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1995-2002 Range Rover P38A
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Discussion Starter #1
2001 4.6L vogue.
I've searched for abs type issues, but only found "clicking" pedal and lots of pedal movement, so:-

Started as "abs type" intervention when turning / parking, with made me think it was a sensor / reluctor, but has developed into banging through the pedal.
Now happens when travelling straight as well.
I sometimes get a heavy banging through the pedal( both feel and audible). This is not the gentle pulsing of ABS.
Occasionally the pedal is hard with little braking, until I push through this.
Often there is a loud "boing" spring noise !
I get a variety of ABS and Traction Control lamps lit, but not in any particular pattern. The startup selftest bit is normal. Sometimes they go out after a short time, sometimes they stay on.

The brakes are physically in good condition with good discs and pads, and free guide pins and calipers.
They don't feel spongy at all.
Vehicle stops in a straight line with the correct braking power on the occasions it doesn't "bang" or "boing".

Oh, and no faults on an OBD tester. Maybe they would only show up on a dealer diagnostic?

I haven't replaced anything yet.

Thoughts?
 

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It's not, that issue only applies to GEMS, the OP has 2001 Bosch.
 

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1995-2002 Range Rover P38A
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I've also experienced that "springy boing" noise and sensation on the brake pedal before, typically in the morning when I'm reversing out the garage and the ABS pump is still doing its thing and i brake to stop. I won't be applying a lot of pressure and I'll hardly be moving when it happens. Also a 2001 Bosch by the way. Rave says "When the ABS system operates, the driver will experience an audible noise from the ABS modulator and vibration transmitted through the brake pedal." so I've always just assumed it was that, not sure why it would go off under the circumstances I've described because I really doubt there were any wheels skidding at the time.
 

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I believe what you are reading about ABS in Rave is that under "Normal ABS" operation, the modulator will do just that....Modulate the brakes. This is felt as a click, or series of fast clicks in the brake pedal as well as very audible clicks from the hydraulics. Try a hard stop on a gravel road, or while turning and going over a bump, and you will hear and feel the ABS system kick in.
The Boing in the morning, which I have heard from both of my 02s is, I believe, the sound that a spring loaded, fast acting shuttle valve in the modulator would make if the system detects braking, and inadequate fluid pressure in the powered side of the system. It will shuttle to bypass ABS and open up ports on the manual, non powered side of the system to continue to provide stopping power, although weak compared to powered operation.
The reason for first thing is that there is simply not enough pressure built up in the system yet by the pump. If I wait about 30 seconds to give the system time to pump up, I never hear the Boing.
I read the entire post mentioned above and now I am very curious about the modulator and it's internal workings. I may need to get one from the wrecking yard, and take it apart to see just what makes it tick....Or Boing!
 

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Discussion Starter #6
Ok folks. re-awakening this thread.....
The strange ABS effects got gradually worse and became undriveable. And un-MOTable, if that's a word.
I replaced both front sensors and a cracked reluctor ring on the left front. Thought I'd solved it when I found the cracked reluctor. No difference.
I replaced the brake booster block with a used, but working unit. No difference. Could both have identical faults?
The warning lamps go out once moving, and when the 6.7Kmh ABS activation speed is exceeded, a continuous soft "chattering" noise can be heard from the direction of the brake booster.
Below this speed, the vehicle brakes perfectly. Above it, all sorts of weird effects appear.
The pedal is often "blocked" and requires considerable force to depress it. A hard banging can be felt through the pedal, and brake power as if un-assisted.
Oh, if I disconnect a wheel sensor to disable the ABS/TC, the vehicle brakes perfectly.
Clearly the ABS system doesn't think there is any fault, otherwise it would disable itself.
I've now got a nanocom.
Live data shows the minimum 1.7Kmh at all four wheels, and all increase steadily until about 6.7Kmh when the ABS comes on. The data freezes at this point.
Live data also shows 2.33V at all four wheel sensors until the chattering noise starts (6.7Kmh ABS on speed), at which point all the voltages go crazy.
Faults give "Left Rear sensor shorted to another sensor" which is obviously not possible, but is apparently a common indication of a faulty ECU.
Bought a used, but "tested" ECU.
Absolutely no difference whatsoever !?*?! Precisely the same symptoms.
Any bright ideas?
rgds Alan
 

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Discussion Starter #7
Ok folks. re-awakening this thread.....
The strange ABS effects got gradually worse and became undriveable. And un-MOTable, if that's a word.
I replaced both front sensors and a cracked reluctor ring on the left front. Thought I'd solved it when I found the cracked reluctor. No difference.
I replaced the brake booster block with a used, but working unit. No difference. Could both have identical faults?
The warning lamps go out once moving, and when the 6.7Kmh ABS activation speed is exceeded, a continuous soft "chattering" noise can be heard from the direction of the brake booster.
Below this speed, the vehicle brakes perfectly. Above it, all sorts of weird effects appear.
The pedal is often "blocked" and requires considerable force to depress it. A hard banging can be felt through the pedal, and brake power as if un-assisted.
Oh, if I disconnect a wheel sensor to disable the ABS/TC, the vehicle brakes perfectly.
Clearly the ABS system doesn't think there is any fault, otherwise it would disable itself.
I've now got a nanocom.
Live data shows the minimum 1.7Kmh at all four wheels, and all increase steadily until about 6.7Kmh when the ABS comes on. The data freezes at this point.
Live data also shows 2.33V at all four wheel sensors until the chattering noise starts (6.7Kmh ABS on speed), at which point all the voltages go crazy.
Faults give "Left Rear sensor shorted to another sensor" which is obviously not possible, but is apparently a common indication of a faulty ECU.
Bought a used, but "tested" ECU.
Absolutely no difference whatsoever !?*?! Precisely the same symptoms.
Any bright ideas?
rgds Alan
Come on folks. Anyone?
I also replaced the bomb with a new one. The pump only runs for 10 to 15 secs on startup, so pressure should be Ok.
Everything I've done had absolutely no effect on the symptoms, so unless I've replaced something with an identical fault, then there must be some other part of the system I've missed?
But what would send the ABS crazy?
Alan
 

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10 to 15 seconds is too short, from zero pressure it should run for 30-40 seconds. Maybe the hose has collapsed internally?
 

