ABS fault - intermitent

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a109

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Hello,
My dads 2004 landmark discovery has a supposedly faulty ABS unit.

The traction control light comes on, so you have to turn the engine off then back on. It will be fine for a day or so.

Went to a "specialist" who replaced the float valves - still the problem persists. They say we need a new ABS unit - but why does the fault disappear when you restart the engine ? Sounds like an electrical problem to me ?

MOT is due in a couple weeks, this will be an MOT failure if the light comes on during the test. So need to resolve this one way or another.

Cheers.
 
You need to plug in a tester to see what the actual problem is before spending loads on things that may not fix it. A common problem is the shuttle valve switches - actually not the switches themselves but the wiring - that usually gives the so-called 3 amigos. Have a look on the D2 boys club.
 
a109;3173819Went to a "specialist" who replaced the float valves - still the problem persists. They say we need a new ABS unit - but why does the fault disappear when you restart the engine ?[/QUOTE said:
I wonder if the "specialist" actually plugged a diagnostic computer into the Disco? You need to see what the faults are. It may be a wheel bearing on the way out, it may be the shuttle valve switch electrical fault (SWS wiring modification which ian referred to).
Click to expand...
 
a109 said:
Hello,
My dads 2004 landmark discovery has a supposedly faulty ABS unit.

The traction control light comes on, so you have to turn the engine off then back on. It will be fine for a day or so.

Went to a "specialist" who replaced the float valves - still the problem persists. They say we need a new ABS unit - but why does the fault disappear when you restart the engine ? Sounds like an electrical problem to me ?

MOT is due in a couple weeks, this will be an MOT failure if the light comes on during the test. So need to resolve this one way or another.

Cheers.
Click to expand...

it's not a very common fault but it happens, there might even not be faults logged in the ECU... provided i understood well and it's not about 3 amigos but only the TC comes on while driving and the car brakes by itself and becomes undriveable

forget about that "ABS unit" for now albeit i'm not sure what they(or you) mean by that , is it about the ABS(slabs) ECU or the modulator? ... if it's about the ECU it's possible to be at fault but before going so far some things must be checked

get it plugged in a proper tester and see if you get signal fault from one of the sensors, if there is something like that check for play in that hub and replace it, or check the sensor's voltage output with tester and if you find one different/lower than the others there is the problem

if no faults logged in the ECU unplug each sensor and measure theyr resistance with ohmeter... there must be between 950 -1100ohm, it's possible to find them all in range(that's why you dont get the 3 amigos) but to find one with a bigger than 100-140 ohm resistance than the other on that axle...in this case you dont get 3 amigos cos it passes the self test but there is the chance that the ECU "considers" that one of the wheels is slipping due to the output difference and activate TC to put brake on that wheel, then the difference being very small the output from the other wheel suddenly gets too high and puts brakes on the other wheel and so on untill the car becomes undriveable as it brakes by itself

for the system to be well balanced and work perfect it's important that the sensor's resistances to be very close to each other and to be all well seated in the hub to not be output differences at the same wheel speed

this happened to me too and found completely different resistances between sensors...bit the bullet and bought 4 wabco sensors and fitted them all... and that was the cure, even bought a spare ECU myself based on other's advice but i dont mind to have one prepared

to make it driveable untill you fix it unplug one sensor from the front and one from the rear and drive it so... you'll get the 3 amigos(and maybe the red brake light too) but the TC will not kick in anymore
 
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i can't understand these guys who just butt in to ask then dissapear... i made a list to myself with those who asked , got the answer then didnt come back to say the outcome........... they'll be back at a point cos the D2 is squeamish enough for that... then i'll ignore them as well:rolleyes:
 
Its very easy to tell a new, genuine forum member. They will usually introduce themselves in some way or another and their post count gets to 10 quite quickly.

However, I can see how regular members who have learned about LR vehicles tgrough a process time, effort and expense, who then are happy to share thier knowledge with said, " new members ", get the hump when the, " new member ", then just takes the FREE information, probably acts upon it, ( undoubtedly saving money on diagnostics ! ), and doesnt have the decency to report back with an outcome let alone a thankyou.

I will quite happily beg the OP,s pardon if this turnes out not to be the case.

For all its p1ss taking the LZ has a MASSIVE wealth of infirmation for the Landrover enthusiast !!!.
 
