fog lights

fog light rewire to operate with parking lights

I combined the gmurphy method with the Danman method to get the fogs to operate with parking lights. They still are disabled w/ high beams and they can be operated with the factory switch. Here's how: Per gmurphy's method, I tapped into the fog relay located on the engine bay firewall (second relay counting from the passenger side). I actually made a tap for the relay pin, but you could tap the yellow wire from the backside as gmurphy suggests. If you tap the relay pin as I did be sure to tap the ground side of the relay coil otherwise you'll pop the tail light fuse when you activate the fogs. After tapping the fog relay, I ran the 22 gauge wire along the firewall (the bonnet seal gasket makes a nice run) to the drivers side of the engine bay. I used black wire so it is not very visable against the black firewall. I then threaded the wire, with the help of a straightened coathanger, rearward thru the open fenderwell to the drivers front door pillar. The wire was run at the same elevation so I now have it at the door pilar above the upper door hinge. I then entered the passenger compartment across the door pillar. I removed the left most vent grill and then threaded the wire behind the dashboard side panel where it intersects the A pillar to get it behind the dash. This is much easier than it sounds and you may find an alternate method of getting the wire behind the dash. I then ran the wire over to the headlight switch, which is under the steering column, and tapped into the sky blue wire at the plug going into the switch per the Danman method. See the EV switch mod described elsewhere for instructions to access the headlight switch. I basically jumped around the body ECU with this procedure instead of digging around the ECU harness to find the yellow and SB wires there. The wire is only exposed for about 3" as it crosses the driver's door pillar near the upper hinge. I secured it there with a small piece of electrical tape.

The fogs now work with either parking lights or low beams via the factory switch and they are disabled with high beam. Since the fogs are activated from the parking light circuit, they are disabled along with the parking lights, if left on when you exit the vehicle.

There is one small quirk with the rewire as described. If you activate the fogs with low beams on you then cannot turn the fogs off with the fog switch. The fog relay remains locked on until the low beams are switched off. This doesn't happen with the parking lights, only with the low beams. To extinguish the fogs with low beams on, you would have to switch off the fog switch and then switch off the low beams momentarily to unlock the fog relay. This doesn't bother me because I don't use the fogs with the low beams very often.

I suspect that if you cut either the yellow or the SK wire instead of just tapping them, the relay would not lock, but this is for more advanced work. I wanted to make the modification entirely reversible so I used taps.

 

Re: fog light rewire to operate with parking lights

<div class='quotetop'>QUOTE(totavi99\";p=\"70732)</div>

Sounds great. Did you by any chance take photos while routing the wires?

 

Sorry, no pictures. This Prius was a gift to my wife (from me) and she gets real nervous when I poke around any part of it. So I did this mod when she was otherwise occupied.

 

I see your problem, which would be a problem with my idea too. When you activate the fogs, the body ECU sends a ground to the relay. With the bypass, you send that ground right back to the fog switch input. Basically it creates a latching relay. Flashing your high beams should kill the fogs when they get latched.

So, for my idea to work right, it won't be enough just to jump the ECU, but the pin to the fog relay output of the ECU needs to be pulled out.

Or, if you add a diode to the jumper, then the relay signal won't feed back to the fog switch.

 

Danman,

Did you ever determine where the Body ECU is located? I'm planning on making this mod.

Thanks,
-bob

 

Nope, not yet. Not up to tearing the dash apart. Sometime soon....

 

DanMan, you're the man! Or, since this site has a strong contingent of feline lovers: The cat's meow!

I wired the fog lights today as per your instructions in which I jumpered the wires (SAB-pin 13 and Y-pin 17) around the Body ECU and disconnected them from the ECU and the fog lights work as expected!

The OEM fog switch turns them on/off when either the tail lights are on or the low beams are on. They are automatically turned off when the high beams go on but I'm not in need of this. My purpose in doing the mod was to have the ability to turn on the fogs without the low beams on. Also, they turn off when I open the door and exit the car.

As for the location of the Body ECU, it's on the backside of the Driver's Side Junction Box. This is the box underneath the dash by the parking brake. If you lay on your back with your head under the dash and against the parking brake, you'll be looking right at bottom of this junction/fuse box. The requisite wires are plugged into B5 Body ECU, which is at the top of the junction box. Unfortunately, it's on the backside and there's no room to splice and dice.

I took a deep breath and unmounted the junction box and disconnected numerous harnesses so that I could drop the box access the two wires from above the box. If you do this, a good stiff drink is needed either before starting or after, once the work is done and everything is reconnected....

In terms of difficulty for the average Prius owner, I'd rate the mod moderate. What may be the easiest and quickest way to do this is to run the wire from the fog relay as per totavi99's method then, just reach up and cut the SAB and Y wires. You can do the latter without even pulling any of the dash panels off.

