The Fiero Subwoofer System

by Oliver Scholz

The story of the Fiero subwoofer system seems to be one of the great mysteries of our time. I keep getting all kinds of questions about this system all the time, so I thought it was about time to try and answer them all. The subwoofer system was available from 86 to 88 only, so to check if a Fiero is (or was) equipped with that system, check the option codes sticker, the "Performance Sound System" has option code UQ6. This sound system was designed to improve the stock sound and came with two-way speakers instead of stock speakers. But don't expect boom that'll pop your sunroof open, GM didn't have that in mind when they made the Fiero amp.

A question that keeps popping up is the location of the various components of the system. Well, the system consists of an amplifier, the speaker in an enclosure, and a slider to control the volume of the sub. The subwoofer enclosure is located under the dash on the passenger side, right next to the chime module. The amplifier is under the carpet on the passenger side under the radio. If you find a Fiero in the yard that has any of these components, go and grab whatever you can, since most of them are discontinued. Even if you can't use it, you'll find a fellow enthusiast who's looking for just that part. Save that valuable part from the crusher, but don't let the yard owner know that part is rare. I was able to snatch the speaker with enclosure and the slider system guts for $15. Just tell them it's a stock speaker.

How does the subwoofer system work? It's actually not that difficult, but a little technical. Bear with me.

The subwoofer amp has eight inputs that are designed to connect directly to the speaker outputs of your radio, and yes, it will also work with aftermarket radios, but more about aftermarket installations later. The signals from all four speakers are added up inside the amp and only the low frequencies are allowed to pass, everything else is filtered out. The amp contains an 8 volt power regulator that supplies that voltage to the slider assembly on the overhead console. The slider is actually only a 10K variable resistor in series with a 10K fixed resistor to ground. This voltage divider feeds a variable voltage back to the amp that controls the volume of the amp. So, the audio signal is NOT fed up to the slider and back down, instead the amp is controlled by a voltage. Finally, the low frequency audio signal is fed to a "power" amplifier that drives the subwoofer speaker. If you are interested in the schematics of the amp, click here.

You don't care about that, since your subwoofer system doesn't work anymore anyway? Well, why don't you fix it? The information in this article along with the schematics on the web may help you do that. If your speaker sounds faint or distorted, it's a good bet that the speaker is shot. Unfortunately the original sub with the part number 1604 1072 was recently discontinued, but I managed to repair mine with white silicone and medical gauze (Oh, and in case you were wondering, it's a 5 1/4" speaker and the impedance is 4 Ohms). You can also have it reconed at:

Simply Speakers
8401 9th st. n. st.
petersburg, fl. 33702
phone: (727) 571 1245

The cost is $35 (including shipping). Thanks to Kim Ylisela for the information!

If the speaker is still fine and the amp is shot, try to verify this with a known good unit. A bad amp can cause a popping or buzzing sound. If you grounded one of the speaker wires, or you didn't disconnect the battery while working on the amp system, you may have fried the power amp IC inside. Normally this would be an AC Delco IC DM133. That one is probably unavailable or expensive. Instead I used a TDA2005M for about $5, it is pin compatible and my amp is better than new now. Oh, the "M" is important, since this is the Mono bridge amp version of the chip. There is also the "S" (Stereo) version which we can not use. The power rating of the stock amp is 20 Watts, by the way. And if you don't like soldering, the same amplifier was used in the 87 (and possible also later) Firebirds, where it is located under the dash on the passenger side, left of where a glovebox would be.

So, you don't have the subwoofer system, but you want one badly or you want to wire it into your existing aftermarket stereo? No problem, we can do that too. Get all the parts from the donor vehicle, not just the the obvious components, but also all the wiring you can get. And if you own an 84 or 85 Fiero, get the heater core cover as well, since this is where the speaker enclosure is mounted to. Also, get the plastic part that the power amp is mounted to under the carpet, since the earlier years didn't have provisions for the amp. The overhead console on the donor vehicle is not the color of your interior? No problem, if yours is an 86-88 Fiero, you can transfer all the "guts" from the subwoofer console over to your console. If you have an 84 or 85, get the console from an 86-88 and you'll be set.

Now the electrical connections. First you need to connect the amp power inputs to switched 12V and ground. If you have a stock GM radio, connect both pink power wires to the E terminal of your radio. If you are using an aftermarket radio, things are bit more complicated. Your radio needs to have an output for a power antenna or other power accessory that is hot (12V) when the radio is on. The problem is that these outputs usually do not have the juice to power an amp, but barely enough for a relay. While this is possible, there is a more elegant solution. The two "power" wires from the subwoofer amp are not really poower inputs. While one actually is the power input, the other is a "mute" circuit. So, you can connect the power input (#13) pink wire to switched battery power, and feed the radio's accessory output to the mute circuit input of the subwoofer amp (#3) pink wire. It works beautifully. When you have installed the speaker and volume slider, all that's left to do is connect the speakers to the amp input. Refer to the helms manual. The since all signals are added up anyway, the connections are not critical, as long as you connect the speakers with the correct polarity. If you don't, the signals may actually cancel each other out! If in doubt, connect the speakers one at a time and verify. The woofer should get louder with each speaker you connect.

One final tip: if you could get all the components but the slider, or the slider is just torn up or you can't feed the wires up to the dome light, you may instead use a standard 10K variable resistor in series with a fixed 10K resistor and mount them somewhere convenient (e.g. next to the cigarette lighter).

I hope this answers some of the questions you have about the subwoofer system. Happy Fieroing!

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