Countax Posted October 26, 2006 Share Posted October 26, 2006 Vehicle is a 300TDi Auto with ABS. I have a strange problem with my brakes and would welcome any advice on possible causes. 1) ABS light should go out when the vehicle reaches 5mph, but on mine it stays on after i start up in the morning or at the end of the day ( so when vehicle has been stood for a while) but if i stop and switch off and do a restart the light goes out at 5mph. this is what i have been doing for the past month or so and have continued to use the vehicle. Tonight i tapped each wheel sensor and i am sure i felt the drivers one move a little. Hopefully this will cure the light problem.....Now to the more worrying problem... 2) When i start the vehicle with my foot on the brake pedal it does its normal 'clunk' and drops a little as the vacum builds up. But the pedal continues to feel spongey and will very slowly feel like it continues to drop. Its as if you were pumping air, maybe its just purgung the servo. When driving the vehicle the ABS works fine, but when I brake i can now feel a low frequency modulation through the pedal. Its not as fast as the ABS cadence braking, and its seems related to the road speed or possibly the vaccum pump oscilations ( its an Auto so the revs drop to idle when braking ). I checked the pads and the rears are getting close to the limit, so i will change them this weekend. I also noticed that on the N/S rear the outer pad is more worn than the inner pad. Could it be: 1) warped discs causing the modulation 2) the rear pads 3) vaccum pump ( how can i test this ?) 4) ABS fault and the reason why the light is coming on in the morning / evening 5) stuck caliper cylinder 6) servo 7) air leak in the vacuum line 8) other? Any advice would be welcome. Nigel Quote Link to comment Share on other sites More sharing options...
Les Henson Posted October 26, 2006 Share Posted October 26, 2006 A warped brake disc will make the pedal pulse as it pushes and then releases the pad, although this is more common on front discs. There may be contact patches on the disc to indicate if this is the problem. To check vaccuum, switch the engine off and wait a while, then pull the pipe out of the servo. If it hisses, then the servo is ok. Do the clamp brake hose trick to determine if there's a fault with a specific axle. Brake faults quite often have to be a tedious process of elimination. Les. Quote Link to comment Share on other sites More sharing options...
Andy Posted October 26, 2006 Share Posted October 26, 2006 how old is the fluid? i've known ABS systems do some strange things due to very old dirty fluid. Quote Link to comment Share on other sites More sharing options...
Countax Posted October 26, 2006 Author Share Posted October 26, 2006 how old is the fluid? i've known ABS systems do some strange things due to very old dirty fluid. Not sure, i will look up the service records. From memory it was either done 2 yrs ago or about 6. It does feel as if its gone hydroscopic. Regardless, i will change it this weekend as it must be due. Nigel Quote Link to comment Share on other sites More sharing options...
Andy Posted October 26, 2006 Share Posted October 26, 2006 if its black then its knackered. i think the problems occur as the water it asorbs isnt compressable which confuses the ABS control unit. Quote Link to comment Share on other sites More sharing options...
Countax Posted October 27, 2006 Author Share Posted October 27, 2006 if its black then its knackered. i think the problems occur as the water it asorbs isnt compressable which confuses the ABS control unit. I just looked at the service records and it was checked, but not renewed in April 2005. The test showed 198 deg C. I guess that means they checked the boiling point. Anyone know if this is well within spec or close to the limit? Car has done 6kmiles since that service. What DOT number fluid does a Disco with ABS use? Les, are you suggesting i clamp off the rears and do a very short drive to see if the modulation is still there? Any easy way to test the front discs for warping? I have a dial guage and magnetic holder. Nigel Quote Link to comment Share on other sites More sharing options...
BogMonster Posted October 27, 2006 Share Posted October 27, 2006 I think normal DOT 4 has a boiling point when new of about 260 C so I would say sub 200 is on the low side, though I don't know if there is a "change it now" figure. If the warping is bad enough to cause pedal pulsing you'll see it with the naked eye, just jack up that corner take the wheel off and spin the hub by hand (or better still get somebody else to) while looking at the disc edge on, you will see if there is any runout worth worrying about, half a mill or so is easily visible when the disc is spinning. Quote Link to comment Share on other sites More sharing options...