Thanks to gmurphy, totavi99, DanMan and all for the collective thought on this one!

 

If you don't want to do any cutting, you might be able to shove a diode between the 2 pins. You'd need a diode so the body ECU fog output doesn't force the fog switch input low causing the fogs to latch on.

If I have my orientation right, the cathode goes to pin 13, and the anode to pin 17.

 

Not that this has anything to do with the topic of rewiring your foglights or anything, but:

The supposed scientific rationale for yellow fog lamps being "better" than white ones is pretty much BS. Except for some *really* rare circumstances involving extremely low-constrast obstacles, you'll get better performance all around from white lights.

a) Re: other cars seeing you: there's just as much yellow light in a white lamp without a yellow filter in front of it as there is in a white light *with* such a filter for the same power output. In fact, there's more light in adjacent even better-penetrating frequencies.

B) it's true that backscatter from white lights is worse, but it turns out that unless the target is very low-constrast the benefit from having the larger amount of direct illumination provided by an unfiltered light on it is considerably greater than the loss of contrast (and even pupil dilation) caused by the backscatter. It might be less comfortable, and you might *feel* less safe, but you're still better off with more light.

If it were possible with standard available car fog light technology to produce yellow light directly rather than filtering a white light, then it could be better, but until you can buy low-pressure sodium fog lamps, you're just sacrificing a ton of light power for little gain.

The only reason to have yellow fog lamps is because they look cool. In fact, that's pretty much the only reason to have fog lamps at all, but that's another story entirely.

 

With regards to writing this up for the knowledge base, I'll gladly do that but let's wait since Danman's suggestion to just use a diode would make this mod very, very easy.

Anyone willing to give this a shot? You don't even have to pull any of the dash apart. You can get to the Body ECU just by laying on your back and reaching up and plugging the diode in. Look for the harness that's circled in yellow on the photo posted above. Should take all of ten minutes.... No tools, either!

I'll play stupid (which I am when it comes to electronics...) Dan, could you give some specifics on what type of diode is needed?

 

Any general purpose 1A (or maybe .5A) rectifier diode will do. You know, the ones with the black cylindrical cases, about the size of a pet food pellet. I am sure there isn't a whole lot of current going through there, the wires are too tiny!

If you put the diode in backwards, you won't do any harm, just the fogs will latch on and can't be turned off until you turn off the taillights.

 

<div class='quotetop'>QUOTE(\"DanMan32\";p=\"79779\")</div>

I can confirm, that your diode solution ( including the orientation ) would work. Altough not quite pretty ( ), I did the mod on Sunday.
http://www.vassfamily.net/ToyotaPrius/FogL...hts/FogMod.html

Thanks,
Attila

 

Attila,

Nice job! Great write up, too!

You may want to add for others that radio station settings (inlcuding XM stations) memory, clock and trip odometer settings are lost when wires are disconnected from the juntion box. Modders may want to write down radio station info for easier programming after the mod.

I have read that there are some owners who use one of the trip odometers for lifetime mileage tally. It would be good to inform them that they would lose this reading.

Lastly, the auto up/down feature on the driver's window also needs to be reset afterwards.

For some who might be queazy about ripping your car apart this much, I think that you still may be able to stick the diode in without having to pull any parts off. A flashlight and a small mirror could help in plugging the diode into the harness.

Great job again, Attila!

 

Thanks Attila for the info. But I wonder how to wire the fog lamps such that they can stay on even when my high beam is on.

I know it is not legal, but this is for an experiment so can any one help me please? Thanks.

 

OK, the reason the fogs go off on high beam is because the low beam switch cuts off fog switch ground feed. If you ground out the dark blue wire going from the low beam switch (pin 16) out of the combination switch, or the orange wire to the ground source side of the fog switch (pin 3), then the fogs won't turn off on high beam, but will still auto off.
By the way, the 2 wires go to the instrument panel J/B, connector 4B. Dark blue wire goes to pin 5, orange wire goes to pin 15.

 

Thanks so much Dan! Will try it out later.

 

<div class='quotetop'>QUOTE(\"bobc\";p=\"82396\")</div>

Yeah, that was my first option as well. I must have been frustrated by the size of my hands ( used all available tools as well ) and little success to stick the diode in, thus decided to do it the "hard" way.
FYI, even after holding the B5 connector in my hands, I tried jamming the diode in, but the connection was very unreliable and unstable. Again, if you file the end of the diode wires down to a hair pin size, you have a better chance ( tried ), but when I turned the connector over and moved it a bit, it still fell out. I hope somebody can work out a technique to do it in a less intrusive way.

I think we may just need to trace the wires to a more accessible location and do the mod there...

Thanks for the power-outage related warning, I'll add that to my site!

Attila

 

The fog switch routes from the body ECU, through the intrument J/B, to the combination switch connector.