3 channel vs. 4 channel rear what is the difference
 

I have a rear out of a 4th gen. It was in the 83 Firebird I am parting out. How can I tell what I have? Where would the abs sensors be mounted? I don't see any unless it was removed? Is there something internally that would let me know what it is?

4ch rears have holes in the brake caliper backing plates where the abs sensors(2) mount in on each side of the rear. Those sensors pick up readings using a cog style ring pressed on the axles.

3ch rears have a hole in the top of the differential housing, above the gears, where a single abs sensor mounts and that picks up its readings off a ring on the carrier.

Also, you should be able to use 3ch axles with 3ch brake backing plates in a 4ch housing. That setup would result in no signs of a place to mount an abs sensor. You would have to look for parts numbers then and ensure its a 4th gen rear by measuring the width (4" wider than a 3rd gen rear).

Thanks. It's definitely a 4th gen rear because it is wider than the 3rd gen rear I put back in the car. Somebody asked me what it was and I wasn't sure since I never had a 4th gen.