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97cummins

Led third brake light

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I bought a new LED third brake light.

When I plug it in, the cargo light comes on and stays on.

I can't find a decent wiring diagram .

I figured out the cargo light and brake light wires.

But I also have two wires acting like grounds?????

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I did some research and come to find out that ram trucks have had that very issue it comes to Installing led third brake lights.

So I dought my head light switch is bad. This only happens when I plug In  the led light.

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Like I said before the cargo lights get there ground from the head light switch. Wired as a constant hot. So if there is a short in the ground lead to ground it will remain on.

 

It might be the bulbs are in backwards for the brake lights. Reverse them and see if they work.

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Dodge uses a stinking 'bulb check circuit' also........  every time I've hooked up LED's in any, the LED is either on about full bright, or dimly so you can see it at night.  In my '92 I have the check engine illuminated enough that you can see it when it's dark.....

Check your wiring to make sure you are wiring up your new 3rd brake light correctly (LED's get wired that same as an incandescent bulb.

 

BUT

 

They do have polarity to them!  You have to wire in the LED with it's positive terminal to power and it's ground to ground.  If you reverse this; it simply won't light.  So for a 194 replacement, simply pull it and reverse it's polarity!)

 

If you wind up with the LED being lit when it's supposed to be off; the simplest fix is to just wire in an incandescent bulb parallel with it.  I had to do this with my turn signals!  But that also caused the OEM flasher to work again (the LED's didn't provide enough of a load to cause it to flash....)

 

If you need specifics on any of this, just shoot me a text / call!!  I'll be glad to help!

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Does it have loose wires or a plug / socket?  If it is a plug-in you will have to work harder if you need to change how it is wired....

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No loose connections.

I ended up cutting the powered wire so the cargo lights are off. Ill end up wiring up a seperate switch for the cargo lights.

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I hear you.  Sure seems odd when you encounter this.  From the way you described the problem; your new cargo light is LED also?  I hooked into the cargo light power (Disconnected the OEM light, and ran wires out of the cab and into my shell) on Meg ('06 Ram) and I remember that every time I checked there was 12V power to the cargo light.  I just assumed it was part of a bulb check circuit because it's never enough to light an incandescent bulb and it doesn't seem to drain the battery.  In Meg, instead of using a diode, I put two bulbs in the cargo light housing and another switch.  I can now turn on the cargo light inside the shell either from the dash or from inside the shell.

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My new cargo light is led. I worked on it for  couple hours after work. I even hooked up a new head light switch to see if it was the switch. Nope. Even completely unhooked the cargo lights were still on. I rechecked the fuses, they were good. What I found odd on the cargo schematic was the wire colors for the cargo light plug matched the head light switch colors. But the head light switch colors didn't match the cargo light colors.

I also couldn't find the two fuses the schematic showed that were constant powered.

If I new that I was going to run into this issue with the new led light. I would have just bought new led bulbs for the stock housing. Oh well for now.

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Again cargo light is constant hot +12V and the ground is switched on and off for control. As for the brake lights they have constant ground but the brake signal comes from the brake light switch.

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I wonder if Dodge uses solid state relays to switch these on and off.........  All that is happening here is there is enough current available to light an LED (But an incandescent won't light).

Do this:  Plug it back in and leave it.  Come back in 30 minutes and again in an hour and see if the cargo light remains on.......?

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On 9/23/2017 at 10:09 PM, 97cummins said:

I also couldn't find the two fuses the schematic showed that were constant powered.

 

Listed right on the wiring diagram where the fuses are either in the power distribution box under the hood or the driver side cab fuse box. Clearly marked on the diagram and even what fuse number you should be able to pull the fuse out and check to see if the lights go out. I still think you got a rouge ground lead and the wire is rubbed through to ground.

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I'm betting it is the bulb check circuit that is putting power (or ground) through the circuit to test the incandescent filament.  I've had this same problem on multiple Dodge pickups.  If it were a short to ground, it would be lighting an incandescent bulb as well as killing the batteries if it sat for a while.......

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Hey Millco... It 1997 technology... There isn't any bulb check electronics just +12V power on one side and switched ground on the other. Nothing more. 

 

Review the diagram above...

eview 

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I believe you!  Maybe I was remembering the check circuitry from my '06......?

I have had this same problem happen with my '92 though.  My dash lights glow dimly for the check engine and another indicator.  To solve this problem with my LED turn signals, I paralleled in incandescent bulbs in with the front LED's and these problems went away (At least on the turn signals.  Dash still lights up........  But that's a different circuit!)

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I already tried the leave the cago lights on and walk away for a while.

No luck, they was still on.

As for the fuses, the fuse numbers in the schematic didnt match up with the fuses in the pd box or side of dash in cab.

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Don't you love that......  I hate it when the color of the wires or fuse is different.  I was just working on my daughters '03 Sentra and the Nissan factory shop manual had the relays reversed for the electric trunk release and the electric windows.  I don't think swapping the location of the relays can be called dyslexia!

I wish I knew what was going on here.  I thought most of my problems with LED's staying on in my '06 was the bulb check circuitry.  I can't say that is what's causing it in my '92.  But then, I have never gotten a bulb out indication in the '06 for any of the bulbs that have burned out over the years.  So, what good is it really to have a bulb out monitor.......... 

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