at each and every contact and cleaned/tested the few that had corrosion on them (PITA).

History: I would be driving and suddenly the brake light circuit error would come up. Eventually it got to a point where it would come any time I started the car and pressed the pedal.

It seems to be a wiring issue perhaps under the carpets or under the fuse box.

Is there any way to know which wires from the rear lights go to whichever wires in the fusebox? It could help me troubleshoot.

Here's what's puzzling: If it was a short circuit, the fuse would blow (which it is not doing). If it's an open circuit, shouldn't the code read "Brake light bulb", rather than "Brake light circuit"?

NEED HELP PLEASE!

Last edited by pitchwite; 08-18-2016 at 01:03 PM.
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Old 08-19-2016, 01:54 PM   #2
pitchwite
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Some more info I've got now:

If I give the rear brake lakes a 9V source (from battery) at the leads of the black connector to the CM module, the brake lights work. So the wiring is FINE!

But, when I bypass the Check Control Module to hardwire the brake switch to the rear right, left, and center lights, the Brake Light Circuit still comes on.

When I connect the brake light switch directly to the rear right brake light only, the voltage at that connection is 11-12 volts ish. Same with the center brake light. But when I connect the switch directly to the rear left brake light switch, the voltage reads 170 volts!!!!!

So that is strange.I have no idea what's going on.

I think I should replace the Control module to rule that out because there might still be a passive signal being sent to the OBC even though I've bypassed the module.

Any other tips? Anyone?
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Old 08-21-2016, 07:35 PM   #3
dcramer
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IIRC, inside the fault module there is a fusable link. It is burned out. you can replace it with a fuse
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Old 08-22-2016, 02:08 AM   #4
pitchwite
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This is something I haven't heard about yet. I just looked into it and you may be right.

I looked at the links inside the CM module (fault module) but they're absolutely pristine and test fine with a multimeter too.

I've read that there's wires near the fusebox which are considered fuseable links. They're wires that act as a fuse with thick insulation, and so if they burn out it's hard to tell/see.

I'm going to wiggle them around tomorrow morning and see if I get better results.

Today my coolant temp sensor stopped responding so I'm really hoping this is the solution to all my problems. I'll report back soon.

Thanks for the suggestion.
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Old 08-23-2016, 01:47 AM   #5
pitchwite
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I checked the Fusable Link near the battery in the trunk and in the CM module. They're in good condition.

I have absolutely no clue what could be left.

Are there any other fuses that are related to the brake lights besides the fusable link and the fuse in the main fuse box?
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Old 08-24-2016, 08:36 PM   #6
windwagen
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170 volts? Thats a typo right?

So if you remove a light bulb, what does the message say? Same thing?

Maybe it is a short... and the CM gives up once the short is detected - you did mention the 9v battery test though.
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Old 08-24-2016, 08:47 PM   #7
pitchwite
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Not a typo. It was reading 170 Volts. I was shocked too.

If I remove a light bulb, the error changes to Brake Light Failure instead of Check Brake Circuit.

I am pretty sure now that the problem is somewhere in the front wiring harness.

I changed the coolant and afterwards, my temp sensor wasn't providing a signal to the IC gauge, and the Window Washer Fluid level sensor was giving me an error.

I played around with the wires and ravaged them and eventually those two things starting working again. But the next morning the car battery was dead. I've wiggled the harness some more and now it's holding a charge.

So, the front wiring harness is HUGE and I really don't want to rebuild it. But I also don't understand why a wiring harness problem would be giving me the 170 Volts for only one of the brake lights through the CM module.

Surely all these findings should point me to one wire in the engine bay that would be doing all of this.
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Old 08-24-2016, 08:53 PM   #8
pitchwite
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The 9V battery test was from the output of the CM module to the brake lights. They worked fine.

So it must be something under the hood or steering wheel.

Last edited by pitchwite; 08-24-2016 at 08:57 PM.
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Old 08-24-2016, 09:04 PM   #9
windwagen
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If it's really 170 volts, then your CM is definitely fried. 170 volts DC will blow up a lot of stuff... It's not so simple to generate 170v from a 12 volt battery system....

What part of town do you live in?
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Old 08-24-2016, 09:37 PM   #10
jabela
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Is it possible you read the meter wrong and it was .170 V? Could be an easy mistake on some digital meters or maybe it auto adjusted the scale to mV?
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