Hacking your stereo with style, part 1: Factory Ford/Lincoln subwoofer transplant.
I'm hoping to make this the first in a short series on getting the most from our factory systems without spending big money. I'm hoping the next one will be a fairly exhaustive parts guide I've been working on for months, including take-aparts, technical specs, and test data.
This is a quick and simple example of installing an OEM subwoofer/amp combo from another Ford/Lincoln Audiophile or Sony premium system. I source as much as I can from my local Pull-a-part lot, as you simply cannot beat the pricing. If you don't have one near you, these OEM parts can usually be found on eBay for not much more.
I didn't think to get pictures until I was halfway through, however most of this is pretty typical if you've ever installed amps or speakers in a car before. My goal is really to shine a little light on some lesser known or murkier details of the OEM parts more than anything.
(The links provided are for example only, I don't endorse or vouch for any particular sellers. Except for parts-express. They're awesome.)
Most of the premium systems in Ford vehicles include an 8" subwoofer in a custom sealed enclosure with an amplifier helpfully attached to it. In older vehicles, the speaker is 4Ω and is driven by about 40W from the single-channel amp. Somewhere around 2005 they switched to a dual voice coil 8", each coil rated [email protected]Ω, however the amp used on these really only puts out about 40Wx2 channels.
At Pull-a-part, these assemblies will run you just under $30. $6 for the speaker, $20 for the amp, and $4-6 for the box, which they call an "interior panel". Not bad at all really for what you're getting.
There are quite a few different form factors these come in, depending on what the donor vehicle is, and some of them are bizarre shapes or extraneous sizes. I've narrowed the field down to the two I think are best suited to fit a Taurus and will allow you to install it in a mostly hidden fashion, just as if it were a factory option.
The size and shape of this box makes it a perfect fit to be installed in exactly the same location in a Taurus that it's found in the Towncar--bolted up underneath the rear deck. It's out of the way, doesn't take up much room, was designed to fit around the other rear deck speakers, and doesn't block the opening between the trunk and cabin when the seats are down (one goal I was aiming for, as I often use that opening for taking lumber home when I'm working on projects).
Although this enclosure is installed in the right side panel of the cargo area in its natural environment, it's still just about the right size and shape to be mounted like the Towncar box in the Taurus. It won't bolt up snug to it and place the speaker right in the large center hole like the other box does, but it doesn't need to. This is actually the box I went with on mine, just because the one I had was newer than the Towncar box I picked up, which only had the older single-coil 8".
While you're there pulling the subwoofer, do yourself a favor and snag 4 of the premium 5x7's to install along with it. At $6 each, they're a great deal. I've actually done a deep dive on these and come up with full spec sheets and impedance/phase/frequency response charts. These are actually not discrete speakers, but little enclosures with JBL component drivers (a 4" full range and 2.5" tweeter) with a full on crossover and Zobel circuit inside. I reverse engineered that circuit and modeled the derived result in XSim as well (I might be OCD). I'll have links to all that data when I post the parts guide.
Long story short, what's relevant now is that these sound good. They're high pass filtered to block lower frequencies, even when used with the Mach RCU, so they pair better with a subwoofer than the stock 5x7's, which get too much bass and muddy that range up and distort. I use 4 of these with the subwoofer, Mach RCU, and Mach tweeters and am very happy with the result.
-Line level converter
The amp takes line-level audio inputs (same as the RCA inputs on aftermarket amps). We'll need a speaker-level to line-level converter for this, same as you would for an aftermarket amp install. An additional benefit of this is that the converter will generally have a gain adjustment on it, which is nice since the factory amps do not.
-DC-DC Buck converter
The remote turn on for this amp should be 5V+ instead of the usual 12V. I've tested a bunch of them, and can verify that they actually will work fine with any voltage between 4 and 12. As I did more research into it, I found it's better to give them 5V, as this line serves as a clipping detection circuit, and setting this input voltage high essentially disables this safety feature. However as long as you're careful not to push things too far, you should be fine. It's easy to give it 5V though, we just need a cheap DC Buck down-converter (cheap as in I literally bought a 2 pack of these on the internet for $3) which we can wire up to the remote lead from the line-level converter. You can also use a cigarette-lighter USB phone charger, as USB is 5V.
