Think I need new rotors, came back from a drive yesterday and when braking on wet grass the fronts locked, but the rears seemed to keep driving the car forward. Climbed underneath this morning and saw this. (the rotor that appears thinnest is also chipped on the edge!)
also asking myself why if the front locked up why didn't the ABS kick in? No warning lights saying anythings wrong either.




the thinnest side has a chip out of it about 3mm on the leading edge.

 

It could be the Accumulator (Black Tennis Ball on the Pump) if the pressure gets low, then you only get Front wheel braking from the Actuator with the Pedal. (As a Safety Device)

While the Rear wheels need the Pump working.

Or maybe the rear Brakes need Bleeding, as I wouldn't have thought a 'chipped Rotor' would have caused that.

 

Dave1109971,

I would consider the 2 issues separately.

That rear disc does look unusual. They seem to be a mismatched pair with that "thin" one being a 3-part composite disc. I've not seen one like that before. Are the pads wearing evenly on that disc? If pad wear seems ok, measure the total thickness of the disc and see if it's still within the defined tolerances. The other disc does seem to be wearing very cleanly with nice edges. I do think that at some time you want to move to a matched pair of discs.

As regards the front locking and not the rear. That sounds strange. IMO, the brake pedal will definitely pulse and shudder under ABS conditions. If you've lost brake pressure on the rear end through accumulator problem, you'll get both the red light and the ABS light illuminating. Are you sure the ABS hasn't been disconnected or the fuse removed / blown? Do your brake warning lights illuminate and extinguish in the appropriate manner on startup?

I would check that everything is in order on the startup sequence, checking pump builds up pressure and the lights illuminate and then extinguish as per defined procedure. If all ok, I would "test" the ABS (very) safely somewhere. I don't think you can rely on wet grass when you can get different braking coefficients happening. If the startup and light extinguishing is not occurring properly, then you need to go through a more detailed diagnostic procedure.

Good luck

Paul

 

Thanks Guys, Yep get correct dash light sequence on start up. On start up I can hear the pressure build up in the accumulator, it depends on time since last start on how much time it takes to build up pressure. The yellow dash light stays on whilst building pressure then goes off when the pump stops (does that sound right?) sat for a couple weeks takes 20-30secs if starting after week or less maybe 10 secs max.

I was aghast when i saw the the rotors! will check the pads and measure the rotors, but my OCD will mean I'll have to replace both with new vented probably. I'm a long time car enthusiast with lots of desire to learn so I'll find someone in the jag club to help drop the rear cage I think. May as well recondition the whole shebang while out.

 

Hi Dave

There is something very Wrong here!

It is inconceivable to have an XJS with Mismatched Rotors.

The Three Part one is OEM while the Single one is Aftermarket.

The Aftermarket ones are a lot better, as when the OEM ones get worn, what looks like a Ring going round the Rotor.

Is in fact just that, a Ring going right round the Rotor and when wear gets to a certain point, guess what happens!

That ring detaches itself from the Rotor and drops onto the Drive Shaft, where it 'Clatters round and round' with a sound like a Telephone 'Ringing' I know this for sure, as it happened to me on my first XJS.

So now is the time to carry a hacksaw, so that you can chop it off 'When!' not if that happens.

My guess is the Previous Owner decided to change the Rotors but without dropping the rear IRS, as he could see what a job from Hell that was going to be.

And while its not Impossible to do so, it is a real Bar Steward of a job or so I hear.

So having just done one of them, he found it so difficult he really didn't want to do the other and as such decided to leave it as it was.

It would be very tempting to try and replace the Rotor that he left, also without removing the IRS, as the single piece one does not appear to be showing any wear.

Obviously that is up to you and how you feel about dropping the IRS.

The thinner one of those Rotors, 'could' also have something to do with your Front Only Braking.

As because of the Thickness differential, maybe most of the Braking, was just on the thicker rear Rotor, somehow tricking the System into thinking that pressure in the Accumulator was low.

In which case the 'Safety System' kicks in, in order to provide you with emergency Braking on the Front.

While the Start Up procedure you describe seems Normal.

 

Thanks guys

OB I think you're on the money regards the abnormal disc creating a false positive for the system. I drive far too 'enthusiastically' to not have perfect brakes . I think Joesphine will have to have a matched pair of rotors. Its funny but the wife (The Honourable Minister for War and Finance) gets such a kick out of being a passenger in her that she has absolutely no problems spending money on the old girl.

I find the idea of dropping the cage quite appealing actually! (certifiable probably) my thinking is its part of owning an earlier jag and as its a disease which means I see a Mark X or SI/II/III XJ6 in future I better get know the how, why when and where!

 

Hi Dave

Yup! Its always better to do the Job right, so I hope you will put up a Photo Post to show 'mere mortals' like me how it is done, as I may have to do the same thing myself on my 'Grey' Car.

 

Dave,

I've been thinking about your problem. If you had "lost" your rear brakes, your ABS would have become disabled and you would have had the ABS light illuminated. And it would have stayed on until your ignition was reset, even if your brakes recovered.

I think what might have happened was not a lockup of the front brakes but an aquaplaning of the front end on the wet grass when you applied the brakes (which would feel like brake lockup). The fact your startup sequence is normal and you never had the ABS light subsequently illuminate would imply that you didn'y lose the rear brakes and teh front didn't lock up.

Go on, drop the rear cage! It's quite therapeutic! But give yourself some days to do it as you'll want to clean and paint the cage, possibly change the mounts, change the pads and handbrake pads, consider rear wheel bearing change, consider radius arm and bush replacement (and check integrity of radius arm floor mounts), consider rear spring and shock replacement, think about any diff refurb, check halfshafts and consider rear exhaust box replacement! I know, I've been there! It's the ideal time to do any of those jobs whilst the cage is down!

Good luck. Let us all know how you get on!

Paul

 

Thanks Paul you prempted my next question about what to refurb while its out! It will be a project for spring I think. Just got to organise some help. What paint do people use when refurbing the cage? or powder coat? Also are there any benefits in going to aluminium calipers when changing to ventilated rotors?

 

Not sure that aluminium calipers are available for the rear. They still have to carry the handbrake assembly. I run vented disc set up on the rear of my '89 and doubt there would be much to gain over that when weight transfer etc. is taken into account. Not a lot wrong with the XJS brakes.
Although the discs in the pics are not a pair I do not see all that much wrong other than the uneven wear, side to side, of the original damped disc. I would check that the caliper piston on the thick side of the disc is not seized though.