My neighbors 90 F350 crew cab has some BIG issues right now....
You hit the brakes and the dash lights light up,the front marker lights light up as well and it kills the signals (makes them freeze)
And if you put the hazards on it lights up both the brake and the signal filiments in the taillights.But there is more to it!! If you have the park lights on then the signals and the brake lights dont work! We have tested the headlight switch, the signal switch, the signal flasher, the hazard flasher, Removed the trailer wiring, replaced the rear taillight wiring harness, and have checked as much of the wiring harness as I possibly could without removing it. Has anyone here had a problem with this before? or do you think you know a place on the wiring that could be the problem, I do think that it is a wiring short but just havent been able to find it! I Need Help!!!

 

Sounds a whole lot like a bad ground. IMHO, the brake lights are missing their ground, and thus using what is called a "floating" ground thru the other light circuits lighting them up in the process.
I am not very knowledgeable in the post 70's truck electrical , but I would check the ground studs and wires for the rear light assemblies. Especially the brake circuits, also check the bulbs and sockets for corrosion.
I do not know if Ford added any switches or relays to those "new fangled" trucks that could go bad and cause these problems

Sparky

 

Also, check for any wiring damage under the truck. Sometines wires get melted insulation. When they touch each other, you can get wierd symptoms. It can be faster to find the damage then to reason out what is really going on.

 

Ok we pulled the wiring harness from where it connects under the hood and checked it all the way back to the taillight connector, we had allready replaced the taillight wiring harness with a spare one that I had, Anyway same problem! are there any wires in the dash area that could cause this grief?

 

Well, now I would concentrate on the steering column. The brake light is getting crossed over to the dash lights, etc. Try unpluging the turn signal switch and step on the brakes. I am still beting on some wire damage, perhaps in the column? Wish I had a schematic of the wiring.

 

You're getting a feedback grounding out problem.

I just checked the schematic and it seems on that year Ford has the cluster lighting sharing the same grounding circuit as the signal lights and brake lights.

It could be at the back of the cluster.
I would remove it & Check the grounding wiring on the back of the cluster.

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I had a very similar problem and after much searching and taking things apart, I found the problem. It was a bad bulb. I would check/replace all 4 two filiment bulbs(front and rear). One of the filiments in the bulb broke and shorted to the other, therefore creating a path between the turn signal/brake light circuit and the park lights (dash too). I would give this a try before doing anything else.

 

I changed the bulbs and still have the problem Now about the cluster? what do you mean? I do speak the same english and all but I just dont understand!!

 

The cluster is the guage assembly or cluster of guages.

 

Hows the saying go... There are no stuid questions? Hoo boy was I asleep or what? If I had put a little thought into it, It would have smacked me in the side of the head!!! now to check the cluster (be right back)!