XYZ Switch/Diff lock complication

Firstly, thanks for welcome messages after my first post on this - OR any other - forum

I have a 2004 discovery 2 TD5 Auto, verified genuine 61k miles. My fourth Discovery since 1995, but my first Auto. Shortly after purchase I discovered that after any long run and after engaging reverse, the 'M' and 'S' green lights would start to flash, as did the gear indicator in the odometer. It cleared every time, simply by turning off and restarting, which the handbook said was a possibility. Initial diagnostics showed no fault as the lights were out when it was tested, but another diagnostic by a really good auto box expert showed "historic" faulty signals to the gearbox ECU, diagnosed as a faulty XYZ switch, which I gather is well known.

That diagnostic showed no other faults, and I have NEVER seen any other warning lights - just the 'two greens'

The car is booked in to a specialist for a new switch tomorrow, because sadly a recent injury has limited my ability to swing spanners under a car, and as this one has a catalyst, dropping the exhaust is a must, but beyond my capability. I also decided to have a new switch fitted because reworking the old one (well within my capabilities) would take time when the car was isolated on a ramp

The complication came this weekend, when I tried the diff lock when stationary - and the same lights didnt just come on, this time they stayed on and couldnt be cleared with the re-start process. I was also locked in 3rd gear, so "limp home" mode applied. ( I suspect this was always the case, but had never tried moving forwards before)

Based on various readings on this and other forums, I tried moving the gear selector vigorously to clear it, with no effect.

I then tried engaging/disengaging the diff lock with the same vigour(!)- still stationary and in high range - and it suddenly cleared itself.

I am advised there is a connection between diff lock switch and engine management, routed through the xyz switch, so intend to proceed with the change of the xyz switch anyway, but would welcome any ideas or experience gained if others had the same symptoms.

I have tried to give as many facts as I can, and would appreciate any thoughts that can be offered.

Thanks for your patient reading!-

A couple of things you should be aware of,,
Fault code P705 faulty XYZ Switch
Is not a definitive diagnosis of a faulty XYZ Switch itself but that a fault has been recorded in the gear position monitoring wiring circuits anywhere from battery to bcu to xyz switch to gear box ecu and this triggers the flashing m+s lights and limp mode which can only be cleared by switching the ignition off completely, not by the fault clearing itself.
The xyz switch is part of the gear monitoring system and has no physical connection to the high / low or diflock systems.
If I were you I would get the fault codes read again as there are 17 or so causes of flashing m+s lights and limp mode.

Sincere thanks - you have confirmed just what was concerning me. I will have them read again.
The 17 possible reasons startled me though.. You are clearly an expert, and am reluctant to trouble you, but could I ask what diagnostic tool you would use to do that, i.e. how do I get closer to the ACTUAL cause!

Thanks again - very much appreciate your help

How good is your battery, I used to get M & S flashing occasionaly and reset by turning off and on. Then ended up TPS sensor fault (among others), local indi said faulty xyz and TPS. After fully charging the battery all the faults cleared. Even though the battery had enough to start the engine at least 10 times the voltage was a bit too low for the ECU's.

I am certainly not an expert but I had an intermittent fault in gear position monitoring system which was very difficult to track down taking several months during which I read up everything I could about the system so have a basic understanding of it and little else to do with automatic transmission.
To see what causes the flashing m+s lights and limp home mode you need a copy of RAVE the D2 workshop manual as used by land rover garages.
If you don't have a copy ,look back in this forum for post of July 12 2019 , land rover discovery 2 1999 v8 haynes book where there are two links that will allow you to down load for free, ignore bit about v8 as it covers both v8 and td5.
When you have workshop manual look up automatic gearbox section 44-19 where you will find a list of fault codes that relate to gear box faults.
As d2 is not obd2 compliant, to read faults you need a dedicated tool, Test book, Hawkeye or Nanocom.
If it was me, I would, if I hadn't had all fault codes cleared when they were last read have them cleared now and have them read when the fault re-occurs that way you can be sure that you are dealing with current and not historic faults.
One thing that puzzles me is I understood that a D2 did not have the facility of diflock even thought the sticker shows it but uses traction control instead.

If the CDL engagement disturbed the XYZ switch's inputs it's most probably chaffed wiring where the harness is clipped to the top of the transfer box(that's the most common) cos the CDL circuit is within the same loom with the gearbox circuit and if there's a short between wires it gets an earth when CDL is locked and mixes up the XYZ inputs.

