I need a little help with the autopilot function. I have no problem loading the FMC and IRS, and I am able to get the autopilot to function normally and fly an entire flight plan with the LNAV and VNAV functions. However, I can't seem to get the autopilot to do a simple task such as heading or altitude deviations.

For instance, I am cruising and receive a descent from ATC and a heading to initiate my approach. I input these into the autopilot and...nothing. So I turn the autopilot off, make the deviations myself and turn the autopilot back on and it just wants to go back to original heading and altitude. This has become extremely frustrating.

Does anyone know of any detailed manual for use of the autopilot, or any other answers to solve this issue? All I need to know is how to get the autopilot to follow my simple requests for deviations from the FMC.

P.S. Turning the LNAV and VNAV switches off only seem to disable the autopilot completely?