# How the *&$^%# do my lights hook work?



## birddseedd (Jul 12, 2010)

04 dodge ram 1500

3 wires. one would assume high beam, low beam and ground. in fact one is even black


*with the plug to the bulb detached... light switch on high beam off.
wire1 13v to black ground wire
wire2 13v to black ground wire

*with the plug to the bulb detached... Light switch on high beam ON.
wire1 13v to black ground wire
wire2 13v to black ground wire

??? why do both of these wires have voltage weather high beam is on or off? is the high beam switch a resistor, only allowing certain voltage across the ground wire, if that's the case wouldn't the voltage being detected across the wires change accordingly?

My first goal is to hook up my plow lights, it would be nice if i can use the high beam vs lo beam (plow lights have 4 wires so i assume can use high and low). it would be real nice if i could use a new set of lights and hookup the turn signals, but thats a different circuit and question.

for now, what is distinguishing high vs low if there is 13 v going to the wires all of the time?

oh, and i tried bridging each wire from the connector to the bulb connector with the volt meter and got no voltage no matter the high beam position ???


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## basher (Nov 13, 2004)

you are low side switched. Are you trying to hard wire your plow lights?


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## birddseedd (Jul 12, 2010)

yes, hardwired


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## basher (Nov 13, 2004)

Don't do it. It could cost you more to fix the truck then the proper parts. If you insist on doing it you are working with a system that is controlled on the ground side. It's the same as high side switched systems if you understand DC power just remember the test light works backward.


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## birddseedd (Jul 12, 2010)

what i dont understand is how it is able to control 2 circuts within the bulb (has 2 filaments) with 1 ground wire.

i can see adding resistance and letting it pull more or less voltage based on the resistance of the switch, but how can it control weather 1 or both filaments are on?


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## birddseedd (Jul 12, 2010)

there are 3 wires hooked up to the truck light, and 3 wires hooked up to the plow light, can i just tap into those 3 wires hooking ground to the trucks light ground and the other 2 all in parallel?


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## birddseedd (Jul 12, 2010)

well. i hooked everything in parallel. iv no idea if it works or not.

a couple times the lights didn't come on until i either turned them off and on a couple times or turned on the high beam.

could be a wiring problem. works for the most part. could a been my dead battery also.

shrugs, ill find out soon enough.


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## PeterD (Jan 13, 2010)

You do want to be very careful... 

1. The lights are controlled by the vehicle computer (PCM/BCM) not the switch. The switch tells the computer to turn on the light, which the computer then does. If you increase the load on the PCM/BCM you risk damage to it. I don't recommend parallel lamp operation.

2. The PCM/ECM sends a small voltage to the lamp filament as a test for a burned out bulb. You need to test for voltages with the lamps connected not disconnected.


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## birddseedd (Jul 12, 2010)

PeterD;1340230 said:


> You do want to be very careful...
> 
> 1. The lights are controlled by the vehicle computer (PCM/BCM) not the switch. The switch tells the computer to turn on the light, which the computer then does. If you increase the load on the PCM/BCM you risk damage to it. I don't recommend parallel lamp operation.
> 
> 2. The PCM/ECM sends a small voltage to the lamp filament as a test for a burned out bulb. You need to test for voltages with the lamps connected not disconnected.


You would be correct, neighbor let me know their both live, with just no amprige untill low or high is chosen. when i had them hooked up in parrellel, which was a gamble, it did mess with the pcm a little bit, they didnt want to come on the first time switching it on, but after setting high or low beam, or changing to one rather, it started working well.

i am just hooking up some relays from the high and low beam wires and pulling juce straight from the battery.


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## PeterD (Jan 13, 2010)

Relays would work best. I'd (personally) suggest switching the relays before turning on the light switch to avoid confusing the computer too... Certainly if I didn't have the Night Saber controllers already, I'd be using relays.


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## birddseedd (Jul 12, 2010)

PeterD;1340748 said:


> Relays would work best. I'd (personally) suggest switching the relays before turning on the light switch to avoid confusing the computer too... Certainly if I didn't have the Night Saber controllers already, I'd be using relays.


the only way to do that really would be to use a different switch but I wanted to control the lights with the normal headlight switch rather than a seperate one. the relays take such a small amperage that it cannot affect the PCM.


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## PeterD (Jan 13, 2010)

Just wire a switch to switch the relay between plow and normal lights. I could do a diagram if you wanted, but the PCM will feed either without any problems, just not both.


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## basher (Nov 13, 2004)

birddseedd;1340812 said:


> the only way to do that really would be to use a different switch but I wanted to control the lights with the normal headlight switch rather than a seperate one. *the relays take such a small amperage that it cannot affect the PCM*.


If you know than why ask than argue with the advice you are given?

Am I to understand you want to run both lights at the same time??????


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## birddseedd (Jul 12, 2010)

well, iv decided to leave both sets of lightw running. i only have spst relays, to switch between the lights i would need dpdt relays and atm its not worth spending money on all new relays. it would be nice to know if a plow light goes out, but when the plow is down the truck headlights shine right in front of the blade wheras the plow lights shine about 10 feet forward for driving. ill just have to be sure to get non reflective paint when i paint the backside of the plow.

thanks for the help everyone.


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## birddseedd (Jul 12, 2010)

i do have another question maybe you can answer peter. in a normall more modern setup you have an electronic control box in the cab correct? is this the same unit as the controls you use to move the plow? do the plow lights take their power from trhis box? if so that would mean that the truck controls do not ever control the plow lights on modern setups?

not that i will have a brand new plow anytime soon, but would be nice to know for when i do nees to install a new one, inwhich time im sure ill have many more questions.


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## PeterD (Jan 13, 2010)

The NightSaber lights use two (identical) modules (one for right, one for left) that are located behind the grill. There is a small two position toggle switch on the dash to switch between main and plow lights. The two modules are somewhat like a relay but have a bit more in them to make them safer for the vehicle's electronics. 

The system is totally separate from the plow electronics. Meyer sells a custom harness for each truck model so it is all plug-and-play so to speak. 

My old truck, (we're talking a while back) I just used relays as I'm suggesting for this application. I used two relays (one right, one left) I scavenged from some old electronic gear. Regular automotive relays only have one contact so they don't work well, but IIRC Radio Shack should have something that will work. If not any of the online supply houses would have what you need: 'DPDT 12 V DC coil relay' Then use one set of contact for low beam and one for high. Avoid the urge to switch ground instead of the hot leads too--you will get some very strange results if you do that!


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