Do you know for sure your 4.6 BeCm is "staying awake?"
The following is a test I did on my 95 4.0 and the car passed the test.
I got the test from http://www.p38a.co.uk
Battery drain on the P38A can often be caused by the BeCM being kept awake by one of a multitude of inputs. These include:
Alarm RF receiver - anything on the right frequency will cause the BeCM to awake. There are known issues with early receivers being prone to interference, and all but the latest can be kept awake by other equipment on the same frequency band.
Bonnet or door open switches.
Pretty much every cabin switch
To test for sleep mode:
Switch off interior lights (hold central button on light above centre of dash for several seconds)
Open Drivers door window and close door.
Insert Ignition Key and turn to position 2
Turn to off and leave the key in the barrel
Open the drivers door and the key-in message will be displayed on the instrument pack
The interior lights should still be off.....
Wait for 2 minutes (sometimes closer to 2 minutes 30 seconds) for the BeCM to go to sleep.
If you can still see the Key-In message then the BeCM is awake.
If the message has gone, and you do something to wake the BeCM (try opening the bonnet), you'll get a bleep from the dash. This indicates that the BeCM has been awakened.
If you do this with an ammeter in series with the battery lead you'll see the current draw around 0.6 to 1A when the BeCM is awake and more like 0.05A when it's asleep.
Of course, you are now left with *what* is awakening the BeCm The following inputs will wake the BeCM:
The Radio RF receiver is a favourite for picking up interference. More on this here.
Another favourite is the bonnet close switch - make sure the bonnet is closed properly.
Armed with this information you can at least get some idea of whether the BeCM is awake and what might be causing it.