560SL Brake Help, Please
 

I'm having a problem with my '87 560SL (70k miles):

After cold start-up, the first application of brake pedal pressure produces a loud "pop & groan", the pedal goes spongy and braking is insufficient. Then the ABS light comes on after driving a few feet.

I checked the booster by applying pressure with the engine off, then starting the engine with pressure held on the pedal. The pedal drops slightly so I assume the booster is ok. Fluid level is ok and no fluid is leaking.

My guesses are caliper or master cylinder, but I haven't yet dug into it. Has anyone experienced the same? Any guesses?

Thanks
Frank

Check your ABS sensors for open circuit. use meter check at end of the sensor plug by pulling the sensor cable at the wheel you shold see open circuit. replace sensor it should fix your problem.

Thanks, I'll do that! Would a bad sensor cause the noise and sudden pedal change?

Frank
yes a bed sensor will turn on the ABS pump when vehicle is moving.

Thanks again for your prompt help. I'll check it in a few minutes and post the results.

Frank

I must be checking at the wrong location, because both front sensors read continuity. I pulled the wires from the sensor at the caliper location and read across the terminals in the sensor plug that's bolted to the caliper.

Did you mean to instead unplug the sensor lead in the engine compartment at the wheelwell and check across the plug there?

It seems odd that both are reading no resistance. I noticed there's a relay for the ABS system - so should the ignition be on during the test?

Thanks,
Frank

Frank
ABS sensor cable connector should be under the hood not by the brake caliper.

Thanks again. I just recognized that I was checking the pad wear sensor and not the ABS sensor.... Dohhh!

Measuring across the male terminals of the flat ABS plug (I hope it's not the large round one), both front sensors indicate open.

What next?

Modulator?

Frank

Closer examination shows paint bubbling on the front of the brake booster so I assume the sealing ring between the booster and master cylinder is leaking. Is that small leak enough to cause the noise and pedal dump I'm experiencing? Should I replace only the sealing ring, or go with a complete master cylinder replacement?

Thanks for any help,
Frank

Frank
Is the female part connector you have check it not the male side.

Both front female connectors show 16.3 ohms.

Further test - attached the VOM leads to the female ends of the sensor connector and spun the tire to measure AC voltage output. Neither sensor showed any voltage output. My understanding of generic ABS research indicates there should be 0.3 to 0.5 VAC generated with approximately 1 wheel revolution/second. Generic research also suggested that resistance should have been ~1k ohms across the sensors instead of the 16.3 I measured at each sensor.

What are the chances that BOTH sensors would show identical readings and BOTH be bad without one having given an indication earlier than the other? The car did mostly sit over the winter, so I also wonder if the sensors just need cleaning?

Frank

Well I found my shop manual and see that it's not the flat plug, but the round plug for the sensors. New testing revealed resistance of 1 ohm at each sensor - within specs listed in the manual.

I then spun the wheels and read up to 0.5VAC generated as they spun up.

There's a long list of tests in the manual - most requiring the diagnostic machine that dealers buy for about $2000. But there are still some things that I can try - like cleaning relay contacts. Guess I should test the rear sensor also.

This job's no fun at all!! I'd much rather install curb feelers, coon tails, chi-chi ball headliner trim, hang dice from the mirror, chains from the rear axles or maybe put one of those chrome boat propellers on the rear end. Yeah, when this is over I think I'll do a beauty treatment!!!!:eek: :)

Frank

Frank

You have pulling the sensor cable at the wheel you shold see open circuit. replace sensor it should fix your problem.

I don't understand your statement "you should see open circuit". Do you mean that an open circuit is correct after I pull the sensor at the wheel?.... or that I should replace the sensor if the circuit is open?

Both sides now read ~1 ohm resistance at the connector with the sensor attached to the wheel. The MB manual specs say anything between 0.85 - 1k ohms (approx. - don't remember the upper limit) is ok. I can see that 1 ohm is borderline, but just don't understand your suggestion. Both sides generate ~0.5VAC on spin-up.

I've now removed the cover from the hydraulic unit module board (behind the passenger side headlight) and cleaned the contacts on the 2 relays located there. Tomorrow I'll take out the passenger side kick cover and clean the contacts on the relay and over-voltage protection modules located there.

I did notice that the outer insulation on the front passenger side sensor wire is disintegrating so the resistance to ground on the sensor wires might be below the 20k ohm limit. I may replace all 3 sensors on principle along with the master cylinder 'cause I really don't want to dig into this again for a long time. It's only money, eh?

By the time I finish this I'll be the definitive ex-spurt on MB ABS systems!!! Well...... maybe not.

Frank:)