280SE - mysterious brake? problem

[s class]

Hi all,

My 280SE had some front rotor problems (well at least that's what I believed). THe symptoms were

a) classic warped rotor type shudder when braking from speed
b) when approaching a stop slowly, with light brake pressure, a sort of cyclical variation in braking effectiveness (presumably as the rotors rotated and the pads contacted high and low spots).

I recently fitted new front rotors, and I took great care to make sure the mounting faces on the hubs were clean, and the mounting bolts torqued up to spec. At the same time I overhauled the front calipers, fitted new rubber hoses, and flushed out the system. I also fitted new ATE pads.

The hydraulic behaviour is perfect - brakes have good bite and the pedal is really firm.

I was disappointed to STILL have both symptoms a) and b) above. I removed the wheels and used a dial gauge to establish that the new discs STILL had some runout (approx 0.1mm). I was satisfied that this explained my problems.

Then this morning I had the (new) front rotors turned on the car, and now they are perfect without runout. As I expected, symptom a) (the high speed shudder) has completely gone, but BIG surprise - symptom b) remains unchanged

I have gone over the system again and I am positive there are no hydraulic issues. I can only think of three possible causes :

1. rear rotor problems (though when driving, the problem DOES come from - or at least appear to come from - the front).

2. cyclical variations in brake booster performance - from vacuum issues?

3. cyclical variations in engine output power when coasting to a halt.

Personally, I fancy 3. at the moment. The engine has developed an uneven idle, the cause of which I'm not sure - I suspect fuel injectors or more probably the fuel distributor. If one sits at a traffic light and then eases off the brake so that the car starts to move slowly forward against the remaining resistance of the brake, the engine is basically at idle, so I guess if the idle is uneven, the power output will come in cyclically varying 'pulses'. The engine is fine when the revs pick up.

Ideas? I'm stumped. I guess I will overhaul the rear calipers and check the rear rotors with the dial guage just to rule out problems there.

 

[WGB]

What tyres are on the car?

Have you checked your rims are true?

It is possible to have lateral run out of tyre and/or wheel as well as a possible out of round tyre.

You can have the tyre shaved on the car to correct out of round conditions but not so easy if there is lateral run out.

Lateral run out could be made worse by bush problems in the front suspension.

In the 1970's people put radial tyres on the old P5 Rover 3 litre which was very specifically set up for cross-ply tyres and the radials caused them to do a little lateral weave at low speed and when braking to a halt, especially with the crappy fabric radials available at that time.

Bill

 

[s class]

Bill,

Thanks for the suggestions. I can answer you as follows.

The enitre front end was rebushed about 30 000km ago, and the rear end about 10 000km ago.

Tyres are Goodyear Eagle 205/70HR14. I had them balanced again yesterday, and while they were at it, I inspected the rims and tyres as they turned. Rims are perfect. The tyres are about as good as one gets - almost zero lateral runout, and some are ever so slightly out of round. One seldom sees tyres better than this.

Related to that, I can comment that when the front rotors were turned this morning, part of the service is they check the front hubs and stub axled for true running, and everything checked out OK.

In further defense of the tyres, I will say that this problem of 'pulsing' at slow speed when coming to a gentle halt has been present for a couple of years already, certainly it was there with the previous set of tyres too. I had always just assumed it was the front rotors which I knew needed replacement anyway. Now that I have replaced them and had them turned, I realise the problem must lie somewhere else.

After I posted this thread from work, I drove home this evening, really paying attention to where the pulsing was coming from, and I now think it could indeed be from the rear. I then realised that I should find a steep hill, coast down it in neutral, and apply the brakes gently. This may make it easier to identify the source - and will at least eliminate/confirm uneven pulling from the engine. I will do this tomorrow. Depending on the findings, I will make the effort of measuring up the rear rotors with a dial gauge.

 

[John S]

It looks like it's time to have your rear rotors checked as there is a one way valve in the vacuum breaking system so engine idle variations won’t have any effect on the vacuum reservoir as you brake.

One simple check for the idle variations is to spray WD40 around your fuel injectors. If the WD40 gets sucked away or a puff of smoke comes out the exhaust you probably have vacuum sealing problems with one or more injectors.

 

[s class]

Some feedhack to close off this thread -

I replaced the rear disks today, and the problems have been solved.

Initially I had been skeptical it was the rear discs, simply because I didn't think warpage was a common issue at the rear. Of course, back when the car had about 300 000km, they were skimmed, and now its another 150 000km later, and I think they were just under the minimum thickness, and therefore warpage happened.

 

[Michel]

Some feedhack to close off this thread -

I replaced the rear disks today, and the problems have been solved.

That was my thought.
But I came late on this thread....

 

[WGB]

Some feedhack to close off this thread -

I replaced the rear disks today, and the problems have been solved.

Initially I had been skeptical it was the rear discs, simply because I didn't think warpage was a common issue at the rear. Of course, back when the car had about 300 000km, they were skimmed, and now its another 150 000km later, and I think they were just under the minimum thickness, and therefore warpage happened.

Going through water when brakes are very hot can warp them too - especially if they are a bit skinny on it.

Bill

 

[Styria]

Hi S-Class, by the time I got around to help you solve the problem, I find it's all done.....and, whatsmore, you did it all by yourself ! - That's even better. I must say though, it never amazes me how various gremlins can befall a car. I hardly ever have some of the problems that you have struck. Anyway, excellent work on your part. Regards Styria