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Discussion Starter #9
10 to 15 seconds is too short, from zero pressure it should run for 30-40 seconds. Maybe the hose has collapsed internally?
Sorry, I wasn't clear.
I can't remember how long it ran for from empty. Probably was something like that.
10 to 15 secs is what it runs for during a normal startup, which is about right I think.
Alan
 

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Not usually. If left standing overnight it'll usually run for the full time. If it runs after pressing the brake pedal, then there's something wrong. Does it kick in if the car is just left idling and the brakes aren't touched?
 

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Discussion Starter #12
Not usually. If left standing overnight it'll usually run for the full time. If it runs after pressing the brake pedal, then there's something wrong. Does it kick in if the car is just left idling and the brakes aren't touched?
Hmmm, It would only need a full run if it leaked down overnight. I would have thought the system should hold pressure better than that.
I've had three P38s and none of them ran any more than 15s in the morning unless they'd been unused for a week.

(If it runs after pressing the brake pedal, then there's something wrong) Pushing the brake uses system pressure and will cause the pump to run at some point, so I assume you mean braking immediately after the pump stops. No, it doesn't. It takes quite a lot of pushes to trigger the pump again. And I've honestly never noticed it running after the initial startup, other than after the occasional brake.
Alan
 

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It should take 3 or 4 presses of the brake pedal before the pump cuts in so it sounds like everything is working as it should. It sounds like an TC problem rather than an ABS problem. 2001 car should have the Wabco D system with 4 wheel traction control. If it has always done it then I would suspect something has been changed for the earlier type but as it's been getting progressively worse that would appear to rule that out. Of the 4 P38s I've owned, all have run for much longer than yours does when left overnight. On my everyday car that I've owned for 10 years, I start the engine, reverse out of my driveway and the TC and ABS lights go out as I pass the bus shelter about 200 yards down the road. It's always taken that long and if anything changed I would assume something was wrong.
 
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Discussion Starter #14
I think the three I've had all took about 100 yards for the lights to go out.
I've had this vehicle for 7 years, and apart from replacing normal wear and tear items like air springs, dampers and ball joints, it's been mostly trouble free.
This problem went from a tiny "kick" through the pedal, to very dynamic effects, gradually over about a year.
I'd be inclined to think this is unlikely to be an electronic problem, as they tend to fail suddenly. Pity, as plugging in a new ECU would have been an easy fix.
I do have a second 2001 P38 which drives fine (on coils), so I'll swap ECUs to rule that out completely, as its easy to do.
I'm still leaning towards something in the modulator block, like a weak solenoid letting go, but I really don't know what's in there, so it's difficult for me to imagine the effects.
Guess I'll just have to replace it with a third one and see if there's any difference.
I know I could take the known good one out the other P38, but that's a last resort, as its a horrible job including bleeding. I don't want to do it three times.
rgds Alan
 

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It seems very odd that you changed one brake modulator for another and still had the exact same fault, which suggests that isn't where the fault lies.
 

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Discussion Starter #16
It seems very odd that you changed one brake modulator for another and still had the exact same fault, which suggests that isn't where the fault lies.
That's what I originally thought too, but the early modulators had a weak point which caused the same result, so I wondered if the later ones could have something similar?


Am I right in thinking that while the ABS / TC system is monitoring, ABS is only active whilst braking, and TC is only active when not braking?
Or that is what is supposed to happen anyway.
I am finding it difficult to believe that any "normal" ABS or TC intrusion could effectively block pedal movement.............. unless..............
the system doesn't know I'm braking...................?
I'm sure I read somewhere in a post about brake pedal switches coming loose?
Could a defective brake pedal position switch confuse the system that much?
I'll check the pedal out tomorrow.
Alan
 

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Interesting theory. The brake light switch has two contacts, one is connected to the BeCM (to operate the brake lights), the cruise control ECU, the ABS ECU and the EAS ECU, while the other contact is connected solely to the ABS ECU on pin 14 on C0504. With the brake pedal pressed there should be 12V appearing at that pin.

Thinking about it though, if the switch had failed so only one contact was doing something, then the fault would just have suddenly appeared, rather than being there and progressively getting worse.
 

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Discussion Starter #18
Interesting theory. The brake light switch has two contacts, one is connected to the BeCM (to operate the brake lights), the cruise control ECU, the ABS ECU and the EAS ECU, while the other contact is connected solely to the ABS ECU on pin 14 on C0504. With the brake pedal pressed there should be 12V appearing at that pin.

Thinking about it though, if the switch had failed so only one contact was doing something, then the fault would just have suddenly appeared, rather than being there and progressively getting worse.
Yes, I tend to agree. It could hardly have failed gradually. Its a switch which is either off or on.
This really has to be something mechanical or hydraulic.
Alan
 
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