75 k20 light issues

[Zach99999]

I am having a issue with my lights when I press the brake all marker lights come on and also the right turn is turning on the left turn basically I'm having all kinds of issues

I read some of forums with people having some issues and I replaced the turn signal switch and also the brake switch so all those are new and for the heck of it I replaced both flasher relay things


I'm lost they were working just fine just happened all of a sudden

 

[Matt69olds]

Check each and every bulb, make sure they are correct and installed correctly. Make sure each bulb has a good ground. When the bulbs aren’t grounded correctly, all kinds of crazy things can happen.

Does the truck have a trailer light plug? If so, check there also.


The most likely place for parking lamps and taillight circuits to “swap” voltages is one of the taillight bulbs. Since the brake and turn signals are the same bulb (just powered by the same wire on different circuits) the bulbs or sockets back feeding can cause the issue your trying to fix.

 

[75gmck25]

Also check all the grounds and make sure the connection is good. This is from memory, but will get you started.
- instrument panel ground on a ‘75 should be a black wire connected to a post on the emergency brake mechanism.
- ground wire at each taillight, and a larger diameter harness ground near the middle of the truck up behind the license plate area. This is a large gauge white wire.
- ground from light harness to radiator support near each headlight. On the passenger side it also connects to the battery negative
- block to firewall ground near back passenger side of engine
- block to frame near fuel pump
- bed to frame up under the truck near the gas tank (passenger side, I think)

Bruce

 

[Zach99999]

I have went through all grounds and cleaned them up bare metal and still having same issue I also replaced all bulbs and cleaned up bulb sockets and I also for the hell of it replaced all glass fuses and flasher relay...I even ran a ground from the rear lights to the battery to see if it was a bad ground and still same issue... it has something to do with the brake lights shorting out something...because if I unplug the harness for rear lights all my lights work just fine up front...but as soon as I hook up that harness to the firewall when I hit brakes the parking lights and side marker lights come on and my rear brake lights stay on u cant see a differnece in brightness when u hit brakes in the back.....I am so so so lost I've been changing everything that has been said in like every forum I've read and still nothing is fixed

 

[Zach99999]

Check each and every bulb, make sure they are correct and installed correctly. Make sure each bulb has a good ground. When the bulbs aren’t grounded correctly, all kinds of crazy things can happen.

Does the truck have a trailer light plug? If so, check there also.


The most likely place for parking lamps and taillight circuits to “swap” voltages is one of the taillight bulbs. Since the brake and turn signals are the same bulb (just powered by the same wire on different circuits) the bulbs or sockets back feeding can cause the issue your trying to fix.

So unthink if I changed the sockets it might fix issue maybe the sockets are bad and back feeding up into my wiring harness

 

[Zach99999]

I have just cut off the whole rear light harness in the back so I just have the harness with the 4 wires no sockets attached and still same thing as long as I have that harness plugged into the firewall my front lights come on and my dash light

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[Zach99999]

When I hit the brakes

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[Turbo4whl]

I have just cut off the whole rear light harness in the back so I just have the harness with the 4 wires no sockets attached and still same thing as long as I have that harness plugged into the firewall my front lights come on and my dash light

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If this is the case, then that harness leading to the rear lights is shorted together. Check it all the way back to the rear.