Radio works, no sound, no amp to be found

[Americandan]

First time owner of a Sport Trac and it came with a nonfunctioning radio... Sorta. Radio powers on but no sound. There's no amp anywhere to be found and I searched this forum for it so it's not in the back of the rear driver's seat. Tried another radio and same results. Checked the fuses and they're all in working order unless I missed something of a fuse but all of them are good (under the hood and by the parking brake. So what am I missing or how do I fixed this? TIA!

 

[J_C]

Do you have the base model radio or premium? That "might" determine if you have a separate amp. On my '98, I didn't even realize I had "premium" sound because I didn't have a subwoofer, but had an occasion to pull the right rear trim panel off the cargo area and there was an amp I never realized I had!

Anyway, can you find a wiring diagram for your radio? If so then you can check whether both of the power sources are present (memory always live and 12V to stereo circuit live with key in run or accessory position.

When the radio should be outputting, you can measure for an AC voltage on the connector to each speaker. Unless the volume is turned down very low it should be a few volts, or if it is going to an amp, closer to 1VAC. With the connector unplugged from the stereo you can also measure for each channel, the resistance. If it's single digit ohms, there's a speaker at the other end. If not, probably going to an amp.

It looks like you can get a 2007-2008 Ford Service Manual DVD ISO from this place:

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I will leave the link but that file is a gauntlet, password protected (password on that page), then when you run it, it states expired and won't work, so I used Nirsoft's "RunAsDate" RunAsDate - Run a program with the specified date/time to set the date to October 2006 so it would keep running, then needed SVG Viewer, which I selected to install but it didn't, and adobe stopped distributing it so I got that from here and install it: Download Adobe SVG Viewer 3.03

Finally after all that, I only found a single wiring diagram, not the multiple that there must be in order to support different tier radios, or was there only one radio available? Anyway, here it is:

 

[Americandan]

Wow! Thank you! Didn't know I might have an amp there. Sooooo... How do I get to it? Do I have to remove the protective cover for the bed?

Thank you!

 

[J_C]

No idea, that's all I have.

 

[bdmaash]

could be lots of thing
- previous owner had aftermarket speaker and thought they were worth somethin - took out speakers
- previous owner had aftermarket stereo and modified wiring for aftermarket radio
these things can be checked - when you replaced head unit was wiring stock or look modified?
speakers can be checked with 9v battery, place battery across speaker terminal - you should hear a pop/scratching noise out of the speaker tested

- if that schematic is for the external amp: disconnect C290 and connect across each speaker to test them (this will determine if it is the speaker or the amp sort of - could still be the head unit - but you swapped it)

edit:
2005 Ford Explorer Radio Wiring Diagram
This is pretty well the same wiring, I don't have my truck DVD at work
looks like c290a is part of the stereo, so internal amp, they just break it out in the schematic to look external, '07 limited wasn't all that fancy, Adrenalin may have had the external to begin

most likely if it is a stock (not higher line) it should have an internal amp, and an output for the subwoofer amp if equipped.


it's better to do some testing and find the problem first, instead of randomly buying expensive parts hoping that you can fix a problem

 

[Americandan]

Truck is completely stock. Dug around and I still can't find it. Is there an amp in the bed of the truck on the side? That would seem an odd place for it.

 

[J_C]

Maybe there is no amp but you can determine that using the multimeter tests I mentioned previously, though kind of hard to access the wiring harness, might be easier to pull whole radio out to measure for the speaker impedance, grab a spare harness pigtail and as much wire as you can get with it off a junkyard donor vehicle and then hook the radio up powered outside the vehicle, coathanger as antenna if necessary (or should just hear static unless it has an auto-mute feature), then measure for the signal output on each respective speaker connector pin.