Range Rover ABS System Overview
ABS Pump and Accumulator Diagnosis
Wheel Speed Sensor Problems
More Information

Photo at right: ABS accumulator on author's 4.0SE is black cylinder at lower right of picture; pump is directly underneath it.

Range Rover ABS Systems Overview

Since 1990, the various Range Rover models have been equipped with a state-of-the-art four channel Wabco Automatic Braking System (ABS) specially developed to cope with off road as well as on road conditions. The first version appeared in the 1990 Classic and has been refined since then. There were a few basic and Hunter models produced through 1992 without ABS, but all others have had some variation of the system.

The system is the envy of other brands, but personally I find its operation less than reassuring in mud or loose gravel -- you cannot lock the wheels until down to one or two miles per hour, and there is no doubt that stopping power is much reduced. The official manual assures us that this tradeoff is worth the extra "control" you get with ABS operation. Yeah, Right!! Actually as the system became more and more spphisticated this problem was reduced, and I have not heard any such complaints from drivers of the Mk III Range Rover.

The Wabco system does not use the normal vacuum-powered booster found on most vehicles (including early ABS euipped Discoverys), but a more expensive system with an electrically powered high pressure hydraulic pump with an accumulator which stores enough energy for 20-30 brake applications. Part of the advantage of this system is that it lends itself perfectly to Electronic Traction Control (ETC). All that is needed is some electrically operated hydraulic valves in the booster block to apply pressure to the brakes on one or other wheel, therefore transferring torque to the pooosite wheel.

The booster pump is a common (and expensive) failure item.  I had to have mine replaced at about 110,000 miles. An even more common problem is getting error messages on the dash due to the wheel speed sensors getting unseated. Both of these problems are dealt with below.

Troy Wilson , who has had bad experiences with his RR 4.0, offers the following diagnostic procedure for the ABS system. (You can disconnect the EAS pump so as not to confuse it for the sound of the ABS pump running.)
1- Discharge the accumulator by pumping the brake pedal about 30 times with the key OFF.
2- Check fluid levels now that it should have all returned to the reservoir.
3- Look for any external leaks.
4- Without depressing the brake pedal, turn the key on, and see how long the ABS pump runs.
5- Then, sit in the car and listen for about 5 minutes to see if the ABS pump runs again.

If your pump only ran for 3 or 4 seconds after being fully discharged, you probably have a bad accumulator (i.e. not able to take the charge.) If your pump ran for a long time (longer than 15 or 20 seconds), and you don't see any leaks, you probably have a bad/weak pump or check valve. If the pump DID cycle on again during your 5-minute wait, you have some sort of internal leakage (probably a seal, or check-valve). If the pump ran continuously, then you probably have a bad pressure switch that never told it to turn off, or your pump is bad (i.e. producing no pressure, even though it is running.)

Other symptoms and what might cause them:
While driving, if the brake pedal is momentarily hard, and the brake warning light flickers when you stab the brakes quickly, the accumulator may be dying (i.e. charge pressure is low). This happened to Troy, whose dealership couldn't find the problem. He cured it with an accumulator from a wrecked Rover from AAA Small Car World (see the parts and service page). If the pedal is hard all the time, there probably is no power assist. If the pump never cycles, you may have a bad pressure switch (i.e. it never tells the pump to turn on), or the pump is bad. If the pump does cycle, then you might have a bad accumulator, or a (probably internal) leak.  The first diagnosis procedure can help you determine which it is.  Or, again, the pump could just be bad.

Suggestions for Electrical Diagnosis with Multimeter:
It should be possible to check the wires going into the pump, to determine if the pressure switch is telling the pump to do the right thing or not (eg voltage appears, but the pump doesn't run, etc.) You should also be able to measure resistance across the pump's windings, Watching the pressure switch with a multimeter, in conjunction with de-pressurizing the system as in step 1 above, should give you a clue as to how the system works.  Then, you could short contacts at the connector to test the logic of the system, or by shorting contacts at the ABS pump relay to test the pump motor itself. [I would recommend getting the ETM before doing these tests to ensure you do not damage any semiconductor components in this testing].

Rebuilding the Pump??
Kirk Fisher reports that in his case failure was due to the brushes wearing out, and it should be possible to keep the Rover going by replacing only the brushes. He is looking into this possibility. John Purnell has a good writeup illustrating his pump rebuild job at this link. If anyone has more information on rebuilding the mechanical or electrical parts of the pump, please email me.

ABS Sensor Problems and Repair

In my experience problems with the wheel speed sensors are even more common than with the booster pump. You can often get an "ABS Fault" or (on vehicles with electronic traction control) a "Traction Failure" message from this cause.

The Hall-Effect sensors are  located in the hub housing  and need to be a certain distance from the reluctor teeth (on the axle) passing under them in order to function properly.  They can be subject to dirt intrusion or more commmonly to being lifted out of position (eg by the wiring harness getting hit by off road obstacles). They are just pressed in to their housings and held in place with simple but expensive spring clips. (Photo at right: left rear hub on my RR 4.0SE. Speed sensor is in rubber boot at top left of picture).

Jeff Johnson illustrates this point by relating his experience with ABS sensor problems on his late model Classic: "Another thing I learned was about the front ABS sensors, I had been getting a fault due to the front sensors working out and getting too much space above the CV joint, I would push them down and in a day or two they would be up again. I asked British Pacific about getting new bushings and seals for them, they didn't have bushings listed on the site and I thought I needed new ones. They said the bushings were something they didn't stock because the cost about $48 each and no one ever bought them. He suggested I just replace the seals and see how it did and since they are only $8 each it sounded like a good idea. I put the new seals in and  never had a problem since, it's been over two weeks and I've been on rough roads as well as off road and they are perfectly fine now. So it seems that's all they need, the bushings were fine and the new seals hold the sensors in place."

More Information

Brake Pad & Rotor Replacement (4.0/4.6)
Bleeding Brake Fluid in ABS System (4.0/4.6)

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