ABS Oddity

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M

Moja

Member
Posts
16
Location
Western Isles
Hi folks, I've been running my 2001 P38 4.0 for almost five years with relatively few problems. This week though the brakes started playing up. As soon as I touch the brake pedal above about 15mph the ABS kicks in. It stays clicking away for several seconds after releasing the pedal. It can make the brake pedal rock hard, with very little braking force. I have to pump the pedal to "break through" the ABS function to an unassisted brake pressure.
I have no "three amigo" indicator lights; they all go off as normal. I've read that if a sensor was at fault then the ABS ECU would pick that up during the start-up and initial drive off sequence, which is why the three lights stay on for that time. Does that seem reasonable? I've disconnected one of the front sensors to test this and it does then show an error, and the lights stay on. Reconnecting the sensor returns things to normal. When I remove the ABS relay the warning lights all come on, along with the "ABS Fault" indication. The brakes work as a normal unassisted system, not great but It will stop the car and I think it shows that there is no brake bleeding issue needing attention.
I'm guessing that the WABCO ECU may be causing the problem, but I can’t find it! The RAVE manual shows it behind the bulkhead, maybe in the footwell near the steering column. I've removed the steering wheel panel but can’t see it anywhere in there. It’s not under either front seat, or in the engine compartment. Where is it?
There are a few threads dealing with ABS, but I cant find one with this particular fault. Does anyone have any advice on what I can do?

Cheers

Kenny
 
Moja said:
Hi folks, I've been running my 2001 P38 4.0 for almost five years with relatively few problems. This week though the brakes started playing up. As soon as I touch the brake pedal above about 15mph the ABS kicks in. It stays clicking away for several seconds after releasing the pedal. It can make the brake pedal rock hard, with very little braking force. I have to pump the pedal to "break through" the ABS function to an unassisted brake pressure.
I have no "three amigo" indicator lights; they all go off as normal. I've read that if a sensor was at fault then the ABS ECU would pick that up during the start-up and initial drive off sequence, which is why the three lights stay on for that time. Does that seem reasonable? I've disconnected one of the front sensors to test this and it does then show an error, and the lights stay on. Reconnecting the sensor returns things to normal. When I remove the ABS relay the warning lights all come on, along with the "ABS Fault" indication. The brakes work as a normal unassisted system, not great but It will stop the car and I think it shows that there is no brake bleeding issue needing attention.
I'm guessing that the WABCO ECU may be causing the problem, but I can’t find it! The RAVE manual shows it behind the bulkhead, maybe in the footwell near the steering column. I've removed the steering wheel panel but can’t see it anywhere in there. It’s not under either front seat, or in the engine compartment. Where is it?
There are a few threads dealing with ABS, but I cant find one with this particular fault. Does anyone have any advice on what I can do?

Cheers

Kenny
Click to expand...


Is the brake pump actually working?
ABS ECU is under the dash on the bulkhea, passenger side assuming the car is RHD.
Could be the ECU has failed but also check for corrosion on the connector.
 
Found the ECU under the glove compartment. The connectors are clean. This gave me the chance to check all the sensor resistance readings easily at the ECU plug. Three read 900 ohms, and are isolated from the chassis, the front near side reads 1200 ohms and has a resistance of 10 Megohms to the chassis. I double checked this at the sensor lead in the engine compartment. I suspect this may need replacing as I believe there should be no resistance reading to the chassis. This sensor is a replacement about four years ago when I had a new hub fitted.
The brake system works perfectly up to 20mph, really solid brake pressure, then the ABS clicking starts. The fluid is no more than two years old, and is at the correct level.
My bet is still on the ECU. Or could it be as simple as the ABS relay?
 
Moja said:
Found the ECU under the glove compartment. The connectors are clean. This gave me the chance to check all the sensor resistance readings easily at the ECU plug. Three read 900 ohms, and are isolated from the chassis, the front near side reads 1200 ohms and has a resistance of 10 Megohms to the chassis. I double checked this at the sensor lead in the engine compartment. I suspect this may need replacing as I believe there should be no resistance reading to the chassis. This sensor is a replacement about four years ago when I had a new hub fitted.
The brake system works perfectly up to 20mph, really solid brake pressure, then the ABS clicking starts. The fluid is no more than two years old, and is at the correct level.
My bet is still on the ECU. Or could it be as simple as the ABS relay?
Click to expand...

Fluid is inconsequential. It cannot instigate ABS operation no matter what it's condition. The ECU is receiving a signal that is telling it one wheel is revolving slower than the other on the same axle when the brakes are applied. It thinks the wheel is locking up. Therefore it is applying ABS to unlock the wheel. Suspect your odd sensor i would think. Could also be a rusty or cracked reluctor ring. You will need to have a look.
 
Moja said:
Three read 900 ohms, and are isolated from the chassis, the front near side reads 1200 ohms and has a resistance of 10 Megohms to the chassis.
Click to expand...

