Accessing switched (accessory/ACC) power for aftermarket devices

Discussion in 'Clarity' started by jorgie393, Oct 16, 2018.

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  1. jorgie393

    jorgie393 Active Member

    For powering aftermarket add-ons, it would be nice to have access to switched power (ACC, or accessory power) so the devices can’t drain the 12V battery when car is off. (Honda calls these circuits ACC so I am going to use that term even though they also use ACC for Adaptive Cruise Control.)

    So far, I haven’t seen a good solution for hardwiring. Of course, the easiest is to plug into the accessory power outlets with an adapter, but sometimes that won’t suit.

    I found a pretty easy way to tap (hardwired) into ACC (switched) circuits. It is not hard; if I was to do again, I’d guess 10-15 min. It took a lot of figuring out, so wanted to share.

    This is the first of 3 posts (overview). The second post describes the existing ACC circuits, and gives the diagrams. Maybe someone will use them to come up with an even better option. The third is my solution.

    Note that this solution taps into the same circuit as the front console accessory socket circuit, and with 18ga wires, so would max out at about 10A/120W. My needs were pretty modest: powering a Thinkware dashcam that had both Parking and On modes, and a BSS1-LPB rear cross-traffic/blind spot sensor. If you need more power (for your big amplifier or hair dryer or whatever) there is a suggestion at the very end.

    Post 2 to follow....
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  3. jorgie393

    jorgie393 Active Member

    2/3: The background (skip to the next post if you just want the solution). Sorry this is so long but a lot of hard-won knowledge.

    The ideal ACC source would be easy to tap into, and also be in a place where one can easily run wires out to the rest of the car to power the gadget of interest.
    Unfortunately, the ACC circuits at first glance don’t suit. See the attached Service Express documents for more details, but essentially the way the ACCs work is:

    There are two ACC circuits originating in the underdash fuse box (formally the Under Dash Fuse/Relay Box, as there are other fuse boxes), just inside the driver’s door. Fuse 29 protects the Front Console Accessory Power (cigarette-lighter style) socket and Fuse 10 protects the Center Console Accessory Power socket (which despite the name is really for the back seats). Unfortunately, the fuses are BEFORE the ACC control relays. This means one can’t just tap into the fuse locations with a fuse tap—if you do, you get "always-on" power, like any other fuse, not switched power.

    After the fuse, the switching is as follows (see Service Express doc DIAG1---the ACC circuit diagram---attached): ACC ON signal comes from the Body Control module; energizes a nameless relay; and this relay then energizes the coils in relays L2 and L3 (also in the Under Dash Fuse/Relay Box), energizing the ACC circuits. These immediately leave the Under Dash Fuse/Relay box via (inaccessible) Connector E in the (inaccessible) back of the box, and go into Wiring Harness BD.

    You can look at the diagrams (see attached Service Express docs DIAG2, DIAG3, DIAG4), but Wiring Harness BD is ugly. It is huge with lots of wires, goes deep under the dashboard, and goes immediately across over the steering wheel, then down behind the center front console. It’s basically inaccessible. In the center of dashboard, after it feeds the climate control and infotainment sections, some conductors split off to power the Front Accessory Power Socket, then the remaining wires join to a new harness that goes to the Center Console Accessory Power Socket (aka the back seat power), and probably the USB power, among other things. See DIAG4 again.

    So the problem: The BD harness is inaccessible and/or scary. Disassembling the Front Accessory Power socket requires taking apart the whole front center dash both over and under the gear shifters (starting with the screen) which is intimidating. Disassembling the Center Console Accessory Power socket is actually pretty easy (see Center Console part of attached Service Express documents-CENTCON1 and CENTCON2), but once you have accessed this, it’s hard to then run wires out again to places in the rest of the car. (Might be a good solution for some applications though).

    There is however, one place that the ACC conductors are available to tap into. See next post (3/3).

    Attached Files:

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  4. jorgie393

    jorgie393 Active Member

    Post 3/3: My workaround.

    Turns out that you can reach the conductors leading to the back of the Front Accessory Power socket without actually taking the Front Accessory Power socket out (and the whole console apart), and can do so in a way that’s still easy to fish outgoing wires around the car. It’s not trivial, but it requires very little disassembly and not much squirming around under the dashboard. Having done it once, I figure 10-15 min to repeat.

    The key piece is the small kick panel next to the accelerator pedal. (IMG1)

    Panel removal: The two trim panels that should come out are 1) the panel under the steering wheel (the Driver’s Dashboard Lower Cover—see Service Express doc) , and 2) the panel just to the right of the accelerator. Basically they both just pull off.

