Not much bass unless I really crank it up (and I have adjusted the radio controls for bass +7 and subwoofer +2 (or whatever the highest setting was).

Not sure if the subwoofer is actually working...

Listen to bass heavy music, turn sub from -7 to +7 (or whatever min and max is). If you can't tell the difference, it probably isn't working.

Can't really tell the difference

Can someone with a sub-equipped car confirm that there is a noticeable variation? I think it was only standard in VDP and XJR models and an option in XJ8's with Sovereign-package, not 100% sure though.

Yes there is.

Lay a sheet of paper over it and turn up the music. It should vibrate violently. If not, there is something wrong. If it does vibrate, maybe the amp is not powering it enough, or maybe it is partially frozen, or something else.

There should be no question if the sub is working or not when you raise and lower the sub volume. It is very easy to hear the difference, as it gets pretty loud, but quite distorted when turned up.

True, but that's why I thought of paper. It will show if it's working at all. Maybe it's weak from a stuck voice coil, or maybe it's completely blown out. It's easier than connecting a tester to the wires. I would first rule out all the settings on the stereo, then a tester at the speaker wire would be my next step in diagnosing, then follow back till you find power...

Don't laugh please, but how do I access the subwoofer?! The top lip of the trunk appears to not have any access hole to it, do I need to remove the back seats or deck by the rear-window?

These subs blow out all the time from drying out. Mine blew out on its own and I need to replace it. Your sub might be blown and will only make a slight cracking noise if it is.

I didn't notice which vehicle you have, but mine is accessible from the trunk/boot. Pull the Carpeted cover to expose the gas tank and SubWoofer. It's a TIGHT fit.

Gas tank?! Should this be on the top lip of the trunk? (under the back window?)

Would you kindly mind snapping a photo?

No problem.. For me, photo's are easier than explaining. I'm painfully aware of how dirty it is. I just went to Reno the other day, and I am going again tomorrow morning, so no reason to clean it right now.

I've also never removed the Sub, so not sure how it comes out, but this gives you access to the wires, and hopefully the bolts that hold it in. Sounds like burmaz might be more help with that part.

Thanks, the pic was very helpful! Will dissect the trunk in a few days after tackling some more pressing Jag repairs and now I know why this car does not have a trunk pass-through!

Okay, examined the subwoofer and it can move up/down freely by hand, so not dry-rotted plus the wires are all perfect (stemming from a wire harness and looking like-new without any corrosion) with no sign of a previous owner messing with them.

What do I do next? Check for current with a voltage tester? Would that even show up? I have a fancy digital true-RMS multimeter to test with, just need to learn how to use it

If no current is making it to the sub, where do I look next, the back of the radio head?

Is there a fuse for this somewhere? I could not find anything like that in the fuse-listing and checked a whole bunch of radio-related ones in the trunk-fusebox (#11, 12, 3, 4, etc.)

So when you touch this speaker/sub while music is playing, you don't feel any vibration in that sub?
What does it feel like??
Make sure you are playing a CD with Music of heavy Bass or a radio station with music of heavy or medium bass in it.
If that also does not help, turn your Treble all the way down on your Radio, and turn Bass up, you should here something.

Doh, actually I never tried touching it while it is on, lol, but there is no audible difference when the radio subwoofer is set to -7 or +7 when playing Run DMC's You Be Illin' (very bass heavy song) and there is a noticeable lack of bass in the music, like it is all generated from the midrange drivers.

Visual inspection completed, I would test for volatage at the SubWoofer connection. DC Voltage and at about 24 volts, just to protect your tester. You will definately see spikes if it is getting power when the music is playing. If no voltage there, follow it back to the amp, then the head unit.

Thanks, 2 questions:

- test by just touching a multimeter probe to each of the 2 subwoofer contacts (in parallel, like you check for voltage) or disconnect a plug and then pass through the multimeter (like you test for amperage)?

- how do I access the amp? I read somewhere else on this forum it is near the CD changer, but where and how do I remove the carpeting? (are there any plastic retaining fasteners?)

I test for voltage. Your meter will jump when playing music.

If I remember correctly, the carpet is tucked behind the tail light (or near it), the cover over the fuel tank, and tucked under the trunk lip. I don't think there are any retainers or anything. AMP is mounted on top of the CD Changer.

Okay, 0v at the subwoofer, which I guess is good since that means the sub probably works.

I removed the side-cover and examined the amp and the two harnesses appear plugged in alright with no sign of previous tampering, so what do I check for now?