I agree with Datatek.

Fix this fault,

Why do you think its ECU or relay when you have found a faulty sensor?
There should be no connection between the sensor and the chassis
 
Thanks everyone, it is indeed the rogue sensor that's causing the fault. I'd been scrabbling around and reading too many forum posts, some of which suggested that the ECU could be causing the problem. I had thought the relay could be at fault because it sounded like a relay chattering on and off for several seconds after releasing the brake pedal, again some forums say that the relay can go faulty.
New sensor on the way.
 
So, a new sensor fitted with no change in the fault. I've checked the wiring from the ECU to the sensor socket, all appears as normal. I've replaced the ECU, and also the ABS relay. Still no joy. As soon as I'm over 20mph the systems starts the ABS clicking, when I drop below 20mph it has no fault. If I disconnect the new front sensor I get Traction Failure and ABS fault lamps, as expected, and the brakes work perfectly. Just like before.
I'm stumped.
 
Moja said:
So, a new sensor fitted with no change in the fault. I've checked the wiring from the ECU to the sensor socket, all appears as normal. I've replaced the ECU, and also the ABS relay. Still no joy. As soon as I'm over 20mph the systems starts the ABS clicking, when I drop below 20mph it has no fault. If I disconnect the new front sensor I get Traction Failure and ABS fault lamps, as expected, and the brakes work perfectly. Just like before.
I'm stumped.
Click to expand...
Was the new sensor a cheapy from Ebay?
 
Moja said:
So, a new sensor fitted with no change in the fault. I've checked the wiring from the ECU to the sensor socket, all appears as normal. I've replaced the ECU, and also the ABS relay. Still no joy. As soon as I'm over 20mph the systems starts the ABS clicking, when I drop below 20mph it has no fault. If I disconnect the new front sensor I get Traction Failure and ABS fault lamps, as expected, and the brakes work perfectly. Just like before.
I'm stumped.
Click to expand...

Check the ABS reluctor ring, (the toothed ring) I have known these to crack.

Is the new sensor sitting right in the hub properly?

Was the new sensor a cheapy from Ebay? +1
 
The reluctor ring will be checked tomorrow. I hadn't realised until today that a Wabco sensor is £150. So the £20 ebay sensor could be a waste of time?
 
This fault is running away with itself. We spotted a crack in the reluctor ring, fitted a new CV joint which comes with a new ring. Now I get an instant "ABS Fault" indication followed by "Traction Failure" after moving off and also again when I switch the engine off. I've read in another thread that there are two brake pedal switches, one for the lights and one for the ABS ECU which if it goes faulty causes this ECU issue. Can anyone confirm that? I can see the brake light switch, it works fine, but I cant locate the second switch. Anyone have any experience of this fault?
 
Moja said:
This fault is running away with itself. We spotted a crack in the reluctor ring, fitted a new CV joint which comes with a new ring. Now I get an instant "ABS Fault" indication followed by "Traction Failure" after moving off and also again when I switch the engine off. I've read in another thread that there are two brake pedal switches, one for the lights and one for the ABS ECU which if it goes faulty causes this ECU issue. Can anyone confirm that? I can see the brake light switch, it works fine, but I cant locate the second switch. Anyone have any experience of this fault?
Click to expand...

That is because both switches are within the brake switch. The brake switch is dual action. When one is made the other is open circuit. Make sure all sensors are pushed in until they are touching reluctor rings.
 
I'm very glad to report that this fault is now cured. I had installed a new ABS sensor before discovering that the reluctor ring had a crack in it. So, after replacing the CV joint we reinserted the new sensor. Turns out that it was a dud. Replaced it with another new sensor, and the fault cleared. Brakes now 100% with functioning ABS and no Traction Failure readings. Thanks for the advice on this.
Now, does anyone know anything about changing the blend motors...?
 
Moja said:
I'm very glad to report that this fault is now cured. I had installed a new ABS sensor before discovering that the reluctor ring had a crack in it. So, after replacing the CV joint we reinserted the new sensor. Turns out that it was a dud. Replaced it with another new sensor, and the fault cleared. Brakes now 100% with functioning ABS and no Traction Failure readings. Thanks for the advice on this.
Now, does anyone know anything about changing the blend motors...?
Click to expand...

Think just about every body on here knows about blend motors !! Just do a search on here or have a look on rangerovers .net plenty of info on changing them (done mine by the way).
 
Moja said:
I'm very glad to report that this fault is now cured. I had installed a new ABS sensor before discovering that the reluctor ring had a crack in it. So, after replacing the CV joint we reinserted the new sensor. Turns out that it was a dud. Replaced it with another new sensor, and the fault cleared. Brakes now 100% with functioning ABS and no Traction Failure readings. Thanks for the advice on this.
Now, does anyone know anything about changing the blend motors...?
Click to expand...

So it would appear post 5 was right on the money then? :);)
 
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