On an AMTT coinbox, the dipswitches on a Mars acceptor need to have 1,2,3,4,5,8 turned "ON". 6,7 should be "OFF". Power coinbox down, then back up. Try inserting a bill to see if it stacks it. If not, what is the LED on the back of the unit doing? How many times does it flash when you try to insert a bill?
You also mentioned a computer issue. What issue are you having? The 2 digit diagnostics link provided above should be the same as what is in the Owners manual for your carwash.