R

ROY SIMPSON

Active Member
Disco 2, 2002 TD5 Automatic, 5 seat pre face lift
Intermittent fault occurring after 60 miles plus driving in dry conditions or 30 miles in the wet.
Fault, flashing m+s lights, limp home mode, no autographics or digital gear selection display.
New battery
New xyz switch (genuine land rover part)
New injector harness
Tried replacement eat ecu no effect
Nanocom fault code P0705 position switch monitoring
IDM 13.7v
BCU switch power 13.6v
BCU relay power 13.6v
BCU shows off for xyz inputs for all gear selector positions
EAT ECU shows open for all xyz inputs
EAT ECU battery 13v, does not drop when fault occurs
Is it possible for the BCU to cut power to the xyz feed ( C0663 pin 10) to protect itself in the event of a short circuit in the wiring harness or is it just a case of testing all the circuits to find the fault.
Any advice would be appreciated
 
Make sure there's not some bad contact or corrosion in the XYZ switch's connector ... did you actually measure for voltage on C0663 -10 and no feed there?, if you have feed there but not at the XYZ switch check at C0681-3 too and verify the switch's earh to be solid
 
Thanks for your reply.
XYZ plug clean and no signs of corrosion.
Stripped out centre console and passenger side glove box to expose as much of the internal wiring as possible but after driving round for2 hour no fault occured, so unable to verify readings you suggest.
Will try again tomorrow.
Many thanks for your interest.
 
As long as the fault is not present the readings will be good off course... that C0681 becomes suspect in this case
 
Fault occurred after dark today so unable to check C0663-10 but

Fault showing, flashing m+s lamps, no autographics or digital gear display.
Turned off ignition and then back on to position 2, cleared fault code P0705 with nanocom, m+s lights went out autographs and digital display came back on.
Without starting the engine selected P,R, N,and D which showed correctly in atuographics and digital display and 13v registered at C0681-3 however when engine when fired up and running, P, R, and N display correctly when selected but when D selected, it flashed twice then original fault reoccured with no voltage reading at C0681- 3
Going to take it to auto electrician tomorrow if I can 're create fault and ask him to check C0663- 10.
Earth to xyz switch is fine no problems P or N and reverse works fine
Many thanks for your advice and any thoughts you have would be welcome
 
Update
No power at C0663 pin 10 when fault occurs.
Auto electrician then powered system from battery through tester and all gear functions available.
Suggested fix is to power system through independent fused feed bypassing BCU and see what happens.
Question
Are there external influences that would cause the BCU to cut power to the xyz switch or is it a fault in the BCU's internal relay.

Any reasons not to try the suggested fix.

Long term a replacement BCU is probably the answer or does anybody repair them.

Any advice would be most appreciated
 
ROY SIMPSON said:
Are there external influences that would cause the BCU to cut power to the xyz switch or is it a fault in the BCU's internal relay.
Click to expand...
this issue needs more investigation especially that it's intermittent, a BCU electronic relay internal fault should be permannent unless there's a dry joint within it which is not uncommon... you may try that direct feed thing and see how it goes, in the worst case you'll get M+S warnings this way too with other fault code, anyway use an ignition feed with max 5A fuse not a direct one and cut that wire to 0663-10 as to not backfeed the BCU cos it can be risky, according to the book the BCU feed to XYZ switch is dependant only on ignition status so go for it

"Gear position switch
On automatic gearbox models, the BCU provides an output which supplies power to the automatic gearbox gear
position switch. The BCU checks for the following inputs before it supplies power:
l Ignition on.
l Auxiliary.
When the ignition is on, the feed to the gear position switch is on. When the ignition is off and auxiliary is off for more
than 30 seconds, feed to the gear position switch is off."
 
Thanks sierrafery for your prompt replies to my questions.
Auto electrician booked for tomorrow to wire fix so will post results once I have been for a long test run.
Looked on internet and Turners advertise as repairing /replacing discovery BCU's, has anyone experience of using them.
 
Update
Auto electrician has wired a fix and it seems to be working but had thrown up a couple of problems .
Not quite sure yet where he took the feed from but appears to be a light green / white wire which probably is the feed to the cigar lighter, which is from the auxiliary feed from ignition switch, which is also the feed, read by the BCU to switch the power on to the XYZ switch.
However when he cut the N/ P ( brown /purple) wire from C0663 to stop a back feed to the BCU this caused the M+S lights to start flashing, so he reconnected the wire on the premise that if the fault occurred cutting the power through this wire, it would also stop any back feed occurring.
The second problem is that with the fix wired into the XYZ feed, the doors will not lock using the key fob, one beep only is sounded., remove the fuse in the fix and all works fine.
Auto electrician also removed BCU, checked the plugs and all looked fine and opened it but could see no obvious faults with the circuits board or relays (2) sealed units.
He is of the opinion that the fault is within the BCU as the two checks the BCU carries out before turning on power to XYZ Switch namely inputs from ignition and auxiliary would throw up other faults as well.
Also I know the auxiliary feed from the ignition switch is working as it shares the same circuit as the cigar lighter which works.
Will update further when locking problem is fixed.
 
