Hey guys im not new to the forum but new to the f150. I just sold my 04 250 and picked up a 1991 f150 Nite edition with 5.0 5 speed that the price i just could not pass up. I have been loving it so far. Cept for this weird electrical problem i am having.

On the way home I noticed the dash lights were not working. So i checked the fuse and it was blown okay easy fix i thought. so i go up to autozone and on the way back in the dark i realize that the running lights are out. So i go back and check all fuses they are all fine. I was lost at that point and started checking for bad wires or grounds. No luck i started to install a radio into the truck and half way through my dash lights go out and my running lights work. The dash lights work when i put the orange/black and blue/maroon radio wires together. But when i have the wires together and turn on the dash lights from the switch they turn off? I am very confused with this.

Also my heat does not work and temp needle only goes to very beginning of normal temperature. I flushed the radiator because coolant was rust color. I changed out thermostat of my street bike and without it it would circulate right when it starts up. It stays still until it gets hot witch leads me to believe the T-stat is good?

Sorry for the long post this problem is driving me crazy lol. I have had great luck with this forum with my old truck and i have learned a lot from this forum . Any help is greatly appreciated.
Thank you for your time,
Austin

 

Light issues will likely be resolved by replacing headlight switch, try to flush heater core with pressure from garden hose, both directions.

 

Okay so i just got back from playing with the switch and when it turn all the way to the right and clicks they come on and so does cargo light. But any other was turned it they dont. I am guessing the switch is bad. Thank you for the quick reply. And for the flushing the heater core were is the heater core?
Thank you for your time.
Austin

 

Which fuse was blown?

Could be a connector problem at the headlight switch. The connector is
know to melt causing poor contact at the connector lugs. Only way
to tell is pull the connector off and look at the connector pins.
Advance Auto, Autozone, Napa etc... all sell replacement pigtails.

The Orange/Black wire powers the LCD back lighting on the original radio
and is hot in Accessories or Run (Only when the headlight switch is Off).

The Blue/Maroon provided background lighting for the factory radio LCD
when the head light switch is On.

When the headlight switch is Off there is no voltage on the Blue/Maroon wire.

When you connected the two wires together with the headlight switch Off
12 volts was placed on the Blue/Maroon wire from the Orange/Black wire
causing the dash lights to light.

When you turned on the headlight switch the Orange/Black wire no longer
has 12 volts so the dash lights went out.

Conclusion is the fuse you replaced is blown again or the headlight switch
connector is bad.

The back lighting on a lot of the new radios don't dim. Are you sure yours
can? If it can only use the Blue/Maroon wire.

Picture of the headlight switch pigtail