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(Solved!) how does drl signal from body ecu work?

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Old 04-01-2014, 12:04 AM   #1
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(Solved!) how does drl signal from body ecu work?

I've been analyzing the drl circuit on 2nd gen and read pretty much everything there is in the forums.

we know the body ecu drl pin is connected to the 12pin flasher relay and that controls the drl.

mods abound about cutting that line and adding switch to enable/disable the drl function. What I have not found is an explanation on how the drl works. The mods are simply trying out a theory and it works, without really understanding why.

The only post I found that questions how drl works is this one

http://www.tacomaworld.com/forum/lig...ml#post2463946

but nobody really gave any good answer.

same as that post, I checked the drl line with a voltmeter, and it has no signal at all, regardless if parking brake is on or off, headlight is on or off. So I have the same question as that post, if there is no signal, how does the relay know when to turn drl on or off???

I verified it is not a ground circuit. connecting one side of relay coil to +12 and the other end to the drl line does nothing.

The only possible explanation left that I can think of is that line puts out a pulse to signal the relay.

my objective for trying to find out how the drl line works is so I can use it to add drl without having to use the 12 pin flasher relay.

I know there are several drl mods out there that does not use the 12pin flasher relay, but I want one that works like the oem drl, that is, drl is off if parking brake is on or headlights are on or turn signals are on, or hazards are on.

perhaps someone with service manual or alldata can find out if there is any info about the drl. any useful info regarding how drl line works is appreciated.

thanks
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Old 04-01-2014, 12:20 PM   #2
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I checked the drl line using an oscilloscope and did not really find anything conclusive.

This is with engine running and headlights off


this is with headlights on

although it looks like it is generating some sort of square wave, I think this is just noise. The level is only around .5v.

If there is anyone local in SF bay area (or non local) can lend me a 12v flasher relay, I can probably reverse engineer this over a weekend.

thanks.
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Old 04-07-2014, 06:34 PM   #3
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Ok, I got it figured out.
The reason I was not getting anything is because the DRL line is only active when engine is running.

It is grounded (about 25 ohms) when headlight+parking brake conditions are satisfied, and open circuit when not. e.g. if I start the engine with parking brake on, it is open circuit, if I release parking brake, it goes to ground, if I turn on headlights, it is open circuit. Turn signal and hazard does not affect it.

Now that I know how that line works, I can build my own drl relay circuit to work similar to the $62 drl relay, except this will cost under $10 using a 4 channel relay board like this
http://www.ebay.com/itm/261256181656

The circuit will be something like this



The relay board will be in driver side kick panel area near the fuse panel, since all the connections go to the fuse panel connectors. The L and R dim filament wires go through the firewall to the corresponding turn signal bulbs dim filament.

With engine running+headlights off+parking brake released, the DRL line goes to ground, it activates relay U1 and power goes through relay U2 (normally closed), then to left and right side bulb relays. If hazard is turned on (which goes from open circuit to ground), it disconnects the power and the DRL bulbs go off and both bulbs will blink on bright filament. If either the left or right turn signal is on, that side DRL will go off and the turn signal bulb will blink, while the other side DRL remains on.

This is the only DRL mod that works exactly the same way as the OEM DRL, except the dim filament is used, this way, there is no risk of melting the lenses. This circuit also works with switchback LED bulbs. See my post on converting regular 3157 LED bulbs to work with tacoma.

http://www.tacomaworld.com/forum/tec...bs-3157ck.html

if you do use this with LED bulbs, you just need to replace the 8 pin flasher with one that works with LED. No need for load resistors.

If your tacoma currently has 12 pin flasher relay, all you need to do to convert to 8 pin version is to get the 8 pin LED flasher and short two pairs of pins on the connectors where the 12 pin relay harness connects to. The 8 pin LED flasher costs under $10 on ebay. There is no 12 pin flasher relay that works with LED, so use this mod if you want to do DRL with LED bulbs right.
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Old 04-07-2014, 06:49 PM   #4
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incidentally, the reason the DRL stealth mode mod works by adding a switch to the DRL line is because you are opening the circuit on the DRL line, which is equivalent to turning on your headlights. You can add a switch to the +12v line coming from IGN fuse (as shown in the diagram above) to disable DRL.

The stealth mode DRL mod is needed since if you are idling in Park, the DRL remains on even if parking brake is re-engaged. So same here, with the switch, you can manually turn off DRL.
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Old 04-08-2014, 10:03 AM   #5
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I was just thinking, if I use an 8 channel relay board instead, I can include the alternating signal mod as well to this mod. The relay board costs around $12 on ebay.

as I mentioned in this post
http://www.tacomaworld.com/forum/tec...ml#post8393006

the mod using relay
http://www.tacomaworld.com/forum/tec...html#post46317
is more properly designed (from technical and engineering standpoint) than the one without. Easier does not always mean it is better or proper.

With 2 relays, I can wire exactly the same as the mod with relays. But that will work only if the parking lights are on. Since I will have 4 more relays with an 8 channel board, I can actually wire the parking lights to alternately blink with the signal light even if the parking lights are off, taking the mod one step further to the next level.

Just the same, everything can be wired from inside driver's kick panel, as the parking light and turn signal wires are all available on the fuse panel.
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Old 04-08-2014, 09:42 PM   #6
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circuit using 8 channel relay board to control DRL and add alternating turn signal.
The alternating turn signal feature works even with parking lights off and engine is not running. I changed the power source to the turn-hazard line, which uses a 15amp fuse.

