Diagnosing M+S light / XYZ switch issue

[Geir Tore Simonsen]

As I wrote in my introduction thread, I bought a 2002 Disco TD5 that sometimes get the M+S light with a P0705 code. It happens easily when it's raining, not so much on a warm and dry day.

The fact that the XYZ switch has been changed according to previous owner, and I get the P0705 code is already a strong indication that there is a problem with the wiring harness. But by principle I'm trying to be as certain as possible before I start taking things apart or replacing parts.

On this picture I have hooked my Picoscope up to the WXYZ signals from the switch, at connector 0678/0728.
https://www.aulro.com/afvb/images/imported/2016/06/368.jpg


I also did compare the signals from the scope with live data in Nanocom, to identify which wire is which signal: https://youtu.be/O9xz0tTr__M


This waveform is from a capture when it had been raining. I drove through a small puddle hoping to provoke the limp mode, and at about the -1m20s mark, it is clear that the high signals get pulled to ground.
https://www.aulro.com/afvb/images/imported/2016/06/369.jpg


Zooming in it is obvious that all the high signals are connected inside the switch, as they all have the same waveform. But I'm wondering where the resistor is in this circuit. According to the wiring diagram, there is nothing but wire between the fuse and switch. The fact that the fuse doesn't blow when the signal gets pulled to ground suggests that there is some kind of resistor inside the switch.
https://www.aulro.com/afvb/images/imported/2016/06/370.jpg


Another test drive, this time only a small glitch at -15minutes. The transmission did not go into limp mode.
https://www.aulro.com/afvb/images/imported/2016/06/371.jpg


The reason nothing happened is probably the duration. I think i read somewhere that it ignores any signal that is of less than 30 or 33 milliseconds duration, this glitch only lasted 3 milliseconds.
https://www.aulro.com/afvb/images/imported/2016/06/372.jpg


So, I'm pretty confident that the wiring on top of the transmission is damaged. I've tried searching for the best way to remove and install it, but haven't found any good howto yet. Some have suggested getting at it from the console inside the car, some have mentioned doing it from underneath. I'm leaning towards removing the exhaust and maybe the propeller shafts to get better access, but I also found out that the arms of my portable two post lift doesn't quite reach where I want to put them on the frame. Not too eager to lie on my back under the car more than necessary, so I'm considering moving one of the posts closer to the other one.

[PhilipA]

Once you work out how to remove the console, I consider it the faster way.
The only difficult part is removing the Tbar and hilow stick.

If the trans is left in Neutral and the clip and nut removed from the XYZ switch, then reassembly should only involve checking that the Tbar light and dash light agree.
You have to drill out about 14 rivets.

The reason I say this is that the transmission loom goes through a steel loop which is located on the RH side of the top of the transmission and would be IMHO very difficult to access from below.

Although I noticed my loom had worn on the LH top where it rubs against a bolt head. You may fix it just by slipping some tube over the loom across the bolt head, but this would be trial and error as it could be fretting on the steel loop..

Regards Philip A
BTW I am extremely impressed by your diagnostic work, although in this case the time may have been better spent by just having a look at the loom, as it is AFAIK a known fault if not all that common.