Electrical Gremlin

[Zephyr]

I am struggling with an electrical problem on my 1994 F250 IDI. Long story short, when I turn on the running lights, I can't turn them off without disconnecting the battery. After disconnecting the battery (main headlight switch off), I can reconnect the battery and no running lights until the headlight switch is turned on and the cycle repeats. I replaced the headlight switch and problem continues. NOW, when I remove the #4 engine compartment fuse (trailer running lights) the problem does not occur so I am suspecting running lights are backfeeding from the trailer circuitry.

I have several questions:

1)Checking power to the #4 fuse, I am getting 12v always on the right terminal. Can someone verify this is correct for me?

2)Are their areas in the wiring from fuse box to trailer receptacle that could be easily evaluated for the cause?

My schematic reading stinks and I am no electrical expert. Any help? Thanks!

 

[IDIBRONCO]

I'm nothing that even remotely resembles an electrical expert, but maybe try a new headlight switch. Strange things can happen when you're dealing with electrical systems.

 

[TNBrett]

Is this a new problem, or a problem that you just noticed on a relatively new to you truck? It sounds like a relay wired up in a latching configuration. Like someone may have tried to add a relay in line to the trailer lights and done it incorrectly.

Maybe look near the engine compartment fuse block, or near the back of the frame with the trailer and taillight wiring.

Sent from my iPhone using Tapatalk

 

[Big Bart]

Zephyr,

I saw a diagram on google images that suggests 94’s have a trailer driving light relay. Guessing this relay or the wiring to it is perhaps your issue. (Why pulling the fuse for trailer running lights has stopped the issue.) Could be the relay is bad, shorting out, or to TNBrett’s point maybe some one has hooked to it and that alteration is feeding power keeping the relay on.

Check your owners manual, should tell you where it is. The power box or fuse cover panel should also tell you. Pull the relay and see if the trucks running lights now go on and off with the fuse put back in.(I am guessing but do not believe this relay is needed for the truck running lamps.) If so next step is replace the relay and test again. If problem solved, it was a bad relay.(Not uncommon for relays to fail over time.) If not now you need to dig a little more with a multi meter.

1) Visually check the relay socket for corrosion, melting, or other issues.
2) Perhaps someone has wired up to the relay wiring or something is shorting our to it. This could be powering the relay or the lighting connectio so now your running lights stay on. Most relays have 4 terminals. Two for powering the relay (12v positive and negative.) and the relay connects the other two together when powered up to complete a circuit.
3) Perhaps someone added a alarm or radio, did some wiring, worked on the trailer connection, added a trailer brake, added a electrical pump, or did some wiring fixes. That some how are back feeding the relay or delivering power to the running light circuit. Less likely because the issue is only when you turn on the lights. Not when you hook back up the battery or start the truck.

So start with that relay and let us know what happens. Also send a couple picks of the relay.

 

[Scotty4]

It does sound like a relay issue. Relay #4 in the fuse panel for a 94.

It could also be loose wiring in the headlight switch connector. Mine looked burnt so I bought a replacement on rock auto. The brown wire controls marker lights, tan/white controls running lights.

Being an 85, my dimmer switch is a different shape, but it's the same premise; connector has female blade terminals in it that can loosen up and not hold the Male ends on the switch.

On my truck, the headlight switch always has power thru the fuse so you can run cargo and dome lights, but everything on ethe switch works with truck off. Fuse 4 on yours is trailer running and normal daytime running lights.

Try the relay and work from there!

You must be registered for see images attach

You must be registered for see images attach

 

[Zephyr]

Update:
As mentioned, the headlight switch is new. I replaced the switch pigtail as there was some plastic scalding between a few terminals. Perhaps associated with 25 years of high amperage current? I changed out the trailer circuit relay.

Still has the same symptoms. There is no chance a previous owner wired something in the circuit as I am the original owner.

Any other thoughts? I am still questioning whether the trailer light fuse (#4) in the under hood circuit box should always be hot. Could someone check theirs with a test light or volt meter?

Thanks, all.

 

[chillman88]

I changed out the trailer circuit relay

This was my first thought. Mine got water in it and did exactly as you describe. But you've already ruled that out....

I have a 91 so I can't check the fuse for you because mine is wired different. It would make sense to me if the trailer fuse is always hot simply because everything is wired to pass through the relay, so that fuse is likely just the power feed to the relay, which should only be connected when the relay is active.

Pull the relay and see if it still does the same thing. If it DOES then the issue would have to be in the wiring before the relay.

 

[Zephyr]

Thanks all for the help! So the guy who wired the recent trailer pigtail also spliced in the license plate light wiring so it was backfeeding through the rear running lights to energize the trailer light relay. The idiot who wired the rear lights....me. Finally solved!