Discovery Td5 BCU programming

I've got a 2000yr td5 Discovery that has developed a problem with starting. The motor had a couple of leaking sunroofs so was a bit damp inside.These have been sorted and with the good weather over last few weeks it has dried out . When I came to start it the starter motor came in on the 2nd ignition position instead of the 3rd. After looking at a wiring diagram I found the starter relay was getting an earth from the BCU as soon as the key was in the 2nd position,triggering the starter motor. I checked the BCU for water ingress but couldn't see anything obvious. I gave it a good clean with electrical contact cleaner anyway, but no change. I noticed my electric windows aren't working also but I'm not sure if this is related.
Can I fit a second hand BCU and programme it to my Discovery using my original keys ?

hoth said:

After looking at a wiring diagram I found the starter relay was getting an earth from the BCU as soon as the key was in the 2nd position,triggering the starter motor.

Click to expand...

You should have read the description of the immobiliser too and then you'd knew that what you found about the BCU is perfectly normal. The BCU provides the earth for the relay's coil on ignition 2'nd position when the vehicle is not immobilised(no earth when it's immobilised) but the live is supposed to come from ignition 3'rd(crank) position so if the relay closes when it gets the earth from the BCU on ignition 2'nd position the problem is that the relay's coil gets live then while it's supposed to get it only on crank. The BCU is OK, you'll have to figure out why it's that live getting there on ignition second position, it can be from the ignition switch or a short within the interior fusebox(it used to suffer from water ingress in various ways)...or in the engine bay fusebox...throubleshooting on that path is needed.

Thanks for the information, I will have another look at it.

I removed the interior fuse box and found evidence of water ingress so I stripped the box down and cleaned the circuit board with alcohol on a cotton bud.
The green came off a treat and after reassembly and refitting everything is back to normal.
Thanks again for the advice. The job took about 1 hour and was a bit fiddly getting connectors out and back in but was quite simple.