‘86 MJ rats nest. Please help! No running lights and signal woes

[cruiser54]

My 90 MJ has blinkers that alternate with the lights on. 

[DzienManche]

On 11/10/2022 at 8:06 PM, gogmorgo said:

Those will properly be called the parking lights. The parking lights are made up of various combinations of marker lights, clearance lights, tail lights, etc., depending on the vehicle. No big deal.
 

The left turn signals blinking dimly indicates a ground issue. What’s actually going on is all the parking lights are blinking. One or more of the dual-filament turn signal bulbs on the right side doesn’t have an adequate ground, so it’s finding it via the other filament and the rest of the lights attached to that circuit.

Would you recommend I change the sockets on the rear side as well?

[gogmorgo]

1 hour ago, DzienManche said:

Would you recommend I change the sockets on the rear side as well?

I don’t recommend changing parts you don’t know are bad. 
 

Any bulb with dual filaments could be causing it. The two filaments share a ground at the lightbulb. If the ground between the lightbulb and the truck is bad, then the powered circuit will find a path to ground through the rest of the lightbulbs in the other (non-powered) circuit. 
There are multiple ways of figuring out which lightbulb ground is causing it. The simplest is just to turn on the turn signal and pull all the turn signal bulbs out (leave them out) and checking after each bulb whether it stops doing it. Once you’ve pulled the bulb out that makes it quit you know the ground for that bulb is bad. Leave that bulb out and put the rest of the bulbs back in, one at a time, checking each time to see if it starts doing it again, and if it does take that bulb back out and keep putting the rest back in. You do this because there may be more than one bad ground but you won’t be able to isolate the bad grounds with more than one bad ground in the same cicuits. When all the bulbs are back in except for the ones that made the markers blink, you can start trying to fix the bad grounds.

Alternatively you can also test the ground at each socket either with a multimeter set either to continuity or resistance, or with a test light on the positive battery terminal (or other known good voltage source). 

 

Would I just go ahead and replace the sockets? No. Absolutely not. Not unless I knew the sockets were bad. Maybe they’re corroded or melted, or contacts are bent… then maybe. But it’s very likely the bad ground is due to a broken wire, corroded ground fastener, corrosion or poor pin contact in a connector, a failed wiring repair… the list goes on. 
Yes it’s nice to have shiny new light sockets. But if you don’t know the socket was a problem, changing the socket is mostly just a waste of time and money. Maybe it fixes the problem, but it might not, and now you’ve added in more connection points, essentially weaknesses in the circuit, and possibly also sockets that are lesser quality than the originals and that won’t give you another thirty years life, and now you’ve made diagnosing the original issue more complicated. And if you don’t get a good splice on the ground side where there was no issue, then you’ve literally just turned a working ground into a bad one and caused the rest of the circuit to malfunction. 
If the taillight sockets are bad, then change them. But if you don’t know they’re bad, then you’re likely going to cause more problems than you’re solving by changing the part.