There are two points in the rear section of an E36 that have wiring issues. One is the pass through in the rear fire wall where the wiring for the rear section of the car comes out into the trunk area. The insulation on the wires gets brittle and there's some chafing and it's possible to get some shorting in that area. The other is the wire harness that goes from the car body to the trunk lid. The point where that harness gets flexed with the trunk opening and closing is a place where wires get broken and shorted. You'll have to cut the covering off of this one to inspect the wiring, but you can use electrical tape to wrap it back up when you're done.

 

I and others found we could slide down the black, outer sheath of the trunk to trunk lid cable to inspect the wiring. We didn't need to cut it. It is worth checking if it has not been checked before to forestall other mischief it can cause. Unless the car has a rear wing with a third brake light, however, the brake light wiring will not go into the trunk lid.

[Sorry, I was rather tired and skipped over your last sentence.]

If you get no voltage at the rear light assemblies, sometimes a current sensing shunt melts open inside the check control module on the outer wall above the driver footwell.

I believe there is also a comb terminal junction between the brake light switch and the check control. If you car has cruise control, does it switch off when you press the brake pedal?

 

I and others found we could slide down the black, outer sheath of the trunk to trunk lid cable to inspect the wiring. We didn't need to cut it. It is worth checking if it has not been checked before to forestall other mischief it can cause. Unless the car has a rear wing with a third brake light, however, the brake light wiring will not go into the trunk lid.

[Sorry, I was rather tired and skipped over your last sentence.]

If you get no voltage at the rear light assemblies, sometimes a current sensing shunt melts open inside the check control module on the outer wall above the driver footwell.

I believe there is also a comb terminal junction between the brake light switch and the check control. If you car has cruise control, does it switch off when you press the brake pedal?

Agreed (except on cabrios where the 3rd light is in the trunk lid). Where the trunk lid harness affected the brake lights is the short in the trunk lid wiring had caused some wire heating and had driven a short in the wiring harness where all the rear end wiring converged in one case I saw.

 

I replaced the brake switch and it still doesn't work and no the cruise still doesn't kick off when you hit the brake. But the OBC now doesn't have the "brake circuit failure" flashing anymore.


I tested plugin that attaches to the brake switch and it doesn't have a steady power it blinks on and off. Is that normal?

 

I replaced the brake switch and it still doesn't work and no the cruise still doesn't kick off when you hit the brake. But the OBC now doesn't have the "brake circuit failure" flashing anymore....

I wonder if the new switch is properly seated. The switch has two pairs of contacts that close one after the other: first the main contacts that power the brake lights, then a second pair of test contacts which ground a test input of the check control module. The grounding of the test input triggers the brake circuit error if the first pair do not provide 12V to the brake lights and another check control input.

 

Hi. I wonder if I could add to this thread (or, subtract, really!). I am looking at my neighbor's 96, or 97 318 sedan. He has a left and right rear brake light, and a third (high-mounted) brake light on the rear dash of the passenger compartment. None of these three lights work when he presses the brake pedal. HOWEVER, there are two additional, smaller lamps/lenses that do light up when he presses the pedal. I inspected the bulbs of the non-working lamps, and they don't look burnt. I don't have a shop manual or anything to guide me (other than these posts).

I'll ask him about the cruise control shut off. BTW, that seems like a very dangerous situation to me!!

What else can I check? Is there a fuse for the brake lamps themselves?

Thanks.
mikey

 

Where are the two bulbs that do light up? The brake light circuit is isolated. There should be no other bulbs that could light with the brake lights.

You need to check for voltage at the bulbs when the pedal is depressed. This sounds very much like it's a ground problem or the classic "wire loom deterioration" in the trunk.

 

Thank you so much for the response!!!

I agree that this seems like a ground issue, although I am not quite sure where to look for the wire loom deterioration. Can you expand on that? Is there a DIY on the internet for this problem/solution?

Here is a pic of what I was trying to say.

 

That photo describes normal brake light operation with the exception of the high mount brake light being out. The only brake lights are the high mount location and where the white arrows are pointing to. Everything else (that is red) is a tail light bulb which does not get brighter when depressing the brake pedal.

 

Thanks for your reply!

I would not have expected that behavior. I would have expected the larger light to be the brake light (for more visibility).

So, perhaps we only have a burnt out high-mount lamp.

And, the reason we became suspicious is the brake lamp on the driver side does not illuminate as bright as the one on the passenger side. When we swapped the bulbs left-to-right, the problem followed the bulb. So perhaps we have a bad high-mount and a bad low-mount brake lamp.

Thanks again!
mikey