So my car runs and drives quite good but there are a few things I will tackle to make it perfect. One thing that I haven't figured out yet is my brakes that are "grabby". Background: 06 XJR 79K km (~50K miles)

OK so since grabby isn't a word I'll do my best to explain it. If anyone has any ideas on why this is happening and how to fix it let me know.

When I press the brake pedal, if I press it very slowly and gently (still safely) it stops what I would say normally, smoothly, albeit slightly slower than normal.

If I press the brakes either hard/quick or even at a medium/normal rate the brakes have a harsh grab at first. It's almost as if there is a boost of brake pressure initially or a caliper or two that are grabbing momentarily. After the initial grab they seem to work fine.

It's very noticeable and I'm sure not normal, I've driven dozens if not a hundred+ vehicles in my life and not experienced this in any other car.

I asked the Jag dealer for service history and it was brutal trying to get information from them, but I got them to admit that the brake fluid was never flushed.

I'm not sure if a brake fluid flush would help but I was thinking that could be a next step to rule out a fluid/air problem. Usually an air problem would give a spongy brake pedal so I really doubt it's fluid related.

I don't want to randomly replace parts. I'd like to find a way to properly diagnose the issue without having to spend $2,000

 

These cars have something called " Emergency Brake Assist". It is supposed to detect emergency braking and "assist" the driver to brake the car. So there may be a fault somewhere in the system, and it is applying the logic incorrectly.

X350 Specs - Press release 21st December 2002

 

Hmmm I suppose that could be. Does anyone have any diagnostic info on that system? I found some PDFs of the service manual but the closest thing I could find was how to bleed the brakes and general info on the electronic park brake.

 

The braking system is fairly robust, as well as complex. And as Fraser has pointed out, the complexity is part of the design. There are numerous factors that can affect it's performance, and the Jaguar service literature has point to point testing procedures. This is one system that you would want someone with experience in this design to properly diagnose. Items that can affect it can be state of engine tune, speed, rate of application, cruise control, various sensors on the braking system, ECM and ABS... the list goes on. One thing, brake flush with new fluid is recommended by Jaguar every two years, regardless of mileage, and the fluid is not common DOT4, but low viscosity ISO-4925 class 6 DOT4, and not available at your local parts store. In the end, yes the braking does feel different in these vehicles than just about anything out there.

 

It happens at any speed, and it happens whether I have traction control on or off, cruise on or off (I can feel the grab if ACC slows me too). My engine runs a touch rough at cold startup but I have new spark plugs to install.

I'd hate to take it to the dealer if there are some initial diagnostics I can do first to help narrow things down. I have above average knowledge on cars and how they work but I am a Jag newbie.

Anyone have access to the factory service manual for this braking system?

 

So I did some more searching in the forum here and one guy had brake issues that was diminished after doing a hard reset on the battery (his words). After a little while his brake power went strange again.

So I think I will do a "hard reset on the battery". How long do I need to leave the battery disconnected for on this car? And, is there anything I need to know after I reconnect the battery for any of the system?

 

My best suggestion is to get a copy of JTIS. (Jaguar's service manual) You can search the threads to get a copy. In the JTIS, the Pre-Delivery Inspection section describes the process that needs to be done after battery reconnection. It goes without saying, that unless I have seen the vehicle or driven it, it would be hard to diagnose a condition of "it feels different" over the Forum. You might seek a local independent shop that does Jaguar specifically, and has knowledge and experience of these vehicles to actually take a test drive to help confirm that there is actually a problem. Buy him lunch.

 

It appears JTIS only works on Windows?? I only have Mac computers.

I can tell you this isn't normal. Now that I know just a tiny bit more from searches here and other places, my hunch is that a sensor such as an accelerometer may be faulty or otherwise be causing some of the issue. You see, if I slow down really gently it's OK but if it's normal or hard braking, there feels like that big boost of brake pressure (surge?) for like a 1/2 second. So I am suspecting something in the EBA system is the cause. If I could just find a good technical explanation of the entire system I could maybe even disconnect a sensor and see if that will affect things. I don't need a tech to tell me if what I'm experiencing is normal, it definitely isn't.

I have an Actron OBD-II scanner CP9580 @ http://amzn.to/2cNFIER but I doubt it's capable of reading advanced Jag stuff. I used it just to read for DTCs when I bought the car, I haven't tried any advanced stuff yet.

 

Well this is interesting... so I took the Jag out for lunch and to run an errand. I don't drive it every day for a # of reasons. But...

After pulling out of the driveway I decided to take it out of sport mode. So I drove around in regular mode and the brake grab issue was at least 50% less pronounced. In other words it's nearly normal! I couldn't believe it.

So, I put it back in sport mode and did a bit more driving. I didn't drive it a ton but I can say that I can brake like a normal car now when either braking lightly or normally (medium pressure). I did have to step on the brakes hard one time because a car was coming and I could feel the harsh grabbing. Overall it's much better, more bearable for sure. It still grabs hard when you jam on the brakes quickly but normal and light brake feel is MUCH improved.

I'm going to continue to monitor the situation but that is really odd. I wonder if the brakes are changed somewhat between sport and regular mode? I have driven in sport mode 99.9% of the time since getting the Jag due to how it shifts.

I didn't reset the battery, I didn't do anything other than take it out of sport mode after leaving the driveway...

Interesting development, I'll keep you posted.

 

According to my copy of the Jaguar technical notes, the 'Sport' mode only affects the transmission, nothing else. I have never had anything like this happen to me, although there is always a first time !

Keep us posted.

 

Hmm I had a very similar issue, more noticeable or pronounced once the car had warmed up somewhat.
This issue ( for me ) turned out to simply be a sticking caliper on once side. A service kit and clean up replacing pistons and seals cured this issue for me


Jim