Author Topic: Brake Light Confusion  (Read 13694 times)

Offline Boone83K10

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Brake Light Confusion
« on: September 12, 2013, 03:19:40 PM »
Just recently my rear lights started acting crazy. Here is what is happening.

Left signal: the driver tail and both (driver/passenger) B/U lights illuminate at exactly the same time.

Right signal: works as normal

apply brakes: both tail lights and both B/U lights illuminate at same time and remain on until you take foot off brake.

hazards: both tail lights and B/U lights illuminate together

With running lights: they come on fine, but everything remains the same as above.


everything is normal on the front.


I have cleaned the ground behind the driver side lamp, there is no ground behind my passenger lamp...

EDIT: there is a hack job of a trailer harness from a PO, it is spliced in after the driver side plug to the rear lamp...
« Last Edit: September 12, 2013, 03:33:49 PM by Boone83K10 »
1966 Mercury Comet - Built 429 (First Car/Show Car)
1983 Chevrolet K10 - Built 350 (Winter Toy)
1992 Chevrolet Lumina Euro (Sold @ 265,000 miles)
2003 Ford Escape 4x4 - Mac Perf. Intake/Exhaust (Wifes' DD)
2007 Mercury Milan - Steeda Tuned (Summer Toy)

Online bd

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Re: Brake Light Confusion
« Reply #1 on: September 12, 2013, 03:55:23 PM »
You most likely have an open ground, so circuits are backfeeding.  Make sure the frame rails are properly grounded to one another and the battery, and that the 'ground splice' in the rear wiring is intact.

Nevertheless, remove the bulbs and inspect inside the sockets for damage and corrosion, or any trace of water.  Reinsert the bulbs with a smear of lithium grease or antioxidant on the bulb bases.  Make sure the bulbs are properly indexed as they are inserted into their respective sockets.  As a last resort, cut the hacked trailer wiring off of the factory harness to isolate it and identify whether it's the culprit.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Boone83K10

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Re: Brake Light Confusion
« Reply #2 on: September 12, 2013, 07:23:49 PM »
Ok I got the trailer harness off. It was using two wire splice caps and then redundantly wrapped a 2nd time around bare wire above the splice. I undid all of that and there were actually no cuts in the original wiring. This was all after the plug for the wiring that comes from the front of the truck.

They didn't even tap into the reverse lights at all with the harness. So I went ahead and made a small incision in the cover. Here is what I found:

-Yellow wire is hot (blinking) when you signal left.
-Dk Green wire is hot (blinking) when you signal right.
-Lt Green is hot (blinking) when you signal left. Lt Green is dead when you signal right.
-Brown (at license plate wire) is hot when running lights on.
-All wires hot when pressing brake or hazards engaged.
-I removed the ground right behind the driver tail light, everything still works (albeit wrong) even though it is not grounded there????

This makes me think it has to be in the dash somewhere.

In reality the B/U lamps are the only culprit. They are getting power somehow when they shouldn't.

Could it be the Neutral Safety Switch? Brake Switch? Headlight switch? or my worst nightmare, inside the column?
1966 Mercury Comet - Built 429 (First Car/Show Car)
1983 Chevrolet K10 - Built 350 (Winter Toy)
1992 Chevrolet Lumina Euro (Sold @ 265,000 miles)
2003 Ford Escape 4x4 - Mac Perf. Intake/Exhaust (Wifes' DD)
2007 Mercury Milan - Steeda Tuned (Summer Toy)

Offline rich weyand

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Re: Brake Light Confusion
« Reply #3 on: September 12, 2013, 08:13:43 PM »
STOP!  Listen a minute, da**it.  Pay attention to bd: "You most likely have an open ground, so circuits are backfeeding."

There should be a ground wire from the battery to the engine.  There should be a ground strap from the passenger rear engine head to the firewall.  There should be a ground from the driver front engine head to the frame rail.

If any of these are missing, then the battery, the cab (including the dash controls), and the frame (including the box and the rear lamp sockets) don't agree on what "0 volts" is, and literally anything can happen.

There should also be a sheet-metal ground from both rear light assemblies to the body.

Check all of those grounds first.  They are easy to check, they are often omitted when people do engine work, and they are by far the most common cause of multiple electrical failure.

None of your voltage measurements mean anything, because, if the grounds are missing, what are you measuring voltage against?  The frame?  Who says that's ground?  Unless everything is grounded to the same "0", then no readings matter.

