1984 XJS 3.6 manual cabriolet

Hello All, Happy easter !! My 3.6 has a DAC 3510 ECU fitted ,which should be for a car to 1985. I have a spare DAC 3810 ECU - which is for a car from 1985 onwards. Can anybody tell me if the 2 ECU’s are intercahngeable - BOTH are for the 8 CU system , BUT my car won’t start with the 3810 !
Rgds
Gerald