Truck Light Gremlins Have Me Puzzled

Lanse · Feb 23, 2017

Morning everyone! So, I"ve been working on a 1984 Ford F700 (same cab/electrical parts as their smaller trucks) and I have all the lights on it setup really nice - new LED
taillights, new marker lights on both sides, etc. Everything was working fine, until something stopped working (obviously).

At the time it stopped, I was replacing the cab roof marker lights, however I"ve since unhooked them completely which doesnt change anything, making this even
stranger...

Currently, the only things that work are the headlights and the turn signals.

All the other lights - just -randomly - stopped working and I cant figure out why! They were functioning before, and now they"re not. All the fuses look just fine and I"ve
already replaced the main lighting off/on switch, which made no difference.

Anyone know know what I should check next? This has me puzzled. Thanks in advance everyone!

rustyfarmall · Feb 23, 2017

Don't know if this applies or not, but quite often the medium duty and heavy duty trucks have a seperate switch for the marker lights.

Bob · Feb 23, 2017

Simple things first...

You said the fuses "look good"... did you actually check the taillight fuse or replace it?

If so, grab a non-powered test light... is there power at both sides of the tailight fuse? Check with the light switch both "on" and "off".

If power that far, check for taillight power at the lightswitch, IIRC, likely the tan/white wire. If power there from the fuse, switched output to taillights is a brown wire.

Also, some medium-duty Ford trucks have another fuseholder behind the seat at the RH corner of the cab, don't know if yours would, but it might be worth a look back there.

tomturkey · Feb 23, 2017

Lighting electrical problems befuddle me. But as learned at this site, grounding is a usual suspect. that truck probably has multiple grounds. Start shining them up and work thru. good luck, its frustrating gobble

Lanse · Feb 23, 2017

Whats strange is the rear lights work just fine - when they're turn signaling (theres only two taillights, they double as turn signals)

Lanse · Feb 23, 2017

But they were all working just fine before - how could one bad ground turn off the entire taillight circuit (except when its signalling) and all the marker lights together? Wouldnt they just use a different ground?

Steve@Advance · Feb 23, 2017

Does that have a trailer tow package?

Not sure about going that far back, but the newer trucks have separate circuits for the trailer. Someone may have wired the truck light to the trailer circuit, if that has failed it would have taken the truck lights out.

Check the owners manual or online for a fuse/circuit breaker map.

Moonlite37 · Feb 23, 2017

T never saw one equipped with the turn signal lights sharing a circuit with tail lights. Most share circuit with Brake lights.

Bob · Feb 23, 2017

There's nothing strange about that at all... turn signals are a completely different circuit (and a different filament in the original taillight bulbs) than the actual tailights. (NOT sure what type of LED setup you may have replaced them with.)

There should be 4 wires going to the back... BROWN for ALL tailights and marker lights, YELLOW for LH brake/turn signal light, GREEN for RH brake/turn signal light, and a fourth wire for the backup lights.

The brown tailight wire needs to be "hot" anytime the lights are on, regardless of whether the turn signals or brake lights are in use.

jon f mn · Feb 23, 2017

That truck should have 2 fuse boxes, one under the hood and one under the dash on the drivers side. Did you check both?

fixerupper · Feb 23, 2017

Sometimes when you lose the ground to all of the taillights, the turn signal bulb will ground through the tail light circuit enough to make the turn signal bulbs light up and sometimes the tail lights will dimly light up too. The tail light circuit along with the front parking lights are now the ground. Shine up a spot on the frame back by the tail lights and ground your tail light ground wire, usually the white one to that shiny spot and see what happens.

old popper · Feb 23, 2017

How did the plug to your light switch look when you replaced the switch? My Ranger pickup (91) the tail lights and dash lights just quit. What I found was the plug to the light switch was loose (broken clip one side). Being loose it melted. I cut the plug from my old explorer out in the weeds and spliced it in. Problem solved.

Dick L · Feb 23, 2017

We all have to find those types of problem from time to time. I break out my probe test light and a jumper wire. I trace the wires from the light back toward the power source after making sure I have a proper ground. My jumper is hooked to the battery so I can put power to the wires and make the lights light to make sure I am tracing the correct wire if needed. They sell a hand held power source tester. With that you do not need the jumper wire.

DENNIS MIN · Feb 23, 2017

Lanse,

As an attempt to assist, were all the replacements a LED? They are polarity oriented. I may have just stated the obvious, and not meant to frustrate further.

D.

Truck Light Gremlins Have Me Puzzled

Lanse

Well-known Member

rustyfarmall

Well-known Member

Bob

Well-known Member

tomturkey

Well-known Member

Lanse

Well-known Member

Lanse

Well-known Member

Steve@Advance

Well-known Member

Moonlite37

Well-known Member

Bob

Well-known Member

jon f mn

Well-known Member

fixerupper

Well-known Member

old popper

Well-known Member

Dick L

Well-known Member

DENNIS MIN

Member

Similar threads