coin changer/bill vaidator issue

I built a machine that vends bundles of firewood.$5 vend, no change, accepts $5 and $1 bills only. 110 volt.About a 6 weeks ago the bill validator would not accept bills(mag pro 30 sa). It is connected to a Mars tcr 6800 coin changer. Both coin changer and bill validtor were bought used. I sent the mag pro to Coin Co and had it repaired. They cleaned the sensors and replaced a wiring harness. When I got it back and reinstalled, same problem, bill validator would not accept bills. I brought the coin changer to a fairly local company and had it bench tested. They said it was good.Reistalled coin changer, same problem, won't accept bills. Double checked all wiring and relays, no luck. Found a guy the next state over and towed to him. He determined the bill validator was the problem.Bought a used Mars 2511 bill vaidator from him, installed and now the machine vends. However each time the machine vends the coin changer dispenses coins twice. Everything else works perfect. Any thoughts? Thanks John