If a distributor is not keyed for installation with only one orientation, it could have been removed previously and rewired. The resultant wiring would hold the correct firing order, but could change the relative placement of the plug towers in relation to the engine. For this reason, it is imperative that you label all wires before disconnecting any of them. Also, before removal, compare the current wiring with the accompanying illustrations.
On the 3.8L and 5.0L engine's ignition system, the distributor is driven off the camshaft and uses no centrifugal or vacuum advance. The distributor operates by using a Hall effect vane switch assembly, causing the ignition coil to be switched on and off by the EEC-IV and TFI-IV modules.
The 4.6L Engine uses no distributor. The ignition system is the EDIS system, which consists of a crankshaft sensor, ignition module ignition coil pack, the spark angle portion of the Powertrain control Module (PCM),and the related wiring. The EDIS eliminates the need for a distributor by using multiple ignition coils.