    The trim panel under the steering wheel “instructions for removal” are in DIAG5. Easily removed by raising the steering wheel, then grasping the panel in the space so exposed. You need to unclip some wiring connectors to free it (and, interestingly, what looks like a footwell light in my Touring model at least). The panel next to the accelerator also just pulls off (no diagram, sorry). See IMG1 and IMG2 for pictures before and after removal.

    At this point, looking in the space next to the accelerator pedals (IMG 2 again) the much-reduced BD harness can be barely seen bending to run towards the back of the car. A small branch splits off and runs behind the spot weld. Nothing more can be seen without a mirror (though see IMG 3 for an around-the corner picture), but if you lie on your left side and reach in with your left hand you can feel that this small branch runs 4” more and goes to the back of the Front Console Accessory Power socket. This just easily wiggles off—no catch to release—and then you have access to a roughly 5” section of conductors, though still wrapped in sheath. I used a razor blade to release the sheath a bit and open the sheath, revealing the white (hot) and black (ground) conductors (IMG 4).

    This is admittedly a bit of a tight spot to do soldering in. So I took the easy way and used two Posi-Tap PTA2022 connectors to tap in two 18-ga wires., which took about 60 sec. per wire. Totally worth it (IMG 5 and 6). I could have just done one tap, off the hot, and gotten a ground connection anywhere, but it was so easy I just tapped the ground here as well. (Side note: Ordering information is at the bottom of this post. I had not used Posi-Tap before, but I love them. Not cheap, but among “vampire connectors”, these are very highly rated in general, even in high-vibration roles like motorcycles. I don’t actually know the size of the conductors that Honda is using, but this size seemed to fit. Require 18-ga for the tap wire though).

    After that, the tap wires can readily be run under the steering wheel and to the fuse box area now that the panel is out (make sure that they are not pinched when the wheel is all the way down). Once they are back to the fuse box area, you can run them anywhere you please. (See IMG 1-2, which were actually taken during reassembly; the red and black wires are visible). Snap the two panels back and you are in business.


    General: Cable ties, snips to shorten them, electrical tape (Super 88 if you want to be fancy), spools of 18-ga wire (red and black), razor blade or similar. Two Posi-Taps PTA2022 as below:

    Posi-tap ordering info:

    I actually ordered the PTA2022 on Amazon as , though that didn’t give the model number. Full details re. wire gauge of tapped and accessory wire for different models are at the PosiProducts website

    Side note: Note that if this 18-ga wired ACC circuit doesn’t give enough power for your monstrous accessory, you can use this still to control a DC-DC relay that only energizes a larger circuit, tapped off any fuse, when the car is on. (for example

    Side note 2: This doesn't require a fuse tap, but in the course of this I determined that the Clarity seems to use "mini" fuses so "mini" taps such as Amazon Part # B0722CGGS8 would be appropriate

    Attached Files:

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  5. jorgie393

    jorgie393 Active Member

    Won’t let me edit for some reason, so I’ll add: you can still maintain power to the front accessory power socket. After you have attached the taps, you can pretty easily blind-fit the plug back onto the contacts on the back of the front socket. Correct polarity is assured by the socket shape.
  6. Hobbesgsr

    Hobbesgsr Active Member

    Wow. Great sleuthing to get ACC switched power.

    I always used to tap into the existing fuses or available open fuses that were switched by ignition and not accessory. For my previous cars most fused circuits are not “always on” and connected directly to the battery but connected either to accessory (first turn click of the key) or ignition (second turn click) just before you engage the starter motor.
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  8. jorgie393

    jorgie393 Active Member

    Yep, there’s clearly a lot of the car that’s powered on only when ignition is on, and that’s just as good. I started with ACC only as I didn’t feel comfortable even with fuse taps on most of the ignition-switched circuits.

    But if any of the fuses on the underdash fuse box are ignition-switched circuits, those would be even long as the switching happens before the fuse, not after.

    I might look into that tomorrow from the circuit diagrams. Great idea.
  9. Hobbesgsr

    Hobbesgsr Active Member

    I just tested and found that open fuse position #2 (next to #1 ACC 7.5A) in fuse panel 1 is switched by Accessory (car doesn't have to be started).

  10. jorgie393

    jorgie393 Active Member

    Thanks Hobbesgr. This is much better. I am going to start a new thread with "Fuse map, and which are switched by relays" to update this.

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