Hello all, chasing a brake/DSC gremlin:
2003 S-Type R with 120K miles, garaged daily driver in Maryland. Over past several months, car has several times suddenly applied/released brake on one or the other of (I believe from sound/motion) the front wheels, while car is moving slowish and usually turning but not sharply. After several on/offs, the DSC unavailable light witl light and stay lit for the remainder of that drive, and the problem won't recur during that drive. I have started turning DSC off at the beginning of each drive as a stopgap up to now. I haven't been able to use my Foxwell NT301 code-reader at the end of any of these episodes, and when used after the car has been off, no stored or pending codes are found (that apply, one pending code for intermittent voltage issue with intake air sensor).
Brake rotors have been replaced twice, last time a few months before this problem began. All hub assemblies are original.

Hope there is a clue in there somewhere!
Thanks

 

Any problems with the parking brake? Before digging too deeply, look through this thread on how a marginal battery can sometimes cause some weird issues. Your symptoms don't quite fit, but it's still worth reviewing:

https://www.jaguarforums.com/forum/s...attery-193787/



And copied from another post just a few minutes ago, this is certainly worth checking:

These cars are very sensitive to low prestart voltage, and various computer modules will act up, even though the battery seems to have enough oomph to power the starter. Two things you can do to confirm this:

1) Before starting the engine, turn on the headlights for one minute and then turn them off. This removes the surface charge from the battery. Next, read the battery voltage. If not 12.6V or above (A VERY high standard), you may experience random faults that make no sense. You'd think low prestart voltage wouldn't matter, but forum experience suggests otherwise.

2) Try hooking up a battery charger every night for a week or so. I understand, it's a nuisance. But this makes sure your battery is fully charged just before each start. If the car now behaves, then you'll know that low prestart voltage was part of the problem. This may not necessarily be just a bad/weak battery, but it could be something slowly draining the battery overnight. But try the battery charger routine for a week and see if there is any improvement. Doesn't cost a thing, just some of your time.

 

Thanks for the input Karl, an interesting read. Battery measured 15.9 cold overnight, 15.8 after start and shutdown, so I don't think I'm going to get off that easy, unfortunately. Pulled the left-front sensor, clean as a whistle, lead looked undamaged. Will pull a rear next to compare. The reports of issues being resolved with hub assembly replacement suggests the "ring" can fail?

 

Typo? Maybe 12.9 and 12.8?

 

No, that's actually what my multimeter read from pole to pole. Considering that for a project at work we just used 10 brand new deep cycle batteries and that the chargers would call them 100% when they were reading ~14V, I don't know what's up with that reading. But I don't think the battery is low.

 

Didn’t know a battery could retain a voltage that high, but maybe that’s the issue. Normally we see low voltage, but high is also considered out of range and could cause trouble.

I’d suggest double checking with another meter to be sure. If accurate, such high voltage may have cooked that poor battery, too. Usually 14.5 while charging is considered the upper limit.

 

Something is very very (catastrophically) wrong if the _battery_ is anything like 15.9!!!

 

A bad alternator can over-charge battery. This condition can cause a battery explosion due to internally shorting it (ask me how I know this).

 

If you don't have another meter available, you could check that one for accuracy by reading the battery voltage on another car.

If your meter is indeed accurate, you'll need to correct the overcharging. You'd be looking at a new regulator (inside the alternator) at the very least. Make sure to get an OEM unit, as most aftermarket regulators don't properly control the red warning on the dash, causing it to always illuminate.

I'd also suspect the battery is trashed, too. Even if it was severely overcharged, I'm amazed it could hold the high voltage for so long at rest. That makes me question the accuracy of the meter, so definitely verify that first.

Is this why your brakes are acting up? Good question, heck if I know. But from Troubleshooting 101, fix any known faults first and see what happens.

Certainly an unusual situation. Please keep us posted.

 

The meter could be off, it is old. I'll bring a good one from work and double check, definitely don't want a battery explosion However, it seems unlikely to me that an off-spec battery could cause the DSC sensors/processor to decide it needs to activate a brake when the car is running, I still suspect a sensor issue. Wonder if brake heating could partially demagnetize the permanent magnets in the hubs...

 

On a good Fluke meter, battery was 12.25 after sitting at work for 11 hours, 12.49 after the 15 minute drive home.

 

Making progress! The suspect voltmeter has been confirmed bad.

Back to your present readings. 12.25v after 11 hours of rest? Too low, the practical prestart limit is 12.6v. Is this the root cause now? Still unknown, but the forum consensus is strange things happen below that number. Up on my soapbox again, but clear up the known faults before digging elsewhere.

I'd suggest the nightly battery charger method for troubleshooting, as detailed in post #2 above.

 

Yes, will try charging. My logical problem is that this is my daily driver, making the exact same commute for the last 14 years. But all parts are aging, perhaps some chain of circuit resistance has pushed the sensor behavior over a threshold.

 

Possibly sticking brake slider pins?
I would remove both front wheels and check that the slider pins are free and moving.
I would remove the pins and lubricate them at any rate.
When was the last time the front brakes were worked on?
.
.
.

 

Late to the game but as you all discovered a battery should not read over 13v at rest. 15v would be ridiculously high so ya, definitely a bad meter! But you knew that!

 

Club, I replaced all rotors/pads with Stoptech drilled/slotted rotors and street pads in May. I reused the front pins because they were in good shape and a slightly better design than the Stoptech ones. The pattern of braking when the problem happens is that I am not braking or lightly braking, and one brake (I believe because of the steering pull) is suddenly applied firmly (not full lock) released quickly, then reapplied, several times, so sticking isn't a good fit.

Aracuda, yes, time to replace that one.

 

Any updates?