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I picked up a 1990 Ford L8000 with the 6.6l Brazilian Diesel. When I bought it about a month ago, all lights worked. After removing several mouse houses, the headlights/running lights will cut on and immediately cut off. Cut the switch off and back on, and same deal. I will be tackling searching for a short to ground soon. After looking through DNTE site, I have not been able to find any wiring diagrams. Did any exist?
The fuse box or panel is located behind the right side dash pad, undo the two thumb screws a open the dash pad like a glove compartment door
Some of these use mini circuit breakers instead of fuses, they will trip out during a short or wiring overload, these breakers automatically reset once the switch is turned off, it the short or overload still exist when the switch is turned back on the breaker will trip out again
These systems are pretty easy to test as all wiring going outside the cab is connected at bus bars on the front of the cab and inside the cab at the right rear corner. If the covers are still in place the wiring description on molded inside the covers
On a 90 model there should be on switch for the head lights and a second switch for the running lights/tail lights, these are separate systems and turning one on should not kill everything
If your loosing head lights I’d first operate the dimmer switch to see if the lights go out on both high and low beam, if they do disconnect the dimmer switch and turn the lights on, probe the wiring plug with a meter or test light, there should be power at the center pin, if so replace the dimmer switch, if not there a short in the wiring inside the cab
This can get pretty long so before I go any farther I what the know what’s working and what’s not working
Thank you for the response. I started diagnostic last night. The headlights and running lights are controlled by one switch. The cab lights are on a second switch that allows blinking. It would seem someone changed this based on what you described and I generally understand about bigger trucks. My suspicion is this was done to install the electric trailer brake controller by the PO. The cab cover for the bus bar is missing but after some cleaning I found some labeling on the bus bars. After removing the panel containing the switches, it looks like there is corrosion in the wire connectors. Once I straightened out the harness, the system started working consistently.
I will check the dimmer the next time I have a minute and then move on to addressing the corrosion.
I may have miss lead you on the switches, mine was a 80’s model that had a separate switch to blink the cab lights, we have a 94 at the fire house but I just turn on both switches and go, hadn’t paid attention to what lights each switch controlled
Electric trailer brakes hook into the brake light wiring only so probably not related
Corrosion could have been causing intermittent lost of contact but if you had mouse nest under the dash there could be a bare wire that got moved while moving the wires around during cleaning
Here’s some photos of the covers that identifies what each stud on the bus bar is for
Drivers side
Passenger side
These are from a 71 model but Ford stayed pretty consistent up thru the years. The 83 model I replaced the cab on still used the same connections
My 87 LTL9000 was also the same
The 90 model 8000 single axle I repaired the wiring only so probably had a small fuse panel behind the right side dash pad instead of the long breaker bar some of the others had, I don’t know if it was a year change or because it was a lighter gvw truck
Last edited by Destroked 450; Nov 21, 2025 at 12:45 PM.
I took the dash apart and traced wires. There were two questionable places that I taped up, but the real culprit was the toggle switch and connector. I disassembled the connector, cleaned the corrosion, and reassembled. That helped immensely but the lights still cut out when going over a bump. If I keep pressure on the switch, it is fine. The dash is broken in multiple places, and I am currently weighing my options if I should source a better used dash or 3D printer repairs for my existing.
@@Destroked 450 - the dimmer switch has a short in it too. Adding that to the list of things to fix.
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