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Third brake light issues.

Brickhead1988

Brickhead1988

Member
Joined
Jun 4, 2011
Location
Prescott, AZ
'88 245

Just to start out. I've read posts all day.
And didn't want to resurrect a 19 y/o thread...

And before you tell me "to hell with the 3rd brake light!"

This is honestly just a step into trying to troubleshoot the cruise control.

So my steps so far:

Swapped out the idiot light relay twice.

Checked voltage at the harness at the fifth door=good.

Checked voltage at the third brake light connector =good.

Checked continuity/ground, bypassed the 1156 connector & verified good working bulbs=Saul Goodman.

Used some poop leds, which obviously set off the derp warning, but they actually work?

At a loss.

If someone has a recommendation on how to proceed further, I'd be happy for the guidance.

And to bypass it would also make me happy! :)

TIA!
 
I think you're missing a few critical bits of info.

I assume you want the 3rd brake light (CHMSL) to work because if it doesn't work properly then cruise doesn't work.
When you test your bulbs on a separate system - they work.
When you test LEDs in your car, they illuminate but the dead bulb warning occurs because not enough current draw.
When you test incandescent bulbs in the CHMSL bulb holder they don't work.

Assuming what I guessed above is true, check your grounds. It sounds like there's enough voltage to get the LEDs running but they don't need much. But there's not enough voltage / enough current to drive an incandescent to illuminate. Likely when the current flows a poor ground is shunting current through the excess resistance. Try measuring voltage relative to ground at various points with an incandescent bulb in the circuit, I'd guess that you don't actually get the voltages you expect.

If that's not it, someone else will reply or else I'll look at the wiring diagram for cruise for an 88.
 
the_hairy_baboon said:
I think you're missing a few critical bits of info.

I assume you want the 3rd brake light (CHMSL) to work because if it doesn't work properly then cruise doesn't work.
When you test your bulbs on a separate system - they work.
When you test LEDs in your car, they illuminate but the dead bulb warning occurs because not enough current draw.
When you test incandescent bulbs in the CHMSL bulb holder they don't work.

Assuming what I guessed above is true, check your grounds. It sounds like there's enough voltage to get the LEDs running but they don't need much. But there's not enough voltage / enough current to drive an incandescent to illuminate. Likely when the current flows a poor ground is shunting current through the excess resistance. Try measuring voltage relative to ground at various points with an incandescent bulb in the circuit, I'd guess that you don't actually get the voltages you expect.

If that's not it, someone else will reply or else I'll look at the wiring diagram for cruise for an 88.
Click to expand...


Thanks hairy baboon. That makes sense. And yes your assumption is correct.
CHMSL? I had to search that one...

Maybe it's an intermittent ground issue. Because other than testing various ground points, everything tested good.

Out of the blue the CHMSL is working again. But no CC. Guess it's time to see if replacing the stock is the issue.
 
Brickhead1988 said:
'88 245
Checked voltage at the harness at the fifth door=good.
Checked voltage at the third brake light connector =good.
Click to expand...
Ah, a wagon.
One of those things with the harnesses that go through the hinges, and bend every time the hatch is opened, with wires that break where you can't see them, causing all kinds of well-documented intermittent problems.

Out of the blue the CHMSL is working again.
Click to expand...
Probably flexed in just the right way so the broken wire is making contact with itself again.

But no CC.
Click to expand...
Usually cruise doesn't really care if the actual bulb is working, it just wants to know if you're stepping on the brake pedal. That's where the switch at the pedal comes in.
 
MasterBlaster said:
Ah, a wagon.
One of those things with the harnesses that go through the hinges, and bend every time the hatch is opened, with wires that break where you can't see them, causing all kinds of well-documented intermittent problems.

Probably flexed in just the right way so the broken wire is making contact with itself again.

Usually cruise doesn't really care if the actual bulb is working, it just wants to know if you're stepping on the brake pedal. That's where the switch at the pedal comes in.
Click to expand...

Thanks MasterBlaster.

The dummy/bulb-out relay doesn't play any into the CC circuit?

Although I didn't run finely stranded, I ran a new ground a few years ago.

Tested the clutch & brake switches. Appears good.

Does anyone have a source for the electrical wiring diagrams? I can't seem to find any & my Bentley is pretty useless in that regard...
 
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