My door opener was growling and shuddering pretty often so I checked the carriage and it was seriously worn.  This is an older door opener (ISD990) and uses magnetic switches. My other door opener, also a Genie (H4000A), is newer and uses the lever type switch, it has no magnet.  Other than that they are interchangeable.  So I took the H4000A carriage (without a magnet) and glued a small magnet to it.  It worked perfectly on my ISD990 door opener with the magnetic switches.  Since they are so inexpensive I bought 2 of these replacement carriages, #36179R, and put one on each of my doors.  In a nutshell, this new carriage will work on both the magnetic switch openers or the lever switch openers.  You may have to readjust the location of the switches but that's a snap.  I could not determine any difference in the 36179R and the 36179R.S other than the '.S' is apparently a replacement for newer magnetic switch units, but, do your research first.