For any HAC4 users out there who've experienced this problem, I thought I'd post my solution.
Problem: Went to start recording data one morning and the speed just wasn't registering. Stopped and checked the magnet, fine, wiggled the mount to see if it was connecting, fine. The HRM was working perfectly...very confusing.
Solution: Replace the battery in the bike mount. The HRM receives a signal in the watch itself, so that's why the HR was ok. You should also check the battery in the transmitter on your fork.
