The "three amigo lights come
Excellent article on the ABS problem by John Robison at RoversNorth .....
Welcome to the techie column for the Fall edition of
the Rover News. In this column, we’re going to look
at some of the common problems with the antilock
brakes on Discovery II models. The Discovery II electronic
braking system, called SLABS (self leveling anti
lock braking), is made by Wabco of Germany. Wabco
is a subsidiary of American Standard, a company better
known to the public for toilets than brakes. In the
automotive field, Wabco specializes in braking and
suspension systems for trucks. According to the company,
two out of three commercial vehicles with
advanced braking systems are equipped with Wabco
products.
The Land Rover system includes four-wheel
antilock braking, hill descent control, and four-wheel
traction control. The SLABS control unit also controls
the self-leveling suspension, if the vehicle has that feature.
The Discovery air suspension is also a Wabco
product. As an aside, Wabco air suspension is also
found in the new Audi A6 and the Mercedes CLS.
One of the most common ABS questions I
hear is, Why do I see the ABS, Traction
Control, and Hill Descent lights coming on?
All three of those systems share a common set of
core components. The wheel speed sensors, the hubs,
the modulator, the controller, and other parts serve all
three systems. So a fault in any one of them will cause
a problem in the other two. It is actually rare to have
a fault that would only disable one of the three systems.
99% of the time, if one is affected, they all are.
To see what’s wrong, you will need to connect a
Land Rover test system and read the faults. These systems
are not OBD II compatible, so a generic scanner
won’t talk to them. At Robison Service, we use the T4
or Autologic tools for this work.
The most common faults are wheel speed
sensor faults. The wheel speed sensors in a Land
Rover are coils that sense the motion of a toothed
wheel that’s a part of the wheel hub. The rotation of
the wheel induces a sine wave signal in the sensor
whose frequency is proportional to the speed, and
whose amplitude increases with speed from 0.5 volts
to more than 5 volts.
If your Rover has a speed sensor fault, there are
two paths to repair. The first is to replace the entire
hub on the affected corner. This is the approach
favored by dealers because the toothed wheel – called
a reluctor ring – and the actual sensor are both part
of the hub. The reluctor can get damaged by rust or
corrosion, and it can also get damaged by a bad wheel
bearing. The only way to service it is to change the
hub.
As of this writing, hubs (front-RND646 / rear-RND694)
cost around $400 and take about three hours to
change.
The sensor can be removed from the hub fairly
easily. If you remove your sensor and look inside you
should be able to see if the reluctor ring is damaged.
The reluctor ring can get damaged if the wheel bearing
gets loose. It can also get damaged by corrosion.
That’s especially true for Rovers that run on beaches.
If you see reluctor ring damage, or corrosion, or if the
hub has any free play at all – you need a complete
assembly. If there is no damage, you may be able to
fix the vehicle by changing the sensor (front-RN292 /
rear-RNH293) alone, a $100 part that’s less than an hour
to swap.
The path you choose should be determined by
examination of the reluctor via the sensor hole. If the
hub looks good, there’s an “8 or 10” odds that a sensor
alone will fix your problem.
Every now and then you will see a Rover that has
wiring problems, usually at the connector between ABS
sensor and body. Always pull it apart and look for
corrosion.
The next common fault in these systems
is called shuttle valve failure. The shuttle valve
is a part of the brake modulator – that big thing in the
location where a master cylinder would be. The modulator
incorporates the functions of an ABS servo and
a brake master cylinder into one unit.
If you have shuttle valve problems, you will see
the three warning lights on the dash and there will be
one or more stored faults for shuttle valve failure.
Land Rover has a test procedure to determine if these
faults result from a failure in the modulator or if they
are caused by wiring troubles in the ABS harness or
grounds. Unless you have corroded grounds and
cables, your trouble is probably in the modulator.
Until now, this problem was addressed by
replacement of the brake modulator (RNH082). That’s a
$1,500 part. As you can imagine, shuttle valve failure
produced a lot of unhappy owners and Land Rover
finally listened up and developed a fix.
As of March 2006, Land Rover sells a shuttle
valve repair kit for under $100. You will have to
remove the modulator and flip it over to install the
valves on a workbench. Removal of the modulator,
replacement of the valve, and refit to the vehicle takes
three hours or so.
This shuttle valve repair is a huge improvement
over the former method of addressing this problem.
The part number for the repair kit is (SW0500030).
If you buy it from a dealer you may also want to ask
for the March 2006 bulletin that gives test and installation
instructions.
Another common problem is a mushy
brake pedal. In my experience, the only explanation
for a mushy pedal is improper bleeding procedure.
Bleeding a Discovery II takes two people and the Land
Rover test system, and it takes the two of them a bit
over half an hour. You need the tester to operate the
pump and valves to make sure all the air is purged
from the modulator.
If you are paying for this service, expect a labor
bill in the range of one and a half hours and $20-30 of
brake fluid. If you are not at a dealer, make sure they
use the correct Castrol LMA fluid. Don’t even start this
process unless the shop has a tester to run the pump
and valves. You could bleed brakes in the field without
one in an emergency, but there is no way to get a
really good pedal without cycling pump and valves.
There is no shortcut for this job. You need two
people and the Land Rover tester.
We see quite a few stop lamp circuit
problems. The usual way this problem manifests
itself is a truck that won’t shift out of park. Discovery
II models have an interlock that prevents shifting out
of park unless the brake is pressed. So, if the brake
light circuit fails, the car won’t go into gear.
If that happens to you, the first step is to check
the stop lamp fuse. We’ve seen several trucks where
the stop lamps were fitted wrong, or the contacts corroded,
and the fuse blew. Also check the trailer connector,
if your Rover has one. A short there can pop
fuses.
If the fuses are good, you should check the stop
lamp switch. It’s located above the brake pedal. If
you are stuck somewhere, it is possible to get out of
park by jumping the switch temporarily with a paper
clip.
Finally, you should check your Rover to
see
