now totally confused - your searchforrecord will only find the first record on unitid and you have just assigned what we advised to a string called swhere and is not used in your searchforrecord and you have ignored Ranmans additional code.
Further your searchforrecord is searching on a field called unitid and your swhere is referencing a field called unit. I'm assuming this is a typo
Code:
Private Sub Combo53_AfterUpdate()
Dim sWhere as string
sWhere = "[unit]='" & Combo53 & "' OR [murphycode]='" & Combo53 & "' OR [plant registration]='" & Combo53 & "'"
DoCmd.SearchForRecord , "", acFirst, sWhere
End Sub