Ivan Posted October 27, 2006 Share Posted October 27, 2006 I seem to remember seeing somewhere that brake fluuid should be changed every 2 years. Can't remember the mileage but, if yours has been in a long time I would change it anyway. Ivan Quote Link to comment Share on other sites More sharing options...
Countax Posted October 27, 2006 Author Share Posted October 27, 2006 I seem to remember seeing somewhere that brake fluuid should be changed every 2 years. Can't remember the mileage but, if yours has been in a long time I would change it anyway.Ivan Thx, I will pick up some DOT 4 on my way home. I have been giving this a bit more thought and i don't feel the modulation through the steering, so my guess is its rear axle related. I will explore if i can clamp off the rear brakes as Les suggested and see if this removes the modulation. I am wondering if i have a sticking cylinder on one of the rear calipers. Nigel Quote Link to comment Share on other sites More sharing options...
Countax Posted October 27, 2006 Author Share Posted October 27, 2006 Thx,I will pick up some DOT 4 on my way home. I have been giving this a bit more thought and i don't feel the modulation through the steering, so my guess is its rear axle related. I will explore if i can clamp off the rear brakes as Les suggested and see if this removes the modulation. I am wondering if i have a sticking cylinder on one of the rear calipers. Nigel Update: The rear discs seem to be the causing the problem. On the N/S disc the inner surface is rusty across the entire face and on the O/S disc the inner surface is 50% rusty. The caliper pistons move in and out ok, so i am not sure why the discs are like they are. I think the calipers have so much carp and rust that the pads cannot get out to the disc. On the O/S, the inner pad was worn slightly curved on its working surface so it looks like it was pivoting on some rust. Anyone have any tips on changing the rear discs, is it easy? any parts like gaskets i should buy at the same time? Nigel Quote Link to comment Share on other sites More sharing options...
Les Henson Posted October 28, 2006 Share Posted October 28, 2006 The problem is most likely your caliper then Nigel. The amount of pressure is considerable, and I don't know that the caliper mounting bracket could prevent the pad from contacting the disc, although I wouldn't be surprised if it was. Perhaps the piston only comes out so far, and then sticks - preventing pad-to-disc contact. Pistons etc are available seperately, as are the seals, so if the problem is your caliper, it can be rebuilt. This is a 90 caliper, but the method is identical. http://forums.lr4x4.com/index.php?showtopic=2851 Les. Quote Link to comment Share on other sites More sharing options...
Countax Posted October 28, 2006 Author Share Posted October 28, 2006 The problem is most likely your caliper then Nigel. The amount of pressure is considerable, and I don't know that the caliper mounting bracket could prevent the pad from contacting the disc, although I wouldn't be surprised if it was. Perhaps the piston only comes out so far, and then sticks - preventing pad-to-disc contact. Pistons etc are available seperately, as are the seals, so if the problem is your caliper, it can be rebuilt. This is a 90 caliper, but the method is identical. http://forums.lr4x4.com/index.php?showtopic=2851 Les. [/quote Well it looks like it was the rusty N/S disc caused by a rusty caliper. I just changed both of the rears discs, gave the caliper a good clean, put new pads in and took it for a run and it seems fine. I will need some new rear metal brake pipes soon so i will leave the fluid change and do it all at once. Nigel Quote Link to comment Share on other sites More sharing options...
SPendrey Posted January 15, 2010 Share Posted January 15, 2010 [/quote Well it looks like it was the rusty N/S disc caused by a rusty caliper. I just changed both of the rears discs, gave the caliper a good clean, put new pads in and took it for a run and it seems fine. I will need some new rear metal brake pipes soon so i will leave the fluid change and do it all at once. Nigel This made me laugh... I own this Disco now, and it still "will need some new rear metal brake pipes soon"! Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.