AFAIK a memory clear only works when the machine's backup memory is corrupted e.g. you can't just do a memory clear out of the blue unlike MK5 and later with the setchips (MK4s doesn't use setchips). Playing around with Fortune Hunter in MAME (MAME doesn't actually have the USA version of 3 Bags Full, but Fortune Hunter is virtually the same game with different graphics), it seems impossible to do a RAM clear without the machine actually having a memory error (aka "12 - METERING ERROR").
If you need to do a switch test to see if the jackpot key is actually doing anything, open the main door, switch the machine off, then wait a few seconds and turn the machine back on while holding down the Robot Test button on the board. This will put it into test mode, which has a button test in the menu which also checks the key switches (unfortunately it doesn't check doors/optics, but these types of errors normally show up in the game itself anyway).