'87 300D. When braking the brake pedal pulses and there is a groaning sound just before the car comes to a complete stop. Pads and rotors(front and rear) have been inspected and are ok. Front caliper sensors are in place. No loss of brake fluid anywhere. the master cylinder is about 3yrs old w/ about 30K on it. ABS light does not come on. The system has not been open since I serviced the master cylinder, so there should be no air in the sytem. Car does not pull to one side or the other during braking either.

Any suggestions?

Does pulse frequency coincide with wheel rpm or is it a mechanical pulse of X-times per second?

Define pads and rotors have been inspected. Did you use a dial gauge? Did you pull the pads to ensure nothing is embedded in the friction material?

Sixto
87 300D

Sounds like rotor warp if the brake pedal feels bumpy when you are applying the brakes.. Have to use a dial gauge on those rotors to find the offender.

I have felt this in my 300SDL, I think it is the ABS activating just before coming to a complete stop. I searched on ABS issues and think my ABS sensor is dirty or acting up. I have yet to pull the wheel or sensor to investigate (too cold in Connecticut). its a possible direction to look.

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Dean

Sixto: visual inspection on pads, plenty pad material left. I did not pull the pads to inspect. Did not use a gauge to measure rotor thickness. Pulsing and groan only occur the second or two prior to coming to a complete stop...5mph or less. No pulsing experienced at other braking times to indicate a warped rotor(s). It's a 1-2 and sometimes 3 irregluar pulses and a groan and the end of the braking sequence to stop.

As I drove home last night, the ABS light came on and stayed on and the pusle and groan stopped completely....no pedal pusle at all during braking. Brake pedal seems firm. Checked again for leaks at the caliphers, master cylinder and ABS unit, with none found.

I vote warped rotor. Get out the dial indicator and magnetic base and find out which one(s).

BTW, I have a warped rotor on my 300D and have for several months. I'm pretty sure it's the right front, but I haven't actually followed my own advice and checked for sure. I'm trying to hold out until the pads wear out and just do it all at once. At the rate I wear out brakes on this car, that may take a while.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

I think your rotors are fine given this updated information.

Clean the inside of the hubs (that's where the sensors do their work) as those can fill up with gunk and trigger the abs way early.

Given that your light came on (that means the system is also disabled) and all of your quirky symptoms stopped, it's clear that the brakes themselves are fine.

You may have a bad abs sensor as well, but I'd start with being sure that everything is spic and span on the backside of those hubs. And wipe off the sensor while you're there, too, of course.

Good luck.

A front wheel reluctor in my 91 300SE somehow got dinged. I filed the couple of cogs back to shape and it was fine. The system can also act goofy when a wheel seal lets go and the reluctor cakes with grime.

Sixto
87 300D

Thanks for the info....Hope to have time this weekend to inspect more thoroughly...between replacing a burned up sump pump in the house, chasing a vacuum leak in my daughters 98 Le Sabre, and installing a new inner and outer tie rods in the wife's minivan, not to mention playing IT specialist when my kids say the computer's broke and I need to do homework, my car gets the last of my attention...

I've rebuilt all 4 calipers with new o-rings and seals...all 4 pistons were pretty grimy. Got them all cleaned up and cleaned out each caliper of gunk and rust...I put new pads on all 4 and new sensors up front, bled the entire system and problem still persists...

I think the rotors are fine otherwise when the ABS light is on and the ABS is disabled, there is no thumping like a warped rotor is prone to do... rotor don't warp then unwarp when the ABS is disabled.....

FYI, the master cylinder and fluid resevoir are only about 2 yr old...

also, just had the fun of replacing the porcupine aka blower motor regulator....fun fun fun

It is something with the ABS system that is causing the pulse. I have a similar issue on my 300E but it only does it when in traffic i.e. stop and go less than 15 mph or so. I have also noticed the ABS light will sometimes come on and the brakes work normally when the light is on.