In fairness it's only been 4 days since the OP, but I agree it's not good manners not to finalise a post one way or another in order for people to see the final solution to a question that may be relevant to them.
Griff
 
to make it driveable untill you fix it unplug one sensor from the front and one from the rear and drive it so... you'll get the 3 amigos(and maybe the red brake light too) but the TC will not kick in anymore
Click to expand...

Yeah, Um NO!

You do realise that these are,

1. Wheel speed sensors. Input is summed to provide information for Engine ECU, SLABS ECU, Auto ECU & BCU
2. ABS sensors.
3. TC sensors.

In short, unplugging will result in incorrect information, no wheel speed signal coming from two wheels on different axles and on opposite sides. This will make the SLABS unit think that the two wheel with signal are slipping and apply the brakes (activate TC) until the speed equalises, which will never happen. Alternately (or in parallel) the ABS will see two wheels "locked" and so not allow use of the brakes on those wheels. So if he does get it moving have fun stopping. The speedo may bot work (nor odometer).


To all, Sierrafery has been peddling assorted "fixes" on assorted sites claiming deree or better electrical knowledge. The last one was a bypass of the ABS Modulator to trick the SLABS ECU into seeing a correct signal without input from the modulator at all using the brake light switch as an input and relays and resistors. All to avoid what we all know as a simple fix, the option B.

I implore you to avoid these as at least you'll have a repair bill, midway a modified braking system no insurance company will pay out on in an accident and at worst someone will get hurt.

I know this is a lot and a first post but this info needs to be out there.
 
D2Au said:
I implore you to avoid these as at least you'll have a repair bill, midway a modified braking system no insurance company will pay out on in an accident and at worst someone will get hurt.
Click to expand...

it's the same like when you drive it with the 3 amigos on when one or two hubs have play in them...the brakes are working normally just no ABS related actions

see the last paraghraph of my post:
to make it driveable untill you fix it unplug one sensor from the front and one from the rear and drive it so... you'll get the 3 amigos(and maybe the red brake light too) but the TC will not kick in anymore
Click to expand...

despite you i don't think that everybody else is an idiot and presumed that the man will know that with 3 amigos on the ABS will not work well.... as i said this thing happened to me too and that was the cure to drive it untill the garage, unplugged two sensors and it was driveable...then replaced all with wabco ones and it's been ok since...why do you all think that the essence of all knowledge is stuck down under?

This will make the SLABS unit think that the two wheel with signal are slipping and apply the brakes (activate TC) until the speed equalises, which will never happen. Alternately (or in parallel) the ABS will see two wheels "locked" and so not allow use of the brakes on those wheels. So if he does get it moving have fun stopping. The speedo may bot work (nor odometer).
Click to expand...
with two sensors unplugged the SLABS "knows" that the fault is present and disables completely the TC and reduces the ABS...that's in real life, if you dont believe that try it yourself on a field where you cant do an accident then

you'll be amazed that i helped a lot of people around here with my idiot solutions and nobody got hurt
 
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D2Au said:
Yet.........
Click to expand...

So we should listen you why?? :confused:

You have one post and start sprouting about a system that you don't understand. If you had ever worked on a Disco 2 with an ABS fault you would know that disconnecting just one of the sensors would disable the system but the car is still drivable.

Serriafery is one of the most knowledgeable people on site when it comes to the discovery 2 and TD5 engines so I think I'll take his advise before asking a 1 hit wonder.
 
and i hope it'll never happen cos that's not my intention, i must insist on this statement
This will make the SLABS unit think that the two wheel with signal are slipping and apply the brakes (activate TC) until the speed equalises, which will never happen. Alternately (or in parallel) the ABS will see two wheels "locked" and so not allow use of the brakes on those wheels. So if he does get it moving have fun stopping. The speedo may bot work (nor odometer).
Click to expand...

The ECU will see two "open circuit" faults on the sensors and trigger the 3 amigos.

1. how would the TC kick in as long as it's disabled by the default mode for wheel speed sensor open circuit

2. the speedo works wit only one sensor connected

i wouldnt have tell such thing without doing it myself first, but these discussions are good and i'll update my signature accordingly
 
I have to say that Sierrafery is one of the most knowledgeable people that I know about the D2 and has saved me a fortune over the 15 months with his and other peoples views and experience. Please ignore the comments from the newbie. Thank you Sierrafery for all your help.
 