The circuit is self explanatory.
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Old 04-12-2014, 09:42 AM   #7
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I just completed a foglight anytime mod and will just be using that as my DRL.

http://www.tacomaworld.com/forum/tec...ytime-mod.html

I won't be doing this DRL mod anymore for the following reasons:

It will be redundant.

The alternating parking lights mod won't work since I just realized the Turn signal goes from +12v to open circuit and not ground, hence it will not activate the relay coils.

I tested wiring power to the DIM filament, and IMHO, it is not bright enough to be seen in broad daylight, which defeats the purpose of DRL.

I can see now why the toyota TSB to fix the melting lens issue is to replace the housing with a different design instead of using the Dim filament. I don't think even the newer models use the Dim filament alone even though all three wires are connected.

Using DIM filament of incandescent bulb is a no go. Using it on an LED switchback may is feasible though, since it is as bright as the amber light.
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Old 06-28-2014, 02:43 PM   #8
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Hey guys. I have an issue with my DRL's which I cant resolve. I live in Canada and DRL are mandatory. When I purchased my 2010 taco from a dealer in Canada that imports from the US they had the DRL's installed for the fed/provincial inspection. I never thought anything of it till I noticed it was my headlight that are on full time, and not the amber lights. Since switching to PIA headlamps I have had one burn out every year from what I can only assume is high heat and long hours. I looked at the thread regarding the relay install and tore my dash apart. It is safe to say that there has been no relay installed. I would like to know how they wired the headlights on full time and and how to reverse it. THen I can do the amber light DRL mod to keep things legal. Anyone able to help? I have a few pics of the wiring behind the fuse box if necessary
Thanks
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Old 06-29-2014, 01:05 AM   #9
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I don't know if using fog lights is allowed as drl in canada. if it is, my suggestion is to just to do fog lights anytime mode instead and use that as drl. I have been using my fogs (with hid) as drl..
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Old 06-29-2014, 10:48 AM   #10
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Quote:
Originally Posted by doughboy View Post
I don't know if using fog lights is allowed as drl in canada. if it is, my suggestion is to just to do fog lights anytime mode instead and use that as drl. I have been using my fogs (with hid) as drl..
doughboy. My headlights are already wired as DRL. I don't want this because it is burning out my PIA lamps every year since they run whenever by key is turned to on. THe problem is I don't know how to reverse the wiring. The marker light mod hasnt been done as you can see in the first image. The only thing that looked out of place (which is only a guess) is the white box with green wires coming out. THat is the second image. I can always turn on my fog lights full time if I can reverse the headlight wiring for DRL. ANy help is appreciated
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Old 06-29-2014, 11:18 PM   #11
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without knowing how the headlights were wired to always come on, it will be hard to figure how to reverse it. but I would look at the headlights relay in the engine compartment and see if it is rewired to always come on.
unmodified fog light circuit will turn on fog lights only if low beam is on. since your headlights are always on, that gives you the impression you can always turn on your fog lights. Once you restore the headlight to normal, then you can consider your options for drl.
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Old 07-19-2014, 09:34 AM   #12
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Quote:
Originally Posted by doughboy View Post
without knowing how the headlights were wired to always come on, it will be hard to figure how to reverse it. but I would look at the headlights relay in the engine compartment and see if it is rewired to always come on.
unmodified fog light circuit will turn on fog lights only if low beam is on. since your headlights are always on, that gives you the impression you can always turn on your fog lights. Once you restore the headlight to normal, then you can consider your options for drl.
I can definitely turn on fogs on with the headlights on and they do turn off when set to high beam. I am going to figure this damn issue out on monday and get it back to stock lighting. Where in the engine compartment is the headlight relay? I sure wish I had a tacoma I could look at that hasn't been modded to compare to! As soon as I have it back to stock I would like to wire the lights that came on my ARB bumper as the DTRL. It has three wires for each light. WHere would you suggest I tap into once I have the DTRL headlight issue resolved? Sorry if this is a bit much. I figure if I can get all the info possible I should be able to tackle it in one go. Thanks for the help so far!
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Old 07-19-2014, 01:31 PM   #13
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The headlight relay is in engine compartment fuse/relay box. When you open the box, the inside of the cover should have a diagram where the headlight relay is located. If it is a 2010, looking at it while standing on the side of your truck, it will the the top rightmost relay, to the left of the series of fuses. Pin 1 is the side of the relay closest to you.
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Old 07-21-2014, 10:10 AM   #14
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Quote:
Originally Posted by doughboy View Post
The headlight relay is in engine compartment fuse/relay box. When you open the box, the inside of the cover should have a diagram where the headlight relay is located. If it is a 2010, looking at it while standing on the side of your truck, it will the the top rightmost relay, to the left of the series of fuses. Pin 1 is the side of the relay closest to you.
Thanks for the help. I did find the mod and it was done right under the steering wheel. It was spliced in by the light switch only visible once I took the column cover off. Really happy to have this resolved
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Old 07-21-2014, 11:14 AM   #15
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ok, so basically their hardwired the headlights at the switch to "ON". I think that was a clever mod. well clever in terms of simplicity, but not functionally, as it caused your bulb burnout problem, but they did not know you were going to use a different kind of bulb.

that mod would cause the headlight relay pin1 to be permanently at GND regardless of headlight switch position.
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