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by Alain Hoffmann

The Anti-Lock Braking and related systems


Figures given here are valid for the 1999 Discovery II

Every model of the new Discovery is equipped with anti-lock brakes, system WABCO, similar to those fitted on the 1998 Freelander. It's a 4-channel system, regulating every wheel independently according to friction of the wheel on the ground and to the load on each wheel (BD-Electronic Brake Distribution). Also it controls the HDC (Hill Descent Control) offroad as well as the ETC (Electronic Traction Control). Both systems work on the brake pistons of a single or multiple wheels in order to give the best performance. The system proved to be good, reporting almost no trouble. I hope this will stay the same once the vehicles get older and past the 7-years lifespan ordinary cars have.

Now to the details on this:

Master Cylinder

Servo-assisted double brake servo. Reservoir capacity: DOT4 fluid, 950 ml on manual and 850 ml on automatic cars.

Wheel brakes

Type Lucas Colette, floating. Front double pistons, vented discs, rear single piston, non vented discs. Due to the mounting system the performance of the discs is better when they are exposed to higher operating temperatures for an extended period of time. Front discs have a diameter of 148,6 mm and a thickness of 25 mm. Minimum is 22 mm.

Rear discs are 152 mm diameter, 12.6 mm thick and must be replaced at 11.7 mm. Maximum runout is 0.15mm for both front and rear. They are not identical to the older models as they have a different wheel speed sensor.

Hand brake

Unchanged. Drum on the transfer box. Watch to replace the bolts holding the backing plate to the trans only by the genuine parts as they are specially heat treated.

ABS modulating system

Very similar to the system on the Freelander but NOT identical. None will work correctly in place of the other, so the mountings are a bit different to make it impossible to interchange both. The system works best if you have a wheel lockup of 10-20%,(optimum tire slip point), not as commonly believed no lockup at all. The ABS tries to limit differences in wheel speed to much below 10% of the average speed. This average speed is calculated as a mix from the 4 wheel sensors. Also the system feels if you are travelling at high speeds or in corners and adjusts itself..

Great news: Not a single piece of the modulator is serviceable, so it has to be replaced as a whole unit! As it holds also the Hill Descent and Traction Control I assume it will cost you an arm and a leg. If you get the new part, it's inside a vacuum sealed and dated box. If the box is no longer sealed or the manufacturing date is over 3 years ago, REFUSE IT. LR themselves have ordered not to mount such parts.

The ABS might go wrong. In this case full braking facility is retained but the anti-lock system does not engage. An flashing signal is shown on the instrument cluster as well as 3 short buzzes can be heard. A good example is a faulty wheel speed sensor. Even after replacing the faulty part you must start the car up to 15 km/h as the system can only verify the components above this speed.

If a single component as a sensor fails the ABS tries to "work at it's best". This means it only shuts off as few systems as possible.

ETC- Electronic traction control

Works remarkably well which is no surprise as the same system is used since many, many years in vehicle trials. The ETC replaces the DIFF LOCK position on other models where you could lock the centre differential. On the new Discovery you can't. The only thing the second lever does is engage low range. Contrary to the Range Rover Classic this transfer box (model 230Q) does not have an visco-coupler unit which was very expensive, difficult to repair and not very long lived. However it still has the electric contacts for informing the system of various conditions. The ETC replaces a limited-slip differential, working the same way in braking one or both wheels. I suppose you CAN'T fit an aftermarket diff lock unit in these cars. The ETC works fully up to 50 km/h, some functions even up to 100 km/h. A tip on the brake pedal switches the system off as long as the brakes are working

We found the first drawbacks of the ETC when offroading in the Auvergne. We had to make a sharp 180° uphill turn. One Td5 didn't make it in the first run, so he had to back up and floor the gas to get some wheelspin to be able to make the sharp turn.

However as soon as the wheels began to spin the computer switched off the power thus making it impossible to get up that hill. Sure, you can say it's a very rare condition but this doesn't change a dime on the fact that it couldn't get where the others were able to go!

So judge for yourself. It it was for me I'd install an override switch at the power supply for the ABS control unit. As soon as power fails the brakes and the other systems should work as usual.

The HDC- Hill Descent Control

This system brakes the vehicle when going downhill in low gear. As any offroader knows it's vital not to step on the brakes when going down a slippery hill- You usually only do that once, at least in the same vehicle. The HDC acts on the brakes via the ABS so to assist the engine compression braking. This system is even more important as you have no central locking diff- one wheel slipping would reduce engine breaking to 0. The system even compensates for wrong gear choice or no gear at all. The HDC is deactivated if the driver hits the brakes. However it still works for 60 seconds if the driver hits the clutch and then reduces progressively. In every day driving it means you can drive in almost any gear in low range and when you hit a downhill or think you're going too fast you simply push the switch and the vehicle slows to between 7 and 14 km/h, depending on gear. You can even change gear going downhill or you can accelerate. Take the foot off the gas pedal and the car slows down. Reverse braking is even only 5,6 km/h. This looks as if there was an engineer who has actually driven the thing offroad. The HDC switches the stop lights on if it works, regardless of the acceleration or deceleration. If the ECU thinks you abuse the system, it can switch it off, showing a flashing signal and then reducing the HDC progressively. The HDC braking is shown a steady green lamp on the cluster. Flashing green means the HDC is disengaging progressively and an orange sign and an acoustic signal show a problem.

Wheel speed sensors

Different from all the other models. It's now part of the inner wheel bearings, front and rear. Not serviceable, must be replaced with the bearing. Relies on a signal from a small wheel with 60 teeth. Gives also the vehicle speed to the instrument cluster

ECU of the ABS-system

Mounted behind the glove box, beside the ACE-ECU and the Body Control Unit-ECU. Access by removing spring clips on the glove boxes side and lowering the box. The brake system unit is the middle one of the 3 boxes. If you don't have the optional SLS (Self Levelling Suspension) you probably still have the same ECU as both controls share the body of the brake control unit. Only the cables are missing. I'd bet the control units are identical inside as this reduces cost. Maybe a way to retrofit the self levelling system?