That's strange .... tell him to try with feed get from there:
Interrior fusebox live.jpg


and put a blocking diode between that and BCU pin, he'll know what i mean.... if no joy you'll have to try with another BCU
 
Update
Door locking problem now fixed by auto electrician wiring' fix' feed to xyz switch through a relay (on reflection I think he took the feed for the fix from the feed for the electric sunroof not installed, also a green / white wire which would be connected to the vehicle security system).
This seems to have fixed all the problems for now.
However prior to the relay being installed it rained and driving in the wet revealed another twist on the original problem.
Flashing m+s lights
P,R,N digital display and autographs normal
When D selected, digital display and autographs start to flash and limp home mode engaged.
Nanocon showed fault P0705, position switch monitoring.
Without turning the engine off, clearing the fault with the nanocom, and the system returned to normal operation.
Turn off engine, restart, same fault returns and again cleared with nanocom.
Strange behaviour, not an xyz switch or onward wiring problem as P,R,N use the same wiring as D to the BCU and EAT ecus.
Has not rained since relay wired in.
Can live with the problem short term as is easily fixed with nanocom but if it persists will look at another BCU but not sure this is the final answer.
Many thanks to sierrafery for all his help and will post further updates as and when.
 
hey guys, have you solved this issue?
I now have similar situation sometimes... after I drive in Reverse, M + S lights start flashing, and it looks that 1st and 2nd gears are not working, it seems that the car starts driving in 3rd already (due to its rev and power).
If I turn off engine and back on, everything is OK.

note: it happens not all the time after reverse... cannot understand under what circumstances...
Nanocom shows P0705 Position Switch Monitoring for Automatic transmission. No faults for the engine...
 
Yes I eventually diagnosed the fault and an auto electrician wired in a fix.
My problem was that the power feed from the bcu to the wxyz circuits in the switch was failing intermittently caused by a failure in the bcu and was difficult to diagnose as it can only be checked when the fault is actively happening.
If you have access to a nanocom or similar, when the fault is active go into the bcu and look at the wxyz
circuits and if they all read open when you run the gear selector lever through D,3,2,1 then this indicates that there is no power coming through the circuits.
Next look at pin8 in connector 0675 which is the connection of the xyz switch to the wiring harness or co681/ co668 pin 3 located under pinnacle by hand break or co 663pin 10 located at bcu behind front passenger glove box and check for 12 volts as this is the circuit that supplies power to the xyz switch for forward gear selection..
If there is no 12 volt feed from co663 pin 10 it is possible to bypass the bcu by wiring in a fused fix to another supply so long as it is live when ignition is switched to position 2 .
 
sandu said:
hey guys, have you solved this issue?
I now have similar situation sometimes... after I drive in Reverse, M + S lights start flashing, and it looks that 1st and 2nd gears are not working, it seems that the car starts driving in 3rd already (due to its rev and power).
If I turn off engine and back on, everything is OK.

note: it happens not all the time after reverse... cannot understand under what circumstances...
Nanocom shows P0705 Position Switch Monitoring for Automatic transmission. No faults for the engine...
Click to expand...
Hi, your problem is most probably with the XYZ switch itself, there are some links with how to repair it, see which one is more complex: https://www.google.ro/search?q=xyz+...y6gLjUAhWka5oKHYT4CvoQsAQIMA&biw=1366&bih=638
 
Hi again,
yes, I've seen the way to repair the switch, but, before unmount it, I just wanted to check some other options as well...

here is what I noticed:
first, I added 600ml of transmission oil in the auto gearbox. then, I've read that it is not bad to overfill a little the gearbox, so I lifted the car a little, and I added 300ml more into the gearbox...
after-that the problem disapear for a while... today I had the similar situation again but, I had it twice, and second time, staying in traffic jam, the transmission light came on...

I suspect that the gearbox oil change has been performed without filter change (by previous owner), because I only paid the oil, so I don't believe he changed the filter by itself...
do you think that changing oil (&filter) could somehow affect this problem?
 
The gearbox's mechanical side or oil level/quality would not trigger flashing M+S lights whatsoever, the warnings are exclusively for management issues also you have a switch related fault code so concentrate on that
 
If you have a nanocom one test you could carry out, but only when the fault is present.
Look at my previous thread Disco2 xyz switch information needed where you will find a table of wxyz feeds for gear position, read the whole thread as you will see that the table is correct when read at BCU but you need to reverse the order from wxyz to zyxw when you read the results at automatic transmission.
When the fault is present, with the nanocom go into Valero bcu body2 where you can read live data from the wxyz circuits, run through the gear positions and check against table, note any discrepancies, the go into the automatic transmission where again you can read the live data from wxyz circuits and again run through the gear positions ( remember to reverse wxyz to zyxw) and note any discrepancies.
If the same discrepancy shows at both BCU and automatic transmission then this indicates that the problem is either in the xyz switch or wiring from switch to header 0724 .
If the discrepancy shows at t only one location this indicates that the problem is either in the wiring from header 0724 to the BCU or EAT ecu or within the ecu itself.
You should also be able to identify the wxyz circuit circuits that are faulty.
Pins on xyz switch connector
Pin 6 blue/white Z output
Pin7 blue/green Y output
Pin 9 blue/purple W output
Pin 10 blue/black X output

This is not fool proof as the fault is intermittent and may have corrected itself by the time you test but this is not difficult and may help.
 
I love reading about this kind of a problem, it is really interesting. The only trouble I have is that I have no clue whatever is going on, it's way way beyond my understanding. I can only admire the posters who can understand it all, I take my hat off to you.
 
Necessity is the mother of invention.
In my case after six months, endless visits to local indie and car electronics experts, about £1000 worth of bills and no further forward in fixing problem I finally decided to investigate the mysteries of the automatic transmission system.
Rave is invaluable in how a system works.
Wiring diagrams and electrical library for circuits and detailed description and pictures of connectors header etc .
Information sheets on Valero body control unit, eat ecu ( electronic automatic transmission) and engine control unit.
Search relevant threads on here.
Post threads on here.
All the above are available on line with a bit of searching.
Finally and probably most difficult find an auto electrician who will do what you ask if you cannot do it yourself as was my case.
 
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