No Rear Lights! (except 3rd brake light...)

[MMcc]

Hey All! I'm building out this '97 burb into an overlander. Hit my first frustrating roadblock - all the others have been "fun". (The truck is one-owner from Phoenix, bone stock & rust free).

Problem
Brake lights, rear turn signals, rear hazards, reverse lights --- they all just stopped working one day.

Attempted Fix
I've done all the easy stuff:

• fuses are all good and have power
• cleaned the ground drivers side rear (right at the bumper)
• read every related post on gmt400
• removed a tail light and looked for power at the plug. Nothing. But the ground here is a'okay.
• replaced the multifunction switch

I have a FEW CLUES and feel like I'm close! But i'm a parts monkey...aspiring passible mechanic.
The truck has three trailer plugs hanging by the hitch. (what.the.hell??)
1) uHaul - all dead
2) damaged 4-pin w/o the plastic block - all dead. I think this one was OEM b/c it has a separate pigtail going to the non-functioning license plate lights (I'm not sure if the license plate lights ever worked).
3) another 4-pin. Two pins have voltage when the break or hazards are engaged!!
So there IS power getting from the front to the back.

The other clue may be a red herring. Two of the wires on the multifunction switch were a little melted at the connector. I've bought the replacement from 1aAuto and hopefully they can help me out with some info on what wires those are.

I'm desperate for direction. Without it, I'm thinking of exposing as much of this wire harness as possible, looking for damage. And if it comes to it, splicing in a chuck of junk-yard harness.

 

[SUBURBAN5]

It sounds like electrical for sure. Did you look at your brake switch by any chance?

 

[HotWheelsBurban]

When this happened on my 99 Burb, it was the multi function switch. All the affected light circuits go through this switch. But since it sounds like you have a hot mess of a wiring harness out back, you need to look closely at that. Might be a wire rubbed through the insulation that's touching somewhere it shouldn't be. Doesn't take much to start frying wires. The factory harness for the rear of the Burb has a connection between halves at the back of the truck, between the bumper and trailer hitch. That connector plug is where the T connector goes for the trailer wiring. Hopkins makes a plug and play setup that goes into this harness and has a 4 prong flat plug( like U haul uses for their equipment). They have other units that have more wiring in them for the bigger connector types.
There isn't much space to work up there, and lots of places for a wire to get pinched. This kinda stuff requires a lot of patience. But it can be done, just take your time with it. Good luck!

 

[Erik the Awful]

And if it comes to it, splicing in a chuck of junk-yard harness.

The rear harness is a simple plug-and-play replacement. If you have a Pull-A-Part nearby, go grab the entire harness off a Suburban. The pickup harnesses are usually hacked pretty bad, but the Suburbans are usually unmolested. Plus, the Suburbans usually have the 4-prong plug built in.
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[MMcc]

The rear harness is a simple plug-and-play replacement.

It might be that easy!? At least...it seems that replacing this portion of the harness is the next step in diagnosing this problem. Do you remember how far up that plug connection is? I saw where the wire loom enters the frame rail in the rear but I didn't see any plug.

brake switch

I have NOT put eyes on brake switch. I did read elsewhere on the forum that a bad break switch could cause the tail lights to go out while the CHMSL still works normally. (Doesn't make a lick of sense but, I'll trust ya?)

 

[MrPink]

you can even get a brand new rear tail light harness for $40-100 depending on where you get one.

 

[SUBURBAN5]

It might be that easy!? At least...it seems that replacing this portion of the harness is the next step in diagnosing this problem. Do you remember how far up that plug connection is? I saw where the wire loom enters the frame rail in the rear but I didn't see any plug.


I have NOT put eyes on brake switch. I did read elsewhere on the forum that a bad break switch could cause the tail lights to go out while the CHMSL still works normally. (Doesn't make a lick of sense but, I'll trust ya?)

Like mentioned try the harness 1st. I got a burb if you needs some reference. I think bad brake switch would leave lights on of not work at all. But I dont think that's your issue. Wires in the back like a bird nest sounds more concerning to me

 

[John Hendrix]

If it were my truck, I would start by first restoring the rear light harness to factory configuration. (Remove any and all wiring that did not come from the factory including any “t” connector trailer harness)

Once all of the aftermarket wiring has been removed, check for voltage signals at the harness connector. The main harness/taillight harness connector is a flat pack connector located on the driver side at or near the left side bumper bracket at the rear of the frame.

Check for the voltage signals on the connector coming from the front.

If they are present at the connector, then the issue is/was in the aftermarket wiring.
If the voltage signals are not present, try replacing the multifunction switch.

It has been my experience that people are generally lazy when it comes to trailer wiring. I’ve seen wires cut and spliced and I’ve seen wires with the outer casing removed and trailer wires simply twisted and left bare.

The issues associated with poor installation range from blown fuses to complete meltdown. I bet that you will find a “shorted” wire in the trailer wiring.

If removing the old wires solves the issue, and you need the trailer wiring, consider using a new “t” connector as this will eliminate the chance of having poor connections and leave the wiring jackets in tact.

One last thing to look at is incorrect bulbs in the individual sockets. This would be a stretch as I believe you stated earlier that they were working and just quit.

I have seen bulbs where someone had manipulated the wire tangs on the bottom and they were shorting within the bulb socket.

Good luck!

 

[MMcc]

Check for the voltage signals on the connector coming from the front.

If they are present at the connector, then the issue is/was in the aftermarket wiring.
If the voltage signals are not present, try replacing the multifunction switch.

Seems like I "should" be able to unplug the taillight harness at that rear of the truck and check for voltage BEFORE digging in all the way and replacing that segment of harness. Right? I say "should" b/c I'm pretty sure that I dont have a plug connection there anymore.
I could cut the harness just upstream of the first branch (to the uHaul plug) and test the bare wires. At that point, either way, I'm pretty well locked in to replacing that harness with a junkyard find.

I'm confident that I'll find power in one of the four, since I there is power going to one of the trailer wires. If not - I'm really stuck b/c the multifunction switch was replaced yesterday.

A new taillight harness can be had from here for $100. No trailer pig tails. For that kind of $$ I'll go crawl in the dirt and see what it's supposed to look like.

Thanks for the help everyone!

 

[MrPink]

they can also be had on ebay for $40-60 as well

Truck Tail Light Wiring Harness Fit 88-98 Chevy GMC Blazer Suburban Tahoe Yukon | eBay

That's the one I bought