Pulse Width Modulation and Jeep Headlights

Sit back, get a coffee, this is gonna be a heavy read. 

After installing my relay wiring harness (here), I ran into a problem with my daytime running lights (DRLs). I noticed it when I drove to my train station in the morning and parked. When I turned off my headlights, there was a loud buzzing from under the hood, right where my new relays were. 

After a quick check, I discovered that it only happened when my DRLs were on. For those not in Canada, DRLs are required (since 1990) on on vehicles. Basically it's headlights or marker lights on while driving and it's automated. To avoid the extra cost of having another set of lights, auto makers were allowed to use the high beams, at a dim setting, as DRLs. 

The method used on the Jeep TJ is called "Pulse Width Modulation" (PWM). In it's simplest form, it basically means that a module turns the high beams on and off really quickly. With a standard headlight, this on off quick action has the effect of not letting the filament reach full brightness, essentially running them at 6v, or half brightness. (I'm really oversimplifying here). When the "On width" is the same as the "Off width", you have a 50% cycle, and 50% voltage (for all intents and purposes). 

Normally, this is not noticeable, and to the naked eye, is just looks like a dim light. 

Since I added a relay to provide direct battery power to my head lights. When the switch is on, no problem, however, when the DRL module is controlling the high beams for that 50% brightness, the high beam relay is being flipped on and off at a rapid rate, causing the buzzing sound.

https://goo.gl/o6fJx8
Photo via www.justanswer.com

The easy solution, disable the DRL module. On a TJ, it's a pager sized box on the drivers side of the firewall. Pull the connector out and that's that. Only downside is that the high beam indicator no longer works. Frankly, if you don't know you have your high beams on, please stop driving. 

Upgrading the headlights (sorta) on the TJ

This isn't really upgrading the lights, it's about upgrading the wiring to the headlights. Stock Jeep (CJ/YJ/TJ apparently JK) have piss poor headlights. I was recently out with my wife on a stretch of road with no streetlights, it was as if I had no headlights. Cars passing me had these beautiful bright lights and I probably could've used a lighter and a mirror
and had better output than my TJ lights.

Enter Relays. Relays a triggered by a switch, but carry far more power through the relay than a switch can handle. It's safer for your fuses and switches, and relays should be used whenever high-draw items are used.
If you check out the diagram to the left, you'll see a basic relay. In this case when the switch is closed on the input to 86, it triggers a spring loaded internal switch, which is capable of much higher amperage, to close between 30 and 87.

After reading around I discovered that Jeep routed power through the switch in the cab, and than to the light. All that extra wire causes a voltage drop, which means less power to the lights themselves. By using a relay, you can have the power go directly from the battery to the lights, and use the switch in the cab to activate the relays. Better for the switch, and better for night driving.

Octane Lighting Harness

For a really good write up, check out Jeepfan.com. While I'm handy with a soldering iron, and not afraid of hacking up my wiring, I found a simple solution. That solution is the Headlight Relay Wiring Harness H4/9003 by Octane Lighting on Amazon.ca. For just over $30 Canadian, I was able to easily add relays, and I didn't have to hack up my harness in the process.

It was a nice February morning, a whopping 3C out (37F). Throwing my hoodie and coveralls on, I tackled the job.

Really easy install. About 30 minutes start to finished. Run the harness, attach to the existing grounds by the headlights, run power and zip tie it all up.

I grabbed a video of me doing the install to try something new. 10 out of 10 would recommend the harness for ease of install. Not too long, not too short, everything was soldered and had heat shrink.

Amazon.com link for the same harness

The video is embedded below.