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"EUDM" fog lights ON when headlights OFF

1 June 2005
Dear All,

As I have an EUDM, I have the switches for fog lights in the center console. I want to be able to turn the front fog lights on while the headlights, the rear lights and the instrument / gauge cluster lights are off.

If I come with 12 v accessories to the fog light switch in the console, will I be able to turn on the foglights with headlights off (if the ignition switch is on the "on" position) ??? Or is there a relay somewhere downstream ?

I also do not want that the position lights, rear lights and the instrument / gauge cluster lights turn on when I switch the fog lights on.

Do somebody have a wire diagram of the EUDM fog lights ? There is nothing in the pdf of the Acura "1991servman" as it is USDM.

Thanks a lot for your help.

If I remember right, my JDM fog light switch only turns on the front fog lights and the front marker/parking lights. The headlights definitely don't have to be on, I don't think the gauge cluster lights up unless the turn signal stalk is switched to the intermediate position, and I think the taillights also stay off but I'd have to re-check those last two.

I'd have to take a pic of my physical electronics manual which I can do this weekend. I think there's a page for fog lights though it may assume Canada-spec.

Do you not have functional fog lights currently and you're wanting to add them with the functionality you described, or does your current fog light switch also activate the headlights/etc.? If the latter is true then the EUDM wiring schema might also force the headlights/etc. on which isn't true for JDM, since the fog switches are the same it would be instead connected downstream from the switch elsewhere in the wiring harness. You'd have to do more invasive wiring changes in that case, there are some threads floating around about disabling the radio/gauge dimming feature for better visibility during the day with DRL's on.
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From my recollection with owning European vehicles, the standard for vehicles in those markets is that the head lights must be on in order to operate the fog lights. If you have a EUDM fog light set-up, that is how they operate. JDM market fog light arrangements may be different based upon Japanese regulations. The North American version of the Electrical Trouble Shooting manuals makes no mention of fog lights, probably because I don't recall fog lights ever being an available option. I am very sure that it would never have been an option on a 1991 - 2001 Canadian Spec NSX because the factory supplied daytime running lights would have prevented fitting fog lights.

If the fog lights were standard equipment on Euro models, the electrical schematic for the fog lights will be set out in the European market factory service manual. If the fog lights are a factory accessory, wiring diagrams are typically not included in the service manual; but, are covered in the Electrical Trouble Shooting Manual (assuming that document exists for European market cars).

You may have better luck with getting a wiring diagram for the fog lights by posting this question on the NSX Great Britain webpage. Once you have a wiring diagram it should be easy to figure out how the fog lights are interlocked with the headlights and it may be relatively easy to by-pass that interlock. SpeedyPartsJapan (MITA) may be able to help you with a wiring diagram as they sell the JDM market fog lights.
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Procuring JDM or EUDM manuals seems to be very difficult and they tend to be incomplete. I pestered Kaz for JDM keyless wiring on the keyless, he spent some time on it, and he stated it simply was omitted from the wiring diagrams. And he didn't seem surprised.

Your best bet is to physically track down the wiring on your car. Honda tends to use the same wiring color throughout the entire harness/loom so you can easily pick it up the next connector down. The harness nexus is in the front left firewall. If you have a toner, it might help.

Minor FYI, in the US, it is a violation to have fog lamps on without headlamps. I suspect it is the same in the EU.

(There was a complete set of NSX manuals in German that shows up on the ebay.de a while back, > €1K. You might consider maintaining a search on that site.)
Well I checked today. When you press the front fog lights switch front position lights + rear lights + instruments / gauge lights switch on. They are all connected somewhere.
Unfortunately I can't supply any wiring but legal information from Germany (your NSX is equipped with beam level control and therefore might be a German registration).

In Germany a rear fog light became mandatory as of registration past 1st January 1991. They are required to be wired as such that they may not light up on their own but only in combination with other lights (side lights, front fog lights and low beam, high beam, front fog lights or any combination of these). If front fog light are installed, the rear fog light needs to work independently.
Front fog lights need to be wired as such that they can only be switched on together with the low beam.

Regarding front fog light wiring, the JDM version (at least the later models, e.g. mine from 1997) do feature front fog lights which are wired as specified above. If you can locate some JDM electrical info that should include the corresponding wiring information.

Since the front fog light switches are low current, a relay must be used somewhere to switch the car to low beam. Checking the fuse/relay box labeling (inside the box) might help in locating the corresponding relay.
Digging through my pile of info about the JDM HID system I found something interesting:

View attachment JDM NSX Pop UP HID Headlight Electrical Diagram.pdf

Note that the JDM NSX does feature front fog lights only.

Page 2, right hand, shows a relay labeled "フロント フォッグライト リレー" meaning "front fog light relay". It's enabled by an OFF/ON switch (the fog light switch), lights up a small lamp (the fog light indicator) and pulls done the line 'B' ("テールライト リレー" meaning "Taillight Relay") via a diode.

The tail light relay (テールライト リレー) can be found on page 1. right hand side. It's activated either by the switch at pin 4 (probably the light switch assembly) or by the fog light switch (フォッグライトスイッチ) wire mentioned above.

The tail light relay enables the tails lights but also a huge number of other systems listed on the bottom right corner of page 1: Front position lights, control lights, radio illumination and many more.

To make things short, you would need to cut the connection between the fog light switch and the rear light relay to work them independently.
According to the large box on page 2, right hand side, that would be the diode included in the fog light switch that needs to be removed/cut.

I'm not aware of the regulations in Switzerland but such a modification could potentially be illegal - in Germany and the USA it is.
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Well my ETM (USDM) didn't have anything about fog lights unsurprisingly, so I took a few pics of some relevant pages just in case they come in handy. Between that and Heineken's helpful posts you should be able to make the necessary wiring harness changes if you still feel it necessary.

Though it's not relevant to anyone in the thread right now, I'll say in general you would never get in trouble for having only fog lights on (no low beams) in the US unless you live in a select few lucky states, most states (at least mine) seem to only specify mounting height and projection distance. Even so, every day I drive there's innumerable other cars with >1 head/tail light out or no lights at night, a Nissan product running only on DRLs at night, or a Civic driving with high beams on all the time, so your use of fog lights becomes a non-issue in any case.







The entire USDM ETM is available as they have been scanned into PDF's. (BTW: NSXprime needs a repository for such things...where is it?)

Not sure if the ETM will help you because fog lamps (aka driving lights) were not available in North America.

It would be the rare patrol cop that knows, or even cares, about fog lamps usage without headlamps. However, if your goal is to make the car more stealthy for smuggling runs, such a vehicle violation will make a solid probable cause pretextual stop possible and THOSE particular cops will know the vehicle code down to extreme ticky tack violations.
Have you fixed it already?

As I have made a similar mod to my DC and EK, you may only switch the ignition plus to continious plus.
Maybe you get yourself the wiring from an Accord, Prelude, Civic (EG, EK) or Integra (DC2). They ve got in the oem fog light wiring an relais included. You can pin this on "conti plus" and shoud just be happy.
Remember to get a good grounding and the correct fuse ;)