QuestionI have suffered two total brake failures in a Freelander with a Conti-Teves MK25 TCS modulator. I currently investugating this "phenomena" of one minute brakes working, then a total failure, and then working again. It is obvious that the inlet valves of the modulator are closing "by mistake".
I was wondering if you have had any experience if this?
AnswerFrom your description, it is not obvious that the valves are closing by mistake. I don't know why you are experiencing intermittent brake failure but without proper testing no answer is obvious, as you say.
Before answering you I searched LR's current list of bulletins and alerts and I found nothnig pertaining to brake failures. So at this moment the problem you are reporting is not something Land Rover has sen previously or is advising service personnel about.
The Land Rover t4 test system does have a routine to test the modulator. Here is a brief quote from the workshop manual:
The hydraulic unit of the modulator consists of a pump and 12 solenoid operated valves, accumulator and damper chambers. During normal braking where ABS intervention is not required, brake fluid passes straight through de-energised inlet valves (normally open). Where ABS intervention is required, pressure is maintained at a wheel by closing the appropriate inlet valve. When pressure needs to be released from a brake circuit, the appropriate outlet valve is opened (when outlet valve is opened the inlet valve must be closed) and the brake fluid is allowed to flow into the reservoir. Brake fluid is returned, via the return pump, to the master cylinder line via the damper chamber
Brake Fluid Pressure - (Inlet)
The hydraulic circuit of the ABS modulator consists of the primary and secondary feeds from the bake master cylinder. These are fed into the modulator by two Ø 6 mm. brake pipes. The input pipes are easily distinguished by their size, compared to the four Ø 4.76 mm. outlet pipes.
The ECU can detect electrical failure of each of the inlet valves and will generate relevant fault codes which can be accessed via T4.
Brake Fluid Pressure - (Outlet)
The hydraulic outlet circuit of the ABS modulator consists of the four pipes leading to the front calipers and rear brake drums. The four pipes transmit the brake fluid usually at the pressure determined by the drivers brake application, but during ABS, EBD, TC and HDC intervention at the pressures modified by the ABS ECU. The pipes are attached by a series of clips into the body and terminate at the caliper/drum via a flexible hose.
The ABS ECU can detect electrical failure of each of the outlet valves and will generate relevant fault codes which can be accessed via T4.
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Now, your suggestion is that the valves are working "by mistake" . . . if the brakes are releasing upon application something is happening but from the symptom alone it's not obvious what.
The most common cause of this situation is an electronic problem whereby the vehicle think it's skidding and begins to cycle the ABS, thereby preventing a quick stop. That is most often caused by bad sensors.
But I've seen bad ABS modulators, and bad master cylinders too. Any of those could be intermittent and any could produce your symptom.
I would strongly encourage you to find a qualified shop and get to the bottom of the problem. Be sure the shop has either an Autologic or T4 test system and teh knowledge to use it. The system can't be diagnosed without one of those tools.
Our shop has both testers and for a situation like this I'd use the T4 because the test procedures in the workshop manual are written for that tool, and I could produce a printout of anything that I found that was in accordance with what LR expects. That would be needed if I were to report a problem to them, which could be the case with a safety issue like this.
At this point I suspect you've got a problem specific to your vehicle. I have heard nothing to suggest that what you are reporting is common to other Freelanders. I'd be interested to hear what you discover.
