2008 F250
All of a sudden no back lights when headlights are on.
What gives??

 

I had this issue on my truck. It was a bare wire going to ground on 02 superduty. Found it on rear drivers side under tail light cover. Fuse also blew. Not a normal labeled fuse.

 

Brake lights are NOT supposed to come on when the headlights are on, those would be the "running", "park", "tail" lights.

So, which lights do not work, the brake lights or the running lights?

 

The 02's circuitry is COMPLETELY different from an 08.

 

the running/park/taillights do not work. brake lights work fine
bulbs are ok and no fuses blown


thoughts?

 

Have you towed anything recently?

 

Do the FRONT parking lamps work?

License plate lamps?

Cab or side markers, if equipped?

Which SPECIFIC fuse(s) did you check?

 

I checked every fuse inside and engine compartment. I will check the other lights.

 

Ok all lights work except the rear parking lights. Every and I mean every fuse wad checked as ok.
Any suggestions???

 

I haven't towed in a few years. Why do you ask?

 

Have you actually checked the tail light bulbs. The bulbs are dual element with separate elements for brake lights and tail lights (two bulbs in one package)?

If the bulbs are verified good, get your meter and start at the tail light and work forward looking for the lost voltage. If wiring is good, you will probably find the problem either at the multifunction switch or the headlight switch.

 

The running lamps do not go through the MFS.

The front running lamps work, according to the vague report, therefore, there is nothing wrong with the headlight switch as they are in parallel with the rear running lamps.

The trailer circuitry is isolated from the vehicle's lighting system and cannot bring it down unless someone did a shade tree hack job on the wiring.

jawdoc,

Please answer each question explicitly, do not try and give a global answer as that suggests you did not thoroughly check the very specific item asked.

1. Do the license plate lamps work or not?
2. Do you have separate side marker lamps? Do they work or not?
3. Does this truck have single or dual rear wheels?
4. Is this truck equipped with the rear parking aid or not?
5. Is this truck equipped with the "standard" box bed or something else?

Again, EXPLICIT answers are needed to effectively troubleshoot. I've done this long enough that I've learned that vague or ambiguous answers are to be avoided as they tend to lead down wrong paths.

 

^^^^^^^^^^^XX2 What project says!^^^^^^^^^

2 other questions that need explicit answers... Do you have decent mechanical/electrical skill and do you have a decent VOM (volt/ohm meter)?

Most untrained folks (not a jab just a fact) go right to catastrophic failure when its often up the troubleshooting food chain towards simple over looked items, e.g. bulbs & bulb sockets. Does the fuse blow when the bulbs are completely removed from the sockets? Dead short if so. You said you "tested" the bulbs now do an ohm & volt test on the sockets. Do a resistance (Ohm) check on them first without power on the circuit then do a volt check. A bad socket will do exactly what you describe. Dont be afraid to Ohm the bulbs either. Double check to see you are in fact installing the exact replacement part number bulbs? Seen this one before too. Wrong bulb that fit the socket.
Answer all these Qs as best you can and we all will help once we have the full crystal ball picture of what were working on.

If you do have decent skills do yo know how to do a voltage drop test? You will need to do this once the simple things check out OK above. You will need to start "ringing" out the harnesses and P & J connectors. A simple test light can be your friend as well but I like to see the actual DCV and resistance values.

 

Ok here goes...
1license plate lights work(one was blown,now replaced)
2 separate side marker lights work(one was missing the other was blown now both replaced and working)
3 single rear wheels
4 yes it has a rear parking aid which hasn't worked in 2 years
5 standard box with a cap. The light in the cap also stopped working.
I have not done any continuity testing. I have traced wiring looking for broken ones, visual inspection of bulbs which all look fine and checked all fuses.

 

Volt ohm meter now coming out. Ill check for continuity.