Discovery 2 td5 electric problem

lohr5000 said:

The key learning for me was that there is a separate switch inside the XYZ switch body which immobilises the starter unless the gearbox is in P/N. Do not assume that just because the gearbox selector lights/in dash gear position display correctly show P, that the separate starter immobiliser switch will be in the correct position.

Click to expand...

Hi... you have a good point and thanks for sharing your experience with this cos it's a really intersting issue... the oddity is that the P/N output from the XYZ switch to the BCU and all others(IP, ECM, etc) comes out on a single wire which splits between all and goes to C0663-11 BCU too and that's the only P/N input for it.

IMO there is only one switch for P/N output, my theory is that the switch's alignment must be perfect as all the XYWZ contacts within it to be in correct position... once earth goes out on the P/N output wire from the switch(C0675 - 5) the contact must be tight cos even if the signal's strenght is enough to light up the indication it's not strong enough for the starter relay coil's management too due to reverse voltage drop also all the other contacts must bbe in the perfect position to not be other unneeded output too which can mix up things... i'm not 100% convinced that i'm right but that's what i can think based on the diagram

but i completely agree with your last statement:

lohr5000 said:

DO NOT ASSUME like I did that because the P indicator was lit up on the gear shift / in the dash display,then the starter inhibitor logic built into the BCU will be OK !!!

Click to expand...

i pointed red arrows to the the P/N signal output/input from XYZ to BCU to sustain my theory:

You could well be correct

It just seems strange that Nanocom has a seperate diagnostic for the park/neutral switch to the WXYZ switching.
And a total of nine wires from the XYZ switch, including the earth.

If the 0/1 switch settings shown in reply #35 for the WXYZ and P/N switch are correct then W is connected when the gearbox is in P, whilst the other switches are unconnected.

If switch W was also used to determine when in Park for the solenoid inhibitor, then my assumption is that this wouldn't work as W is also connected when the gearbox is in N and D .

The earth switching for the starter solenoid relay is shown as being within the BCU. So does that mean there is another relay on the BCU which is controlled by the BCU. So not just a straight pass through from the BG wire on the XYZ switch.
I think there must be an internal relay in the BCU as there is mention in your earlier reply about the BCU preventing the starter from cranking if it receives an engine running message from the engine ECU. It would have to open that relay to stop the cranking.

If it is a single switch controlling both the gearshift indicators and the starter inhibitor, then I would have thought the signal strength from the XYZ switch wouldn't make much difference. If it is strong enough for the BCU to drive the logic for the gearshift indicator light outputs, it should also be strong enough to drive the logic which controls the internal BCU relay.



They do say that a little knowledge is a dangerous thing and I fall 100% into that description!!!!!
So please feel free to pull apart my thoughts on this.

The main thing is I am very happy to have got the thing working correctly now as I was worried that someone may have started the Disco in gear by accident. I had temporarily bridged pin 85 on the starter relay to earth to bypass the BCU controlled earthing of the relay.

Interesting to share ideas though and I am so happy that your earlier replies to the OP's question pointed me in the direction of the BG wire and starter solenoid immobilisation via the BCU.

lohr5000 said:

The earth switching for the starter solenoid relay is shown as being within the BCU. So does that mean there is another relay on the BCU which is controlled by the BCU. So not just a straight pass through from the BG wire on the XYZ switch.

Click to expand...

I dont know exactly cos i've never been able to get a BCU internal diagram but i'm sure it's not just a straight pass, there is an electronic switching circuit within the BCU which needs "solid" earth input for relay's management while a LED or the LCD would work with weak earth signal too.