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Old October 21st, 2022, 08:53 AM   #1
InvisiBill
EX500 full of EX250 parts
 
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MOTM - Aug '15
Headlight delay relay bypass

While planning a bi-xenon/LED projector upgrade for my EX500, I realized that the stock H4 headlight with its high/low switching between two filaments didn't align very well with a light that uses a single bulb and switches a shield for high/low. In researching things, I learned that the OEM junction box has a second headlight output that bypasses the relay circuit. This can be used for retrofits like mine, troubleshooting headlight relay issues, or even bypassing the relay in a pinch.

First, a little background on the headlight relay. There are some aftermarket "headlight relay" kits available. These use the factory wiring to trigger a relay, which then feeds power to the headlight directly from the battery on upgraded wiring. This ensures that your bulb is getting the highest power it can, and therefore putting out as much light as possible, by avoiding electrical losses in the small factory wiring.

The OEM headlight relay does not function this way. Inside the junction box, the headlight relay actually interrupts the headlight output. The relay is triggered by two sources. One is the output of the relay itself. This just means that once the relay is triggered and feeding power through, it will stay latched and keep feeding power through, until incoming power on the headlight circuit is completely stopped by turning off the ignition. The other trigger is a branch of one of the yellow wires between the stator and the regulator/rectifier. This has the effect of keeping the headlight turned off until the engine turns over and the stator starts generating power. It leaves more electrical power available for starting the bike, and avoids flickering the light as it starts.




According to Partzilla, the 26021-1089 junction box was used in the Pregen at least for '95-'07. I assume that if it wasn't used for all years, there was some insignificant change made to it, and other years are electrically compatible. It was also used on some bigger Ninjas and Vulcans.


Here's what the EX500 wiring going into it looks like. In the 8-pin connector, the white wire is constant power which feeds the ACC and FAN fuses. Brown is switched power that feeds HEAD, TAIL, IG, and HORN. (Older EX500s used a different harness, with a second switched brown wire feeding ACC/FAN instead of the constant white.)

Blue/black goes to the fan, white/green goes to the turn signal switch, red/blue goes to the tail/running lights, and blue/yellow goes to the headlight switch.


The headlight fuse is in the lower right. The left side of it is connected to the pin where the brown wire comes in. The right side of the fuse splits in two. It runs up that long bar on the right edge of the PCB to the headlight relay circuit at the top. It also goes out to the unused upper right pin next to the brown wire. That upper right pin is ignition-switched, HEAD-fused, un-headlight-relayed power.

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I highly suggest using this pin only for headlight-related things. If you wire up your $2 knockoff phone charger to it and it blows the fuse, your headlight will lose power too. Don't risk your ability to see at night for something stupid. Use the ACC circuit, or a completely separate addon circuit, for low-priority accessories.
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You can use the upper right pin for quickly testing your headlight circuit. If you're having weird headlight issues, this will let you isolate the relay circuit and ensure that the fuse and incoming power are working properly (not much different from testing at the right side of the fuse). In an emergency, you could move the blue/yellow headlight wire's pin to this position (or make a short jumper between the two) to bypass a failed relay (since it's on the junction box's PCB, you can't just buy a replacement at an autoparts store).


My plan for converting to a bi-xenon/LED projector setup was to add a new wire in place of the blue/yellow to power the actual bulb. It would retain the startup-delay and I could use larger wire to ensure the light's output wasn't being limited by the small OEM wiring and switch contacts. I would then move the blue/yellow wire to the upper right location so the high/low switch would still get power from the OEM setup, and I could use the high/low pins at the original H4 headlight connector to control the high/low shield in the projector. This would result in the single bulb always having power regardless of the high/low switch's position, high/low would still be controlled through the OEM switch and wiring, all of it would be fused through HEAD, and it would require no permanent modifications to the OEM harness.


Bonus: The lower left pin is connected to the ACC fuse. It looks like the EX250 uses this for the accessory leads found at the front and rear of the bike. On the EX500, it's completely unused. Just keep in mind that it's powered by the upper left wire. Depending on the year/model, it may be either ignition-switched or always-on.
Attached Images
File Type: jpg junctionbox.jpg (130.5 KB, 24 views)
File Type: jpg fuseboxconnectorstop.jpg (89.8 KB, 25 views)
File Type: jpg junctionboxpcbtop.jpg (124.4 KB, 26 views)
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Old October 21st, 2022, 10:13 AM   #2
DannoXYZ
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awesome! Thank you!!
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Old October 21st, 2022, 03:09 PM   #3
CZroe
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Name: J.Emmett Turner
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Motorcycle(s): '08 CP Blue EX250J, '97 unpainted EX250F, 2nd '97 unpainted EX250F (no engine), '07 black EX250F

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This is genuinely helpful for me, but for whatever reason I can’t hit the thumbs up on my phone. After trying repeatedly it ended up thumbing down and now I can’t even undo that.

Hopefully a PC will let me undo the mess when I can get to one. Ugh.
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