Again, thanks to all for taking the time and trouble.

I am having a re-diagnostic done today and will go from there - but with the wisdom of all this input! Just to answer a couple of points made, though, system voltage at the cigar lighter is 14.2 with the engine running, and on all but one of the occasions it occurred after a long run and before switching off. That said, I havent measured it at the BCU plug, and will do ig it persists. The Diff Lock puzzle is answered by the fact that it was reintroduced towards the end of the production run - around 2003 I believe - in response to constant complaint about removing it. I have personal experience of life without it on my 2000 yr manual, which relied on traction control. On one occasion when all 4 wheels lost traction it actually applied brakes to all 4 wheels!

I suspect this could take a while to resolve but will advise when it is. Thanks again in the meantime.

Oh, and can anyone advise the easiest way to access the transfer case "p" clip to examine it? Thanks

You have to remove the central console and follow the loom down toward the boxes

The most common problem is around that clip at the middle of the pic(that's atached to the TB) and as you can see the two small female spades at the left(for CDL) near the XYZ switch and solenoid connectors are coming through that loom from the other connectors which are under the console so if there's a short in the wiring when CDL is selected the circuit gets an earth which can mix up things in the XYZ inputs hence the oddity with M & S + CDL

Brilliant! Many thanks again

Sierra rides again!

Well I think I should at least provide an update after all that help....
The xyz switch was changed for a genuine LR part by a Gearbox specialist, who had first rechecked with his (autologic) diagnostics again and found the same archived record of missing switch input, or words similar to that, which he felt was confirmation of the xyz switch. He looked a bit doubtful when I suggested he checked the loom over the transfer case, but he did anyway, and reported that it moved freely with no signs of external chafing to the heat proof loom at the point where it was on the "P" clip.
To his credit, he also tested for 40 miles+ after changing the switch, with lots of R-N-D movements, but without any problem. Since then I have done over 100 miles with lots of P-R-N-D, and no problem at all. I have used High and low ranges, but I have avoided using the diff lock at this stage since I want to do several hundred miles without incident so I can be absolutely clear that the diff lock is the "trigger" if (or when!) those two damned green lights reappear.
Given that it was failing every couple of days, without any diff lock input, before the switch was changed I am concluding that it WAS faulty, but am not assuming that replacing it was a total fix - yet!
In the meantime, MANY thanks again for the input from all, Sierrafery in particular for taking the time to send images, etc. My first experience with forums has been brilliant and I just hope I can return the favours at some point
Watch this space......

sierrafery said:

The most common problem is around that clip at the middle of the pic(that's atached to the TB) and as you can see the two small female spades at the left(for CDL) near the XYZ switch and solenoid connectors are coming through that loom from the other connectors which are under the console so if there's a short in the wiring when CDL is selected the circuit gets an earth which can mix up things in the XYZ inputs hence the oddity with M & S + CDL

Click to expand...

Hi sierrafery,

referring to the 2 female spade connectors on the left of your picture, are they redundant on some vehicles? As i have just removed and repaired my loom, (it had rubbed on the sharp edge on top of transfer box) however there appears to be nothing to connect them to. They were not connected when i took the loom out. My vehicle is a 2004 td5 with factory CDL.

Thanks.

Those are for the diff-lock switch

My diff lock light still comes on, when engaged.

Then those are unused plugs which were supposed to be for the transfer box temp sensor(not fitted on UK specs)

Can I ask if the MIL light came on? If not the diagnostic chart in RAVE (44-19) would point to inadequate battery voltage. All other faults appear to give an MIL light as well.

sierrafery said:

Then those are unused plugs which were supposed to be for the transfer box temp sensor(not fitted on UK specs)

Click to expand...

Ok, thank you for the response, I can get on with reassembly now.

Si Click said:

Can I ask if the MIL light came on? If not the diagnostic chart in RAVE (44-19) would point to inadequate battery voltage. All other faults appear to give an MIL light as well.

Click to expand...

I had the green m & s lights on with flashing gear change lights and blinking lcd display on speedo, put it down to a pattern of wet conditions, as it only happened after rain or driving through some deep puddles, so took the loom out to check for p clip damage (As advised by sierrafery) but found the wire had chafed through on the sharp edge on top of T box.