You're headed off on a wild goose chase until you make sure everything agrees on what "0" is.
Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift

Offline Boone83K10

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Re: Brake Light Confusion
« Reply #4 on: September 12, 2013, 08:30:30 PM »
Quote
There should be a ground wire from the battery to the engine.  There should be a ground strap from the passenger rear engine head to the firewall.  There should be a ground from the driver front engine head to the frame rail.

All 3 of those were checked and present. All those have bare metal showing.

Quote
There should also be a sheet-metal ground from both rear light assemblies to the body.

Trucks only have one ground, behind the driver side tail light. Suburbans have two grounds. Mine has bare metal showing.

1966 Mercury Comet - Built 429 (First Car/Show Car)
1983 Chevrolet K10 - Built 350 (Winter Toy)
1992 Chevrolet Lumina Euro (Sold @ 265,000 miles)
2003 Ford Escape 4x4 - Mac Perf. Intake/Exhaust (Wifes' DD)
2007 Mercury Milan - Steeda Tuned (Summer Toy)

Offline rich weyand

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Re: Brake Light Confusion
« Reply #5 on: September 12, 2013, 08:53:41 PM »
OK!  My suggestion is to run a wire from the battery (-) terminal to your negative voltmeter clip, then check all the voltages to battery ground.  Check also the voltages of the grounds at both tail light assemblies.  Also check the voltage of the ground block under the dash to make sure it is zero.

This diagram shows both tail lights having a sheet-metal ground, though it also says "typical wiring".
http://www.73-87chevytrucks.com/techinfo/wiring_diagrams/81-87_chass_rr_light.jpg

Alternative failure mode: it could be that the backup lights wire and the driver's side tail light wire are shorted to each other somewhere in the run back from the dash.  Could also be under the dash.  This would be the LT GRN wire shorted to the YEL wire in the diagram.  That would give you what you have, I think.

I would follow those wires back and inspect for chafed wiring or IPO wiring changes between there and the dash.
Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift

Offline rich weyand

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Re: Brake Light Confusion
« Reply #6 on: September 12, 2013, 08:56:21 PM »
Question.  If you put the truck in reverse with the lights off, does the driver's side running light light up as well?  That would also be a symptom of a LT GRN to YEL short in the wiring.

I think you can check this with the ignition on and the parking brake set, but the engine not running.
Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift

Offline Boone83K10

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Re: Brake Light Confusion
« Reply #7 on: September 12, 2013, 09:04:53 PM »
lights off and in reverse light up both B/U lights and both tail lights.

« Last Edit: September 12, 2013, 09:11:40 PM by Boone83K10 »
1966 Mercury Comet - Built 429 (First Car/Show Car)
1983 Chevrolet K10 - Built 350 (Winter Toy)
1992 Chevrolet Lumina Euro (Sold @ 265,000 miles)
2003 Ford Escape 4x4 - Mac Perf. Intake/Exhaust (Wifes' DD)
2007 Mercury Milan - Steeda Tuned (Summer Toy)

Offline Boone83K10

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Re: Brake Light Confusion
« Reply #8 on: September 12, 2013, 09:16:43 PM »
wait, doesn't that suggest that I have a short between the LT Green and the brown as well as a short between the Yellow and Lt Green????

Because the Dk Green is separate and works fine when I signal right...
1966 Mercury Comet - Built 429 (First Car/Show Car)
1983 Chevrolet K10 - Built 350 (Winter Toy)
1992 Chevrolet Lumina Euro (Sold @ 265,000 miles)
2003 Ford Escape 4x4 - Mac Perf. Intake/Exhaust (Wifes' DD)
2007 Mercury Milan - Steeda Tuned (Summer Toy)

Offline rich weyand

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Re: Brake Light Confusion
« Reply #9 on: September 12, 2013, 09:47:14 PM »
Yeah, sounds like the DK GRN wire is the only one not shorted.  His trailer wiring, if it was really cra**y, may have melted insulation in the harness and shorted these three all together.  Follow the harness back and inspect carefully.
Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift

Offline Boone83K10

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Re: Brake Light Confusion
« Reply #10 on: September 12, 2013, 10:26:45 PM »
Yeah I tested the plug from the cab.. post #18 and #24 light up when I signal left.