I agree. At least he shows his thoughts and how he comes to his conclusions rather than just making blanket statements. I've had a huge amount of help from him.
 
as here is the place where i was attacked i must say that i remember a statement of a guy who named me idiot from that thread which was erased on aulro forum(thanks to the moderators), something like that(quoting from memory so it might not be 100% accurate) :
"the SVSs are working through a huge range of resistances..... the ECU modulates (i dont remember what) based on SVS input...." something like that

i can't go there to be alone against all odds and to be told to ''shut up and learn" so i cropped and edited with red the relevant part from an official WABCO document which shows the SVSs and maybe i'm really idiot but i can't see more that 3 scenarios: both closed(brakes on), both open(brakes off), one opened/ one closed(which IMO is for diagnostic purposes for the case that one valve is sticking as i got 3 amigos when i simulated that)
SVSSWITCHES.jpg
, ,

if somebody wants to see the whole document it's attached, not one word about SVSs there, the description of all the functions are referring to ABS sensor inputs and inlet/outlet solenoid valves ...as about the huge importance of those switches

as i always want to learn can somebody here(which is a more friendly place) explain me for what else than brakes on/off signal are those switches used by the system... i'm speaking strictly about the shulttle valve SWITCHES not about the VALVES knowing the only statements from RAVE about these switches:
70-14 DESCRIPTION AND OPERATION said:
Passages within the ABS modulator, separated into primary and secondary circuits, connect to the various internal
components that control the supply of hydraulic pressure to the brakes:
l Shuttle valves and non return valves control the flow through the internal circuits.
l Shuttle valve switches, connected in series to the SLABS ECU, provide a brakes on/off signal.
l A damper chamber and restrictor are included in each circuit to refine system operation.
l Inlet and outlet solenoid valves control the flow to the individual brakes.
l An expansion chamber is connected to each circuit to absorb pressure.
l A return pump is connected to both circuits to provide a pressure source.
The ABS modulator has three operating modes: Normal braking, ABS braking and active braking.
Normal braking mode
When the brake pedal is pressed, pressurised fluid from the master cylinder assembly moves the shuttle valves to
open lines 'A' and close the shuttle valve switches. Pressurised fluid then flows through the open inlet solenoid valves
to operate the brakes. The closed shuttle valve switches supply a brakes on signal to the SLABS ECU. If the SLABS ECU determines that EBD is necessary, it energises the inlet solenoid valves for the brakes of one axle. The inlet solenoid valves close to isolate the brakes from any further increase in hydraulic pressure.
Click to expand...

that's all no other reference about them, other that a table with the resistance limits within they must work

i'm really just trying to find out things, nothing else:rolleyes:
 

Attachments

  • WABCO_ABS_unit.pdf
    1.2 MB · Views: 311
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Thanks for all the replies, I have a busy week ahead by the looks of things!
I will look at the wheel speed sensors first and see what happens.

This is probably one for the main dealers ?
 
D2Au said:
Yet.........
Click to expand...

ever! ;)

Plenty of cars in the world run safely without ABS still. No problems.....

I reckon most if not all the manufacturers handbooks for ABS equipped vehicles specifically state not to rely on the system - it's up to the driver to avoid the accidents, ABS won't stop you quicker etc. etc.

I am on the learning curve for the D2, but the likes of SF have provided a huge amount of information which is greatly appreciated.
 
a109 said:
Thanks for all the replies, I have a busy week ahead by the looks of things!
I will look at the wheel speed sensors first and see what happens.

This is probably one for the main dealers ?
Click to expand...
A main dealer solution will be very expensive because of the stupidly high parts costs that Tata are forever increasing...Plus main dealers labour rates are usually much higher too.
A decent LR independent is probably the best bet as they should be able to offer more choice in terms of decent aftermarket parts and also be aware of workarounds such as the shuttle valve mod.
If you do choose to do it yourself,most garages should be able to diagnose it for you for a reasonable cost,and now even a Snap On scanner is more than capable of diagnosing Slabs faults on DII's.Just be careful where you buy parts from though,I have seen e bay hubs which fitted perfectly,but had the wrong number of teeth on the reluctor ring - caused some very strange faults with the TC coming on at 60mph...
 
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