I can not figure out why I have no rear running lights or side marker lights. The brake lights, turn signal, and back up lights all work. The fuses all look good. I'm at a loss and do not want to get stopped or much more rear ended at night.

 

Test for voltage to ground at the pigtails on both of 'em.

Turn on the lights and probe 'em with a test light.

And, check the GROUNDS! GROUNDS, GROUNDS, GROUNDS!

 

X2 on GROUNDS GROUNDS GROUNDS

I would bet that your other lights are finding a ground path through the running lights when they are not on, but normal grounds are bad, so running lights won't work.

 

Also, did you check the basics? Like are the bulbs good?
The sidemarker lights are notorious for getting a loose connection between the bulb and socket, so the quick test is just to jiggle things around some. If you've already done that, then it's on to the next test.

And for the tail lamps, bulbs first (well, "first" being AFTER checking the ground aspects), then check the condition of the sockets. They get corroded back there quite quickly sometimes.

Also common is a pinched/buggered wire. Follow the harness along the frame and make sure there is nothing pinched, or you don't have an exhaust pipe running too close. Many dual exhausts have this issue when the installer is not as careful as they should be. Plenty of room under a full-size. But sometimes the tubes still end up running right up against the brake lines, fuel lines, and wires.

There is no fuse for the tail lamps. The circuit protection is in the form of a circuit-breaker built in to the headlight switch. Usually, if the headlights are working you are not having an issue with the breaker.

But what about the connector at the switch? Or the connector near the firewall heading down and to the back? Either one could have an issue after 40 years or so, which means they should be checked.
The headlight switches often have a wire getting hot for one reason or another and burning up a bit. Not enough to see smoke or smell, but it deteriorates over years and then finally will not pass electricity. Happens often enough to make it worth turning into a contortionist and getting under the dash, or pulling the switch down to take a better look in the daylight.

Just some more things to check before pulling too much hair out!

Paul

 

I just had this problem last week. Brake and turn signal lights worked, but no running lights. The wiring was in rough shape so I cut the harness just behind the cab and rebuilt it. I also had a new set of rear LED lights laying around and installed them on my flatbed. All worked great until I wired the last side marker light. It had a bad internal ground that prevented brake and running lights from working. Ground, Ground, Ground

 

Also check the grounds IN the sockets...sometimes the ground wire breaks loose from the tab in the socket. Otherwise....check grounds again. Are you sensing a common theme yet?

 

 

Thank you all for the advise, not much a fan of wiring or have any real knowledge about it. Going to have to learn though.

 

Tn, I hate wiring also. But it always seems that once I get into it, I enjoy it. DC wiring is pretty simple. Just remember that anywhere there's a ground, ensure you've got clean metal. Use the correct size wire. Use quality connectors (I always use heat shrink type connectors, and I add heat shrink tubing). Use the right tools to make the job right, easy, and fast. With old vehicles, it's imperative to have a good wiring kit. The stuff is cheap.


Assuming your wiring is same as mine, it all runs down the drivers side frame rail. Find a spot to cut the wires that leaves you plenty of usable wire on the upstream side. Use a multimeter (they're cheap) to find out which wire does what, by turning on lights, stepping on brake pedal, going through turn signals, and setting shifter in reverse for back-up lights. If you're alone, you can use the video function on your phone (assuming you have one) to video the meter while you're stepping on the brake pedal. The other functions you can do alone. Mark each wire's function with a sticky label.


Move to the actual light assemblies. If they're old and dry rotted I replace them. Remove the lights, disassemble them and look for loose connections, loose bulb sockets, and bad wiring. Look for any ground wire points and remove them, clean the contact surface, inspect the wire and terminal, then reattach. Now just run new wires from where you labeled the old wire. Most likely you'll find One bad ground, and that's all it takes.


There should be two wires common to both light assemblies: Running lights and Reverse lights. You'll need to do a T splice on these, in order to run to the passenger side. Use a T splice. Don't just strip and tape the splice.