During a recent road trip and after having used the cruise control numerous times, during a subsequent use, I got several fault warnings on the odometer display.
At first, the warning of "Limp home Mode" appeared. Next, a warning of "Trac Mode unavailable" appeared. The Limp home Mode then re-appeared and the car started to decelerate and would not respond to throttle inputs. I then toggled the cruise control button on and off, then leaving it engaged. Pulling over to the side of the road, I turned the car off for several seconds and then restarted it. The car would not restart. I turned to the key to the off position and waited almost a minute before attempting to restart it. On this second attempt, the car did restart and ran normally. I did not attempt to re-engage the cruise control.

I am wondering what my next step should be at diagnosis or whether I should leave the problem to the professionals. I have yet to plug my OBD II scanner into the car but will try that next.
I'm not anxious to go down the rabbit hole of having a mechanic spend hours at diagnosis and then have the repair done for an astronomical cost. Any thoughts?

 

My guess that it comes down to an ABS issue - and while it could be a defective ABS sensor (or even dirt on the "sprockets", which trigger those 4 sensors) or corrosion on contacts on the way from the sensor to the ABS module, I feel that on the X308 the most likely issue is that the 2 big solder points (the point for power supply) on the ABS module might have developed cold solder points over time, as they seem to have auto-soldered the complete module back then during the production, i.e the same time in the solder bath for all the small and those 2 big solder points - and it just was not long enough for those 2 big points. The module needs to come out, then it needs to be opened, even though it was designed never to be opened. Then those 2 points need be get re-soldered. I did that already on my X308.

I assume this is that the guy is telling you at time 1:48 in the following video:

I would not know, what he is saying, as I am listing to music just now...

 

When it didn’t restart, did the starter engage the engine and cranked over and over or did the starter not work at all?

Any faults in the modules?

 

Brake light switch: do a search and read them all.

 

Why are we guessing when we don't even know what car it is?
AJ26?
AJ27?

 

Bob, it’s a 2000 XJR. It says so in their profile.

 

I had a limp mode fault where the throttle would stop working with the cruise control 'armed' it was the TPS on the throttle body (not the separate one on the end that reads the cable).

 

Thank you for all of you who responded.

In answer to your questions; the starter did engage and cranked the engine over but didn't start. During the subsequent longer pause, it did restart on the first try which is as normal.

As this was a few weeks ago, I didn't see any "module" faults come up. What was on the odometer display was "ABS FAULT" and TRAC MODE DISABLED" I think.
The brake light switch is an interesting comment; I installed LED bulbs in my tail lights and each time I use the headlights, the "CHECK BRAKE LIGHTS" message comes on in the odd display. I figured that since the LED's don't draw as much current as the conventional bulbs, that must be triggering the warning.

 

As suggested above, keep the brake light switch in mind. I had trouble with my cruise years ago (though not quite like you described). I replaced the brake light switch because of a forum topic at the time, and I haven't had any trouble since. I wasn't using LED lights anyplace, but based on other forum comments over the years, I think they cause more trouble than they're worth because the car freaks out when it senses unexpected voltage from the tailllights/brake lights. Or any voltage variation for that matter.

 

The brake light switch has two switches in it.
The first one turns off the cruise control & the second one puts the brake lights on. What we seem to see is if one fails the brake lights can still work but it throws up a cruise related error.

 

Thanks again for all the tips.
I did a scan with my OBD II scanner and found no faults and everything showed normal.
Where is brake light switch located? Under the dash, as I suspect?
I'll give that a look and see if it solves the problem.
I might add, also, that this fault appeared after I had made the Vegas to L.A. trek and used the cruise control frequently.
Otherwise, the cruise control doesn't get a lot of use around town, as the car is my daily driver.

 

You should have a code in the ECM if it’s a brake light switch.

 

I have had this fault on a previous xj8 2000 the fault was the switch behind the brake pedal.
it was replaced and all was well again. I found the car started as normal but warning lights came on when I braked and the cruise control would stop working.
I hope this helps mark

 

Thank you, Mark.
I'll check this switch as part of the rest of my diagnostics.

Is this switch actuated by the brake pedal itself or is it just where it happens to be located? When I take look, this question may answer itself.
Thanks, again.
Gary J.

 

Hi Gary
I think it is activated by the brake pedal as the warning lights don’t come on until I use the brakes.
I hope this is of use to you I presume it is a common fault as it happened on two of my xj8s.
good luck and happy driving.
mark

 

Reread #10: The switch lights up your brake lights when you depress the brake and it turns off the cruise control when you hit the brakes. Easy to clean; easy to replace; hard to get to unless you are small and more agile than watash . .

 

An old chestnut, but worth repeating, again and again . . . use search, as any reference to BPPS will include . . .

.https://www.jaguarforums.com/forum/xj-xj8-xjr-x308-27/brake-pedal-position-switch-bpps-jaguar-xj8-x308-sport-pictures-280881/

This includes all you could need on X308 BPPS including pics, access, low cost repair or replacement of microwitches, and feedback from other respected forum members.

Cheers

 

Ahh handy, I'll take a look through that & should be able to fix the one I've got on the shelf since I replaced it. I can't fault the microswitches when putting a meter on them so it annoyed me that I couldn't say why it broke.