I'd say look for an early (1988-1995) hydraulically controlled A-518. You can wire the OD to engage with a switch or even engage automatically using pressure switches. Be aware that the non lockup units (1988-1992) are much easier to wire through a switch. You just need 1 switch to engage the OD unit. The later units (1992-1996) with the lock up converters would need 2 switches (one for OD, the other for the converter). That would be cumbersome to control with switches alone, so you really would need pressure switches and/or a timer for the lock up converter.