Only one side of brake lights work..

[arby87]

So when I bought this truck- 87 V10, 4" lift, 350 (supposedly), TH400, NP 208- most of the lights worked- including the brake lights and rear blinkers.

Now only the passenger side rear lights work for brake and blinker- nothing on the driver's. Even after replacing bulbs, though the current ones look fine. 

So before you jump to grounds, today I did go in and clean up some of the grounds (worked wonders for the headlights, but they are a topic for another thread) and that made no difference for the rear tails- yes, I am talking about the ground behind the rear tail light assembly.

Now the harness has been hacked into previously for a trailer, and there is one wire hanging out of that mess that looks like it might have been a ground, but I am pretty sure that was already busted off before when the lights were working. I know the rear harness itself has to be getting power because the rear passenger does work.

Should I be looking into another area- like fuse box- or should I keep concentrating on the issue being in the rear driver tail light assembly? I was under the impression if one side of the rear is working, then everything that controls those lights from the front end (brake light switch, blinker control) should be fine.

Recent work to the rear of the truck was the complete replacement of the rear axle assembly and drive line.

The main harness up under the dash is a huge hack mess....

[VileZambonie]

Okay so before you dismiss the ground circuit, without it the system is either open or finding a backfeed. You can have the greatest connection to the bed but the bed doesn't necessarily have the best path to ground. I always run an auxiliary path from the bed directly to the frame, the frame to the body, both to the engine, one from the trans and directly back to B-. If you have this your ground circuit should be fine.

For the control side of the circuit, all you need to do is break out your test light connected to a verified B- and check operation at each point in the tail light harness. The harness breaks out of the bulkhead connector and goes to a connector series at the back of the bed. Those are your test points and the rest is process of elimination. The factory wiring diagrams are in the tech section.

[arby87]

On a related note, what gauge of wire should be going from the battery negative terminal to the frame?

My main ground wires go from the negative terminal to the radiator support (shared by the passenger head light), alternator bracket, and frame.

The wire going to the frame looks like it might be 8-16 gauge.

I will need to look and see how bad off the bed to frame ground is- everything is a corroded mess and I need a stiffer wire brush and some emery cloth. Might have to dig up the Dremel and some wire brush wheels....

[VileZambonie]

Your main B- cable should go to the cylinder block or to the alternator bracket on the cylinder head. It is okay to add additional grounds.

[arby87]

So I am good to go with a B- connection between the radiator support (sheet metal) and engine ground (alternator bracket), and the third line added to the frame below the motor is just extra.

Is there a normal area for the bed to frame ground?

Thought the bolts keeping the bed on the frame would have been grounded enough, but looking at how corroded they are....

[arby87]

Finally got the wife to let me off the hook to get into this issue- wasn't a ground problem at all.

I discovered that not all automotive test lights dim based on voltage seen across them. The newer light I picked up from O'Reilly was lighting up when I probed the light socket (same brightness as when tested against the battery- LED bulb), but my multi-meter was only showing around 1 volt. So the 1 V was enough to set off the test light, but not the actual tail light bulb.

Lesson learned- multi meter still a better tool to use than a test light (least a newer LED one).

So I tore into the harness to determine where the voltage drop was happening. As stated in other posts, this truck first started life towing- then as a mud bogger- and the original owner had hacked the rear harness up pretty good for trailer lighting. I decided to rip the trailer harness wiring out since I am not ready to tow and the harness was all messed up from the cab any way.

As soon as I pulled apart all the electrical tape three of the 4 wires (yellow, dark green and brown) fell apart from the crimped butt connectors that were used to hack into the harness. Looked like because those were used and not protected well enough from water, the next owner that mud bogged the truck just got a ton of water and mud (now dirt) into everything. Whipped out the soldering iron and heat shrink and repaired best I could from under the truck.

Still not enough voltage. Tested at the rear connector and verified I was getting enough volts from the front of the truck, which I was, so I knew it was between the rear connector and the light enclosure.

Taking the enclosure apart and pulling the harness back out, I found an unprotected spade connector setup on the yellow wire that was corroded to heck about two feet back from the light enclosure. It kind of scares me to see this harness hacked as much as it has been just in the rear. I cut the spade setup out, and because a thunderstorm was rolling in I took a short cut with a heat shrink butt connector followed up by electrical tape.

Next up- attacking my front head light issue.   

[bd]

...The newer [test] light ... was lighting up when I probed the light socket (same brightness as when tested against the battery- LED bulb), but my multi-meter was only showing around 1 volt. So the 1 V was enough to set off the test light, but not the actual tail light bulb.

Lesson learned- multi meter still a better tool to use than a test light (least a newer LED one)....

A c t u a l l y - an incandescent test light is the better tool to use for tracing power in non-microprocessor based circuitry.  LED test lights are high impedance devices that require <0.6 volt and ħ20 milliamps (ħ0.02 ampere) to illuminate brightly.  LED current demand is so slight (effective resistance is so great) that excessive resistance elsewhere in a circuit can pale in comparison, causing virtually all of the available voltage to drop across the LED test light, illuminating it.  In contrast, the incandescent test light draws enough current (effective resistance is relatively low) that excessive resistance elsewhere in a circuit will dominate the voltage drop (use the majority of voltage) in that circuit, so that the incandescent test light will not illuminate at all, or at most very dimly.  Voltmeters, out of necessity, are also high impedance devices - the more sensitive (better) the voltmeter, the greater its impedance (effective resistance).  Voltmeters and LED test lights have high resistance purposely to prevent current loading of the circuit being measured/tested, so as not to distort the voltage drop or current flow characteristics within the circuit.  The bottom line is that LED test lights are fine when properly applied, such as probing microprocessor circuits that are sensitive to increased current flow due to the introduction of a test light.

So what it boils down to is you inadvertently used the wrong tool.  It's a common error.  LED test lights should be sold only as specialty diagnostic tools, not marketed for routine repair work.  In the future, use an incandescent test light and you will enjoy better, more consistent diagnostic results.

Now that that's all out of the way, glad to hear you got it sorted out and working properly.  Weather, dust and salt can wreak all manner of havoc.

[roundhouse]

Butt connectors always fail

I've had much better success with normal residential wire nuts
And if it's exposed to the weather , use the epoxy filled wire  nuts
I've had some epoxy filled wire nuts on the fuel injection harness of my 74 bronco that I installed in 1999 and no problems

There's also a very high quality butt connector , lemme dig up the name .....



Ahh
Here's it is , these are pricy but are high quality
https://www.posi-products.com

[zieg85]

I have chased this problem before and found out the turn signal switch was worn out.

[VileZambonie]

Solder and heat shrink your repairs.