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73-87 Chevy _ GMC Trucks => Electrical => Topic started by: Boone83K10 on September 12, 2013, 03:19:40 PM
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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...
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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.
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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?
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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.
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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.
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.
(http://www.gmsquarebody.com/forum/attachment.php?attachmentid=14248&d=1379019917)
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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.
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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.
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lights off and in reverse light up both B/U lights and both tail lights.
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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...
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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.
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Yeah I tested the plug from the cab.. post #18 and #24 light up when I signal left.
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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 (http://www.73-87chevytrucks.com/techinfo/7387CKMans//Wiring/ST386_83_1983_GM_Wiring_Manual_CK_P_G_10_to_30_and_ST.pdf) 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.
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...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.
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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.
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...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.
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does it unplug? or do I just probe it with a test light?
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Remove the retaining bolt from the center of the bulkhead connector, unplug the complete connector from the firewall socket, separate the rear lamp harness plug from the main body of the connector by sliding it sideways, then reconnect the main body of the bulkhead connector to the firewall - you have to reconnect the bulkhead connector to power the cab before testing. Lay the rear lamp harness to the side and probe the firewall socket as you did at the rear of the vehicle. If the results mimic those you attained before, the 'short' is in the under-dash wiring.
Checking the 'ground' at the rear of the bed should be performed using a voltmeter as Rich Weyand explained earlier. Using a test light could be misleading.
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sounds scary
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DISCONNECT THE BATTERY, then....
Remove the retaining bolt from the center of the bulkhead connector, unplug the complete connector from the firewall socket, separate the rear lamp harness plug from the main body of the connector by sliding it sideways, then reconnect the main body of the bulkhead connector to the firewall ...
then reconnect the battery and do your testing.
Reverse procedure to reinstall that portion of the plug.
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this?
(http://img.photobucket.com/albums/v466/Pontiac1976/Pontiac1976%20truck%20parts/Wireing1984setup035IE01.jpg)
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That's what it looks like. But, you probably won't need to pry it apart. It should separate fairly easily. There's a foam gasket sandwiched between the connector and socket that may try to tear as you unplug it (a remnant of the gasket is pictured around the bolt), so take your time.
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ok I took it off and tested at the firewall. All things are good. It is sending the correct signals through the firewall. So that means it is somewhere from there to the rear!
All the protective sheathing is in very good condition. I guess i will have to start somewhere. Could it be the rear plug possibly?
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plugs usually fail open, wires usually fail shorted. you're shorted, so I'm betting on the harness.
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I began to pull the wiring off the frame rail... this is very bad. This truck has true dual exhaust from when it had the engine swap. They ran the driver pipe too close to the frame rail above the axle and it did this...
(https://lh5.googleusercontent.com/-FvqnUTGZ-ZU/UjTNL6azpfI/AAAAAAAABDA/3LcSz8Hykw4/w885-h664-no/20130914_164454%255B1%255D.jpg)
(https://lh5.googleusercontent.com/-vN3uGaob34E/UjTNIY_Hk1I/AAAAAAAABC4/xdV06NkvS-0/w498-h664-no/20130914_164451%255B1%255D.jpg)
Now what do I do, anyone want to sell me the entire rear harness?? I don't feel up to running entirely new wiring from one end to the other.
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Just splice in a replacement. Start up on the firewall, which is somewhat protected, about 18" off the connector. Splice in a four-wire, direct-burial stranded cable. Cover each solder joint with heat-shrink tubing, then coat the four splices in a bunch with silicone bathtub sealer. Run the cable to the back, cut the factory splices off, and splice the cable to the existing wiring there the same way.
Here's something you can use. Then you're not running separate wires, and they have more protection against water, oil, and gas.
http://www.amazon.com/WHITE-Increments-Speaker-Outdoor-Lighting/dp/B0091L60RG/ref=pd_sim_sbs_e_23
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If the only damage is over the rear axle then repair that section. Or, string new cable from the front of the bed to the rear harness plug. Clean the copper strands of the old wires thoroughly so solder can adhere. Using uninsulated tinned copper butt splices, crimp, solder and heat-shrink the old wires to the new wires then tape it up. Use double wall, self-sealing, heat shrink tubing to seal the connections from water and dirt.
As you run the cable to the rear be sure to route it further from the exhaust and clamp it in place with Adele clamps. For extra protection, you can procure some Syn-flex air line of the appropriate inside diameter and use that for conduit.
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Actually, I think I will just purchase and install a complete new rear wire harness rather than patch the old one.
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my bad?
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Even with a brand new harness, you'll need to take precautions against frying the new one!
If you're determined to install a new harness, have you checked with the store here? You can email (webmaster@73-87chevytrucks.com) or PM Captkaos (http://forum.73-87chevytrucks.com/smforum/index.php?action=profile;u=2) for availability and price.
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either way will work one will just cost more money and you would still have to figure out how to run the wires to keep them away from the heat.
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This thread actually hurt my head to read. lol
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I fixed it!!
Honestly was my first attempt at major electrical work. I'm not very good at soldering but I got better with each wire. It's soldered, heat shrink, and wires taped with loom over it. I also bought some wire harness clips and moved the wires higher in the frame rail away from exhaust. All lights function correctly. I am pretty impressed with myself. Anyway thank you to for all the help.
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Awesome, I have an electrical nightmare on my 2 Wheel Horse Garden tractors. I don't have much hair left to pull out but I hope to have the same successful results and feeling of accomplishment as you. Congrats!!!!
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Good job!!