That's not necessarily a bad sign. Crunches are kind of notoriously bad for your back. Some people actually say you should never do them, though I'm not gonna go that far. I'd guess you just have weak back muscles and iffy technique.

The best way I've ever found to work out my core is just by doing full body work that uses it. But if you want to spot train, I'd suggest doing things other than crunches to strengthen your abs and back. Some ideas you could google: supermans, v-sits, tuck-ups, bicycle kicks.

Also, might sound weird, but something that helped my core strength and stability a LOT were headstands/handstands. Start with tripod headstands, go to yoga headstands, forearm stands, etc. Just make sure you have technique right at each step and can hold for a while before you move on.

sit on a pillow then.

Why on earth are you guys doing crunches?  What are you expecting them to achieve?

There are many beneficial exercises listed in the 'Exercises' section of APK
http://www.americanparkour.com/content/category/5/25/386/

Specifically targeting abs (and your core), I would recommend:
Quadrupedal Movement
    Basic, Gallop and Ground Kongs
Knees to Elbows
L-Sit
Windshield Wipers (Obliques)
Handstands

Many parkour conditioning drills will develop multiple sets of muscles, like your core, and help toward a goal of practical strength, not just aesthetics!

On a side note: pull-ups and climb-ups and are always another good thing to practice!

nice man. ill do these. in answer to Chris, because its the only abb exorcise that i know of. i was hoping to get suggestions on new abb exercises

Forget the crunches, seriously! Also, don't be fooled by those phony videos that say "GET A 6-PACK IN 3 WEEKS FROM DOING ___ 6 MINUTES A DAY!1!!1!"

If you want proper abs, clean up your diet, period. Abs are something along the lines of 90% diet (I may be a little off) and the rest exercise and genetics. So just eat healthier, and blend that with enough activity and you will be good.

Although, I see no reason to have a six pack. It proves nothing at all besides looks. I mean if you get one, all the better to you, but I don't believe it's a goal you should necessarily be striving for. Don't you want to be stronger and faster and now just look like you are?

I have somewhat of a 8-pack but I still am a pretty weak guy...

Try holding a 45lbs weight in front of you at arms length.

My back used to hurt when I started doing them but after a while it stopped. Mostly it hurted because I was doing them in the floor but then I used a mat and haven't got any problems since then.

Seriously? Crunches don't do anything? I've been doing sit ups, leg lifts, and crunches for about 2 months now. If that doesn't help me then what can I do?

I definitely wouldn't recommend this.  This would put you way over the healthy limit for stress on a disc and could potentially lead to disc herniations in the future which are no bueno.  When you hold out a weight at arms length it is something like 20 times its original weight on your spine.

You're doing abs work and your back is hurting?

This means your hip flexors are engaged and tight, pulling on your lumbar spine.

Stretch out your hip flexors, and also DO CORE EXERCISES FOR YOUR LOWER BACK. Do not allow your back to arch during abs exercises, and cut a few of those out.

Crunches are useless so get rid of those. SItups should be replaced with someting else. Leg raises are fine.