1966 Mercury Comet - Built 429 (First Car/Show Car)
1983 Chevrolet K10 - Built 350 (Winter Toy)
1992 Chevrolet Lumina Euro (Sold @ 265,000 miles)
2003 Ford Escape 4x4 - Mac Perf. Intake/Exhaust (Wifes' DD)
2007 Mercury Milan - Steeda Tuned (Summer Toy)

Online bd

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Re: Brake Light Confusion
« Reply #11 on: September 12, 2013, 11:46:12 PM »
Ok...  I went ahead and made a small incision in the cover.  Here is what I found:

-Yellow wire is hot (blinking) when you signal left.
-Dk Green wire is hot (blinking) when you signal right.
-Lt Green is hot (blinking) when you signal left.  Lt Green is dead when you signal right.
-Brown (at license plate wire) is hot when running lights on.
-All wires hot when pressing brake or hazards engaged. **
-I removed the ground right behind the driver tail light, everything still works (albeit wrong) even though it is not grounded there???? **
lights off and in reverse light up both B/U lights and both tail lights. **

Wait, doesn't that suggest that I have a short between the LT Green and the Brown as well as a short between the Yellow and Lt Green????

IMO you have more than one thing wrong.  I still think you have a ground issue with the rear lights due to the results you posted above (marked with **). 

Trucks only have one ground, behind the driver side tail light....  Mine has bare metal showing.

Did you verify the 'bare metal' is grounded to the battery?  Make sure the bed is properly grounded to the frame; and the frame to the battery.  Chasing poor grounds can be tedious - don't make the error of assumption.  Remain systematic.  Refer to the 1983 Wiring Manual in our Technical Pages for model specific detail.  Keep chipping away at it and you'll find the problem.  Hopefully our input will make it a little easier.

In addition to a floating ground, the backup light circuit is shorted to the left turn circuit....

I tested the plug from the cab... post #18 and #24 light up when I signal left.

Did you make ^^^^^ tests with the rear lamp harness unplugged from the bulkhead connector?  If so, your backup-to-left turn short is under the dash.  But, since the backup lights don't run to the turn switch, you can relax - the problem isn't in the column.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Online bd

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Re: Brake Light Confusion
« Reply #12 on: September 13, 2013, 12:03:53 AM »
...it could be that the backup light wire and the [left turn] wire are shorted to each other somewhere in the run back....

It's not uncommon for the rear lamp harness to be melted and partially fused by exhaust heat, especially if the plastic harness retainers have broken, so the harness isn't secured within and protected by the frame rail.  If the results you attained when testing circuits 18 and 24 at the firewall connector were achieved with the rear harness unplugged, the short is under the dash.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Boone83K10

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Re: Brake Light Confusion
« Reply #13 on: September 13, 2013, 06:05:52 AM »
the entire harness is still attached to the frame with the shield still in very good condition. I tested the plug from the cab at the rear of the truck, I performed no test at the bulkhead connector. The bulkhead still has all the "goo" on all the wires protecting it from elements, but that doesn't mean something isn't wrong there. How would one test the bulkhead connection with the goo still there?

the bed of the truck has never been removed so I assume it still makes a good ground, but like you said make no assumptions. This truck only has 2 rust spots on it.

When I meant all wires hot when pressing brake, I meant that tails and B/U lights, the license plate does not light up.
1966 Mercury Comet - Built 429 (First Car/Show Car)
1983 Chevrolet K10 - Built 350 (Winter Toy)
1992 Chevrolet Lumina Euro (Sold @ 265,000 miles)
2003 Ford Escape 4x4 - Mac Perf. Intake/Exhaust (Wifes' DD)
2007 Mercury Milan - Steeda Tuned (Summer Toy)

Online bd

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Re: Brake Light Confusion
« Reply #14 on: September 13, 2013, 08:44:42 AM »
...I tested the plug from the cab at the rear of the truck, I performed no test at the bulkhead connector. The bulkhead still has all the "goo" on all the wires protecting it from elements....  How would one test the bulkhead connection with the goo still there?

Repeat your test at the firewall bulkhead with the rear lamp harness unplugged.  That will either eliminate or identify the chassis run as a source of issue.  The 'goo' is just a dark antioxidant.  You can procure more from Home Depot or any electrical supply store.  Trucklite also makes it.  It tends to thicken over time.  You can use a little spray solvent like starting fluid or B12 to remove the old 'goo' if it has hardened, but probably won't be necessary.  The antioxidant is not conductive, but neither is it intended as an insulator.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)