FYI simplest installation DEI 545T for automatic headlights

In my last post, I offered instructions on the simplest way to tap into wires for installing automatic headlights, including which wire controls headlights & parking lights and which wire to tap into for ignition +12V. That post is here: FYI wiring for automatic headlights | PriusChat

In this post, I will show you how in install a DEI 545T to automatically turn on your headlights and parking lights when it gets dark. You will only be making 3 wire connections to the vehicle (power, ground and light control). There are different ways to wire this up. Each way interacts differently with the car's light switch and changes what functions the DEI module performs. At the end, I will discuss why chose this wiring method and the details on how it works. .

PREP THE DEI MODULE

Open the small access cover on the DEI 545T module and cut the red wire. You may have to fish inside to pull the wire out enough to cut. The module is factory set to "daytime running light" mode. That means any time you turn on the ignition, your headlights come on. (This is probably done for you Canadians. But if this is all you want to do, a $5 relay is all you need, not a $30 control module.) Cutting this wire enables the the module to control the headlights depending on daytime / nighttime.



On the larger white connector, cut and cap off all the wires except RED, YELLOW and BLACK. Twist the RED and YELLOW wires together.



On the smaller white connector, cut off the BLUE wire. Separate the 2 relays (lift one up while pulling down on the other) and toss the relay that was connected to that little blue wire. We only need 1 relay.



INSTALL THE MODULE IN THE CAR

Pop open the panel to the lower left of the steering wheel. This is the area you had accessed before when tapping into the headlight control wires on the BCM (linked above). The DEI module fits nicely in this area.


Connect the +12V IGNITION wire from the car to the RED and YELLOW wires on the larger white connector (you had twisted together earlier).



Connect the GROUND wire from the car to the BLACK wire on the larger white connector and the PURPLE wire on the relay. (For the car's ground wire, I used a ring terminal on a bolt inside this area. You can choose any bolt that screws into the steel dash frame.)



Connect the car's HEADLIGHT CONTROL wire to the GREEN wire on the relay.



Install optical sensor. I installed it on top of the dash, left side. You can pry the trim in the picture up and tuck the wire inside. It's also easy to drop the connector end of the wire down into the area you will be installing the DEI module.



Plug all the connectors into the DEI module (larger white, smaller white, on/off switch, sensor). Zip tie the unit and relay to one of the wire harnesses found in that area so nothing rattles around.

A note on the switch: The kit comes with an ugly toggle switch. I used a small rocker switch instead (pictured below). Just be aware that the DEI module's design is such that any switch you use works backwards. That is, in the off position (open) the DEI module is on. In the on (closed) position, the module is off. Just letting you know so you don't install the switch upside-down.




How it works, why I did it this way...

The way I wired this up works as follows: 1) If I turn on the DEI module’s switch is on and the headlight switch is OFF, the DEI module will automatically turn on headlights and parking lights when it gets dark. 2) If I turn ON the car’s parking light or headlight switch, it will turn on the parking lights or headlights regardless of whether the DEI module is on or off and regardless of whether the module thinks it’s night or day.

Some people have wired it up so that the DEI module takes complete control of the lights, meaning it ignores the car’s light switch entirely. What if you’re driving in the daytime and you want to turn on your headlights? In California, there are some stretches of highway where you’re required to turn on the headlights during the day. To manually control your lights with the car's light switch, you’d have to feel around and find the DEI module’s on/off switch to turn it off. But that won’t work while you’re driving neither because the DEI module will not turn off until you have turned off your ignition.

Another note on connecting wires together... I hate butt connectors and T-Taps. They will inevitably come loose. In these pictures, I used solder and shrink tubing in this install because this is my woman's car and I want to impress her with my soldering skills. The only other thing I would ever use are CRIMP CAPS. I think they're almost as good as solder connections. Again, butt connectors and T-Taps suck. Don't use them.