Headlight common.....help

[rmcnamee]

Hello all,

I'm installing my Fisher MM2 snow plow on my 2004 3500 and I have a question about headlight commons. The plow headlight installation instructions ask what type of "common" the truck (generic question) is equipped with. Are these trucks using a "Positive" or "Negative" headlight common?



Thanks

Ryan

 

[rbattelle]

Should be negative. All US-made vehicles use a negative common, as far as I know.

 

[rmcnamee]

Thanks for the reply.



Ryan

 

[tbrudder]

Originally posted by rbattelle

Should be negative. All US-made vehicles use a negative common, as far as I know.

Actually, I'm not so sure. I hesitate to comment here, as such is the case, but you might end up frying something, so I'll stick my neck out a bit.



If this negative-common refers to hot-side switching, wherein one of the three terminals on the headlamp goes to ground, and the +12V is switched between the low- and high-beam terminals, then yes, that was common for all US vehicles.



The opposite is ground-side switching (positive-common, I suppose)--one terminal is constant +12V, and the ground is switched between the other two terminals. This is more common in Japanese vehicles--my old 4Runner was such a system.



I recall a recent conversation, however, with a fellow emergency-services member. He'd burned up his headlight switch by installing a wig-wag unit on his Honda, which had ground-side switching. Another conversation and subsequent investigation led me to believe that many domestic auto makers, including Chrysler, have switched to this scheme.



Assuming we're talking about the same thing, you may want to verify this. Should be able to use a multimeter. Probe the connector to the headlamp, one terminal at a time. Turn the headlights on low beam, measure all of them; then turn on high beam, measure all again. One terminal should stay the same--either +12V or ground (negative), while the other two switch. If one stays ground, it's hot-side switching, which I assume to be your 'negative common'. If it stays at +12V, it's the opposite.



Be careful, or you could melt something.



--Ty

 

[CIverson]

All I know is that the headlight uses the opposite of what the 2nd gen's did. I helped my buddy install my snoway plow and the kit was more expensive because of the new setup.

 

[rbattelle]

tbrudder-



Wise comments.

 

[lmills]

Mid ways through the 2nd generation dodge switched to a common positive. I believe they are the same way on the 3rd gen's also. I had the same issue when i hooked up the lights for my plow. There is always one hot, but the negative is what does the jumping from low to high beam. I hope this helps.

 

[TImler]

They are negative. I'm 100% sure on that. HOWEVER, you need to be extremely careful with this system. Unlike the 2nd gen systems and most headlight systems on american made vehicles, these trucks dont use the typical headlight switch to a relay to activate the headlights. The entire system in run through a computer (don't you just love technology) that activates the lights. For those of you in the emergency service sector you may have figured this out already by having to install an "isolated" headlight flasher. If you don't, you can kiss your main computer system on your truck goodbye and take your kid's christmas presents back cuz it's going to cost ya. In case you don't believe me, do this simple test. Turn on your turn signal and watch the flash rate reflecting off of a building or another vehicle. You will see that the flasher that you hear in the dash is for your own amusement. The actual flash rate of the bulbs on the exterior of the truck are completely different because they are regulated through the above mentioned computer. My suggestion is to see if your plow manufacturer has a "plug and play" wiring package available, I know many of them do. This way you can run the power directly into the plows light control system which "isolates" the plow light system and won't fry yours. Hope this helps.

 

[tbrudder]

Originally posted by TImler

They are negative. I'm 100% sure on that. HOWEVER, you need to be extremely careful with this system. Unlike the 2nd gen systems and most headlight systems on american made vehicles, these trucks dont use the typical headlight switch to a relay to activate the headlights. The entire system in run through a computer (don't you just love technology) that activates the lights.

Hmm. That sounds a bit odd to me. Granted, _everything_ is run by the computer now, but the lights? I'd be interested to hear more about this, as I'm having trouble figuring out how you could kill the ECM by adding a second path to hot/ground. I guess I could understand if the ECM closed some relays going to the lights/turn signals (not sure why they'd want to do it that way, but who knows), though the relays themselves would isolate the computer from voltage transients.



But to do damage, I'd think there would need to be a direct connection, and I just can't see how they'd do that. I'll hazard a guess that the ECM runs at TTL levels--5V, as I recall. The signals are generally very low amperage, due to the small wire sizes. Any signal line that's supposed to control something (say, the grid heaters) runs through a relay because of this--I'd say generally less than a half-amp capacity.



Headlights are, what, 55W? That's about nine amps--I just can't see that coming straight from the ECM. And imagine if you had the high-beams on, all the parking lights, and the hazards--that's, what, 25 amps or so? That is a lot of current for the

ECM to source (or more likely sink--everyone seems to be going to ground-controlled stuff these days).



I just don't see how it could be set up this way. With relays, yeah I guess it could be done. But then you're isolating the ECM anyway, so it's a non-issue.



Then again, I'm by no means an expert in automotive electronics, and I assume you're talking about the 3rd gen, of which I have no experience anyway.



Now I'm interested.

Could you expound a bit?



--Ty

 

[JPM]

Ty, It true, there is no fuse for the headlights only a "computer' to control and monitor the circuits.

 

[tbrudder]

Originally posted by JPM

Ty, It true, there is no fuse for the headlights only a "computer' to control and monitor the circuits.

Hmm, interesting...

Now I really wonder how this is implemented. Seems like if you had, say, a headlight short to ground, you'd blow your whole engine computer.



I don't see how they could get by without fuses somewhere, and relays to switch the heavy loads. Crazy stuff.



--Ty

 

[Prairie Dog]

There are never fuses for headlights - just breakers.