I have added a relay in my wiring harness to give power to my front marker lights so they turn on when I start the motor. What I recently realized is this is also turning on the tail lights at the same time and I would like to change that.

My initial thought was to find the wire for my tail lights and trace it to the headlight switch (because that's where I think the power is coming in from the marker lights to the tail lights). I found one wire to the tail light and traced it to the headlight switch, I then removed it from the headlight switch and turned on the key. The tail light came on even though I thought it had no power. Obviously I am not getting something correct. So I stopped and put things back together for now.

Anyone have suggestions on what I should do here? Do I need to remove more than one wire from the headlight switch?

Also, what's the best way of wiring this up so my tail lights only go on when my headlight switch is on? What wires should I use to do this?

 

I see 3 options here:

1. Completely disconnect the front marker from the park light circuit. It will be on any time the truck is on "run" anyway. Only problem is if you want to use front marker in an actual "parking light" scenario, i.e. ignition off.

2. I think a diode can work for this somehow, but I'm not an electrical engineer (yet!) so I'm not sure of the details on this.

3.


You already have Relay 2, so Relay 1 would be the only addition to your existing setup.

 

All of your running lights are tied together. Usually ran in a circuit to b the fuse box then a wire up to the head light switch.
The easiest way to do what you want to do is pull your marker lights out cut the power wire from the plug. Leave enough room so if later you can splice them back together. Take a wire run from one light to the other. Find a keyed hot wire put a fuse able link in line and run it to the wire you ran from one marker to the other. I'm not an electrical professional by all means but I have had wired up any lights and changed configurations

 

If you want the marker lights to come on with either the tail lights or the relay you can put a diode in the original power line to the marker lights. Cut the wire between the headlight switch and the marker lights (but make sure it's where the wire is only feeding the marker lights) and solder in a diode. The end of the diode marked with a bar goes toward the lights, the opposite end is toward the switch.

This will keep the marker lights from back-feeding the system when they are getting power from the relay. They will still back-feed as far as the diode, which is why you want to put it closer to the marker lights. If the "T" to the tail lights is between the diode and the marker lights you'll still be back-feeding them.

One thing to keep in mind is that there's a 0.7 volt drop across a diode, so doing this will mean that your marker lights get 0.7V less when they are only running on the headlight switch. Not a big deal in your case because they will still get full voltage any time the engine is running. And maybe not a big deal anyway if you aren't worried about your marker lights only being about 95% as bright as they were before. But just something to keep in mind.

 

My issue is not so much how to wire it but where the wires go from the tail light into the cab (subford???). I need to backtrack a little and say the tail light wire doesn't go to the headlight switch. With the switch out (disconnected) and after I turn the key on, the tail lights still go on, but the marker lights are off.

So far I have traced the tail light wire in the harness along the frame rail to just below the brake booster. It goes into a connector that has about 20 wires through the firewall. I see where it comes in the cab and goes up and behind the instrument cluster, but I lose it there. I was thinking it went over to the RABS computer and I looked there but didn't see it (at the tail light I found a brown wire on the passenger side that has power, so I was looking for a brown wire in the cab).

I am not sure if that wire is tied into the other tail light wire, my truck is in the garage and I can't get to the driver's side now to test that wire.

I could use the wire at the point it goes into the cab but I'd like to understand where it ends up - it might be better to tap into it there.

 

It's gets even more confusing...I removed the 20 pin connector I mentioned (is actually 24 pin) and I was trying to identify the tail light wire there by testing for continuity between the tail light wire and the 24 pin connector. I found No less than 4 wires with continuity.

Maybe tomorrow I'll go to the junk yard and pull that harness and take it apart.

 

I'm not sure you even need to be dealing with the rear tail lights. Consider this updated diagram:


Note how the front markers and the rear tails are originally part of the same circuit. (The dashed line would complete this factory condition.) This means that you could make the "break" and insert the relay circuit anywhere from bumper to bumper. Therefore, I think you can do everything necessary under the hood, right behind the headlight assembly.

 

Yes, you're right, I could do that. But as I said above, I'd like to understand where these wires go before I go mucking it up. Just makes me feel better knowing if I am going to affect anything else.

 

I found the reason why I was getting continuity between so many wires was due to the tail/brake light bulbs (stock 3157). After I removed those from the harness, I was able to find the one wire as I would think it would work. So I put a relay on it and it seems to work fine.