-Low pass filter
One last issue we need to deal with, is the fact that these amps do not do any crossover filtering--they output whatever signal they get. In the factory system, their inputs are already filtered from the head unit or DSP so the sub only gets low frequencies. There's several options for this, however I thought the easiest route was using a Harrison Labs FMOD inline pre-amp filter plug. It is used on the low-level side, you simply plug it in between the RCA cable and amp. Downside-these cost $25 for a pair, but because this amp has only one input channel, and is technically putting out mono, I only needed one of the FMod plugs, giving me an extra one to use on another install. So I considered this a $12 expense for the project.
I bought the 200Hz low-pass variety. https://www.parts-express.com/harris...s-rca--266-258
-Three 3/8" carriage bolts, 8-10" in length, two washers and a nut or wingnut for each.
-Some foam or weatherstripping to pad parts of the sub box that might come into contact with the deck or other metal, to prevent rattling.
-Some 14 gauge wire to run power/ground, some thinner wire for splicing in your line-level converter, turn-on lead, etc.
A grounding terminal and a plastic electrical outlet box, to contain the electronics. Or wing it and come up with your own placement.
The first thing I did was lay the sub box on a piece of cardboard, traced the outline, and marked the mounting holes that already existed (3 of them on the Aviator box, 4 on the Towncar box). I cut this out and punched out the holes in the cardboard and now I have a template to lay on my rear deck so I can get it positioned where I want it, and then mark where I needed to drill holes into the metal (and not into anything important) to drop my carriage bolts through. This sub box wasn't the right shape to bolt up tight to the underside of the deck, so it was going to essentially hang by these three bolts, and I'd tighten them up as much as appropriate to keep the thing secure. If you get the box from a Towncar, you should be able to direct-bolt that one right to the underdeck after drilling holes for it, fairly close to the way it was installed in that car.
Next, take down the rear seats. Take out the plastic trim plugs so you get the carpet and padding out of the way on both the top and bottom of the rear deck, Position your template on top of the now-exposed metal. I was trying to get the speaker itself as centered under that middle hole in the deck as I could, while keeping the bolt holes out of the way of the other speakers, however since this is a subwoofer, it really doesn't require that. The first picture here shows what I mean. The box is already mounted in this pic, you can see the speaker grill, mostly centered in the middle hole. The amp attaches to the box on the right side of this picture.
Something that's not obvious in this picture, the rear window defroster relay used to hang on a bracket right into the center of that middle hole. I had to pop that out, and luckily its cables are long enough to allow it to be moved out of the way. You can see it in this picture just below the right speaker, ziptied down to the other cables.
Once marked, I drilled each hole twice. Once with a smaller drill bit, then with the 3/8". That 3/8 bit is so big, it's easier to get it through metal if it's started on an existing smaller hole. I put a washer on my carriage bolts before dropping them down through the hole, just as extra support so they wouldn't pull through (not that we need to be getting these all that tight, but better safe than sorry). Get the box in position, get the bolts through each mounting hole, add another washer underneath and your nut/wingnut, and tighten them up just enough to keep it there for now.
Next I got my converters in order and ready to splice. I decided to mount them in an electrical box to keep them secure and tidy. I took the circuit board of the line-level converter out of its case, because I soldered the remote lead out of the Scoche to the 12v+ input of the buck converter, then took two lengths of wire, one soldiered to the 12v- on the buck, which would share a ground with the line-level converter and the amp, and the other to the output+ on the buck, which will be tied to the turn-on lead to the amp. Output- of the buck is not used. Used some 3M to stick them to the bottom of my box, orienting the inputs on the left side, and outputs on the right.
The best spot I found to mount the electronics was the driver side end of the rear deck. This gives us direct access to both the rear speaker wires we need to tap for signal and the wire harness going to the RCU to tap for power, and if you needed, the audio-system-on lead from the head unit. Also there's a couple large holes here perfect to run our wires through to the sub, and this little cubby area remains clear even after the upper carpet is put back down--almost like Ford intended for things to fit there.
At this point comes lots of tedious cable splicing. Unplug the harness from the new amp and bring it up top to make things easier. When cutting cables to splice to the harness be sure to give yourself more than enough length to run it back through the hole and around to the amp. RightRear+ is orange/red stripe, RR- brown/pink stripe, LR+ gray/light blue, LR- tan/yellow stripe. Even though the amp is mono, we want a full left and right signal combined to get to it.
