Reverse lights only work with parking/headlights on

[Cstoliker]

98 Ford E Super duty Tioga. Inspection time I was checking the lights an noticed the reverse lights only come on when parking/headlights are on.
I re wired all the parking lights a while ago. The reverse lights have good ground 00.2-3 ohm from out side of bulb to frame. So it's not pulling the ground through a parking light bulb.

All other lights work normally. The reverse lights only get +12v at the socket when the transition is in reverse and the parking lights are on.

Is there something in the steering column that could be casing this?

[Cstoliker]

Naa. It's an odd problem but at least it's consistent. I think I may have it figured out. Looks like one of the wires was grounded by a misplaced screw. I'll have to wait till I get more fuses to see if I got it.

[MEXICOWANDERER]

THE MAGIC WORDS

"I re wired all the parking lights a while ago"

Lose a multiple ground or part of one and the fun begins. The reverse lamps are an indicator to others behind you that you aren't going to go forward or even stay still.

Yank your reverse lights fuse and see if you can finagle them to come on using various switching. Yank your tail light fuse and see if you can get the reverse lights still come on. Something is powering the reverse lights to come on and try to use reverse circuit troubleshooting to identify the interloper.

[mapguy]

Do weird things happen if you wiggle the headlight switch? All of these lighting circuits go through the headlight switch on a ford.

[Sea_scape]

To test try running a separate group wire from frame to light in question. Lights can be back fed through another function and other bulbs. It is possible you are measuring the path to ground through other bulbs, and it will be low ohms.

Grounding can be a real pia. Never assume a ground is good based upon an ohm meter.

Your symptoms sound like it may be a backfield, but on a vehicle who can really guess. Lots of trial and error.

[Cstoliker]

As indicated above first think I checked was ground. 00.2 ohms from outside of bulb to frame.
The rear marker lights were not working after I did some major roof repair up front and cut the wires leading back. I then re ran a new wire along the underside. Single 16ga wire from what I thought was 12+ running lights. I'll start there.

[j-d]

Re-Reading, Some of the discussion involves Trailer Wiring. Used to be, Trailer was just wired into Chassis, but Ford has had supplemental trailer wiring for years. Anything's possible, but I doubt the Tioga coach body was wired as a Trailer.
What did the re-wiring you did involve? What were you trying to create or solve? It's a good troubleshooting precept to suspect recent work and modifications.

[j-d]

Backup lights usually come on anytime the ignintion key is in RUN and the shifter's in R. We have an OP who added a switch to turn them on anytime to provide light to work on the towbar, clever idea.

I'm not about to say This is the Problem This Time, but I can promise you that Loose or Weak Grounds can cause all manner of Gremlins. Try making up a couple temporary grounds. Backup Lamp Bulb Shell to Known Good Ground, and Marker Light Lamp Shell to Known Good Ground. Known Good, like the Chassis.

[westend]

It would seem you have the reverse light circuit powering the marker light circuit. Pretty indicative of a wire shorting or a miswiring.

[Cstoliker]

Lol I was thinking the same thing. But it more a safety thing to warn others you about to run them over.

Reverse lights are totally useless on a 30 foot RV anyway. But I gotta keep it legal. In the off chance the inspector checks them with the other lights off. 🙂

[hershey]

Isn't it normal for backup lights to only work when park/headlights are on?
You don't need a backup light in the daytime.
Darn, now I have to go outside and see what happens on my Suzuki in the morning 🙂

[Cstoliker]

Ok the problem deepens. Replaced #14 fuse engine compartment. And reverse lights work normally. But marker/parking lights won't turn off once turned on!
Marker lights turn off when #14 fuse is pulled. Or "D" relay is removed. They can then be reinserted and the marker lights will remain off untill turned on from inside cab.
I swiched "C" & "D" relays. Same results.