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Passing wires through the firewall

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Old 07-10-2016, 12:12 AM   #16
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Ah, touché on the wordplay As far as this next part...I have always ran the ground the shortest distance possible to a good body/frame connection. However, I am far from perfect and am always open to the idea that I have been doing something incorrectly all these years, so I looked into it. Both Crutchfield and Rockford Fosgate state that the ground should be a very short distance form the amplifier, RF goes one step further and specifies not to wire the ground all the way to the battery.
http://www.crutchfield.com/S-n6Y3M0t...ion_guide.html
http://www.rockfordfosgate.com/suppo...er/std_alp.php
With these two sources, I didn't think it was needed to search further.


Same here, shall we continue...


Exactly what the plan was if there wasn't a better way, so it shall be.


Now don't get me wrong, this may very well be true in a competition system, and may even have some sort of advantage in a larger power draw situation. But the two links I posted were from "pros" as far as I'm concerned and the amplifier I am running is roughly 300watts (had it forever, don't remember specifics).

If it's any consolation, I have been kicking around the idea of getting one of these sets of beefy battery/ground cables that are all the rage here.

And...back on topic...I am not running a wire from the front right, full left to the driver side, full right again to run down the passenger side, even with a bigger wire to compensate for lost draw. That is counterproductive. I do appreciate the I put though.


Edit: the RF link doesn't seem to be working, here's a screenshot from my phone; best I can do right now...

Thank you... I was just about to do the same. I've never heard of anyone running a ground from the amplifier back to the battery. As you said, maybe that's what is done for a competition system, but for less than 1000w?

BlueRidgeMark, historically I've seen you give sound advice (no pun intended, for those who remember that company), but I've never heard of anyone running a ground wire to the battery for any accessory at all. Maybe a little more insight as to the logic of this? I'm just really curious at this point.
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Old 07-10-2016, 04:27 AM   #17
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I am adding a Relay box and my devices will be ground from a single buss in the engine compartment.....
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Old 07-11-2016, 09:45 PM   #18
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BlueRidgeMark, historically I've seen you give sound advice (no pun intended, for those who remember that company), but I've never heard of anyone running a ground wire to the battery for any accessory at all. Maybe a little more insight as to the logic of this? I'm just really curious at this point.

The reasoning for using a short grounding wire is that it's possible to create what is called a "ground loop", which acts as a nice antenna to pick up noise from your system, like alternator noise.

However, this is because most amps are grounded by their case, though not well enough to carry the power that is required. Your ground wire is then a second ground point, and that's where you get into ground loop territory.

If the case is not grounded, you are better off running matched ground wire to the battery (or better yet, a common ground point near the battery).

Think about this: If a long ground wire would pick up noise, why would a long power were NOT pick up that same noise?

As for Crutchfield, remember they are instructing to the lowest common denominator level. Not that they are wrong (they're very good), but they aren't always recommending what a knowledgeable pro would do, but what will work for most of their customers without getting them in trouble.

You also have to consider the age of the vehicle. Our old rust buckets aren't exactly going to have the most trustworthy electrical connectivity.

However, that said, the point about it being only 300 watts is quite valid. The suggested method is probably fine for a small amp like that. I should have asked about the size of the amp.

Heh. I remember when 100 watts was a huge amp!
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Old 07-12-2016, 03:08 AM   #19
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Hmm okay, I suppose I can see that. However, I think maybe I personally would save that as an alternate in case I did experience a noise problem from a ground loop. I'm glad you mentioned that.

None of my amplifiers are rated for that much power though. I have a 400w 4 channel amp for my main speakers and a 300w 2ch bridgeable for the 10" sub. Frankly, I wouldn't want anymore power than that.
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Old 09-28-2017, 10:23 PM   #20
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So I'd like to breath some new life in this thread.

Just picked up a 95 XJ for my daughter, and we're going to have a blast gnarlying it up for her. I'm mounting some 100W bumper lights via a relay harness for now, but we'll likely add other goodies later. I need to run the switch wiring through the firewall. Existing wire bundle grommets are pristine and packed pretty tight, I'd rather not mess with that. There are no other unused bungs or grommets on the firewall. So ... I'm planning to drill a new pass through and grommet/goop the wires through.

Earlier in this thread it was recommended to drill high on the firewall through the passenger side, but with the heater/AC box on the inside and various stuff cluttering that area up in the engine bay, I see no good spots to drill.

On the driver side however there seems to be a pretty good spot; right between the throttle cable pass-through and the brake booster. See the picture below. Looks easy enough to access with a drill, wide open on the inside too, and sufficiently high to keep wires well hidden.

So before I start drilling, figured I'd confer with the brain trust here. Wadayasay? Good plan? Am I overlooking something?



Last edited by wristwister; 10-11-2017 at 03:39 PM.
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Old 09-29-2017, 11:52 PM   #21
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As long as there are no obstructions on either side of the firewall, no moving parts nearby, etc. I think you're good to go, man. The ONLY negative I could see with that location is that the exhaust manifold is on that same side. So it gets hotter than the other side. That being said, just run it through some split loom and keep it up away from the heat (like over the booster) and you should be alright.

Congrats on the purchase for your daughter, btw. That'll be me in about 10 years.
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Old 10-01-2017, 01:48 AM   #22
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Other side of booster......
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Old 10-01-2017, 02:04 AM   #23
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HA! And below it too...
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Old 10-01-2017, 02:37 AM   #24
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HA! And below it too...
Belows better random pick off google...lol...
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Old 10-01-2017, 09:48 PM   #25
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Belows better random pick off google...lol...
Yeah I actually looked for that plug that's circled in the pic, mine doesn't have it. Does have the one below though.

Good find, Gary
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Old 10-01-2017, 11:39 PM   #26
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Drilled, grommeted, wires passed through, and gooped, between the throttle cable and brake booster like I showed above. Outboard side of the booster is a bit more crowded on my XJ, no unused grommets or bungs there.
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Old 11-02-2017, 04:02 PM   #27
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On my 98, I also used the grommet that routes to just above the pedals in the cabin and it fits 0/1 oversized wire (huge), plus a few other small gauge wires. I just used RTV to seal it up and no major leaks
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Old 12-02-2017, 04:12 PM   #28
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This is the best way through the firewall. It completely prevents water and the possibility of chafing through the insulation. Plus it also gives you a nice 12+ common on both sides to run accessories. I would run anything between 4awg and 0/1 awg from battery to the post, then 8 awg to amp. The larger wire to the post would be to support future loads. Also, for the ground a nice way to get best of both worlds is to run a battery ground cable to the rear and run it to the chassis, the place your amp ground on the same post. This creates a battery AND chassis ground.
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Old 12-05-2017, 02:41 AM   #29
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This is the best way through the firewall. It completely prevents water and the possibility of chafing through the insulation. Plus it also gives you a nice 12+ common on both sides to run accessories. I would run anything between 4awg and 0/1 awg from battery to the post, then 8 awg to amp. The larger wire to the post would be to support future loads. Also, for the ground a nice way to get best of both worlds is to run a battery ground cable to the rear and run it to the chassis, the place your amp ground on the same post. This creates a battery AND chassis ground.
I like that. Do you have a product name or a link, perhaps?
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Old 12-05-2017, 09:10 AM   #30
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I like that. Do you have a product name or a link, perhaps?
Firewall pass through post.
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