I spliced the Scoche inputs to the speaker wires where they ran horizontally over to their speakers. I tapped into power right at the large bundle of wires coming out of the trunk that runs down behind the seat on its way up front. The power wire you want is thick gauge and all yellow. This one is powered full time, is thick enough to handle some current draw, and is fused at 20 amps. I tapped the amp and the Scoche into it. I drilled a hole just above this into the sheet metal to bolt in a ground terminal, and attached the ground for the amp, Scoche, and buck converter.
If you are going to get your buck converter input from the audio-system-on lead, it is in the bundle with the yellow cable, it's green w/ black stripe.
Here is a photo of the input pigtail going to the amp:
This next graphic lists the pinouts of several different versions of this amp, from different vehicles. They're nearly all the same, with just slight variations in wire colors and location of a couple wires.
Once you've seen a couple of them, it's easy to tell which wires are what without even looking at a diagram. The power and ground are both the heaviest gauge. The audio inputs are always separated from the rest and covered in EMI foil. The remaining small-gauge wire will be the remote-on lead. Outputs to the speakers are on a second harness.
Plug the RCA cable to the Scoche's output, then the Y-adapter, then the FMod to that (female end is the input side), then to the amp input. Did I forget to mention you should have soldiered the amp's input wires into that RCA plug already?
A closer shot of the electronics, after splicing and running the wires back out the hole to the amp:
We're about done at this point.
Before plugging the amp harness back in to the amp, we need to start the system up and do a quick check with the multimeter that the expected voltages and polarities are correct.
We also may need to adjust the buck converter. Many of them (like the one I used here) are variable output--and they come from the factory set to 12v. Stick your multimeter ground probe to your 12v ground, and the positive probe to the buck's + output. Take a tiny screwdriver and turn the tiny setscrew on the blue potentiometer counterclockwise. You may have to make several full turns before you see the voltage start to drop. Get it down to approximately 5V. (it doesn't need to be exact, the Ford electrical specs say +/- 1.2V is fine)
Now you're ready to plug the amp harness in.
Start with the gain on the line-level converter set low, and slowly adjust up to find where you want it. I found the halfway mark on the gain was a little too much for my taste and backed it down.
A tip to adjust gain on any amp to keep it below the point of distortion without using specialized equipment: take a spare tweeter you don't care about, and wire up a cheap capacitor to the positive side, somewhere around 2-3uF. The point is to block all low frequencies below 4-10KHz (approximate is fine for this). I put alligator clips on the other end of the wires to make it easier, and attach this tweeter to the amp's output without the subwoofer attached. It's probably best to do this without any of the other speakers playing either so you can hear it clearly. Play some bass-heavy music with the volume at around the max level you would typically play it. Start with the gain setting low, and slowly start increasing it. When you turn it high enough to cause distortion, you will hear it on that tweeter, and you know to back it down a little. Distortion and clipping will create a lot of noise in the high frequency range that's getting past your little high-pass filter, even if no high frequencies are being sent to the amp, and this is part of what would be killing your subwoofer. Tune it down until that tweeter stays silent and you should be safe.
Attach foam to the sub box in places where it looks like it will touch anything, and start tightening those nuts up to get box nice and snug against the bottom of the rear deck. You'll probably want to come back with a dremel and cut the excess length off those 10" bolts. You can use shorter ones to start with, but they will be much harder to get through the mounting holes without help, and you'll have less room to get the harness on and off.
You will have to cut and/or adjust the padding and carpeting the box now displaces. I just cut a hole around it, as I wasn't overly worried about the aesthetics of my trunk.
I suppose I could have also simply cut bolt holes in that carpeting instead, left it in place and mounted the sub box right underneath it. That's probably the smarter thing to do actually. Well, next time I do this it'll be a little nicer.
This system won't win any bass competitions, but it sounds good for the money, and it is an appropriate amount of bass when used with a stock or scavenged OEM upgraded system. It still gets deep, it can thump--but it's not overpowering. Being a sealed box, it's clean and tight. If you wanted, you could crank up the gain on the line-level converter more than I did--but watch out for distortion levels.
I like this a lot, junkyard BOLO's for other Ford components that will work in our cars. I appreciate you taking the time to share your audio knowledge and do a detailed write-up on the installation too. Nice work!