This one I'm kind of at a loss for. A while back I was showing my car off to a friend so I floored it, got up to maybe 60, then hard braked to slow back down to safe speeds. Hard braking to the level of seeing a deer and tripping ABS on dry road. During braking, all my lights dimmed hard, stereo cut out and took a moment to turn back on (like it does when first being turned on), i got a battery warning and light, and maybe a few other warning lights that flashed for a moment as the gauge cluster woke back up. It hadn't happened for a while since then, so I'd forgotten about it.

Between then and today, I had replaced my alternator for charging issues. Replacement has been working without issue, but it is a junkyard alt.

Today, I was driving a bit enthusiastically, working out the new tires and getting familiar with the sport shifting mode. Man I'd forgotten how scary fast these things can go, 60 seems to come a lot sooner than 5 seconds. Bit of buying/repair anxiety alleviated there. However, after accelerating then braking hard it did the same thing, power and lights cut out for a moment, batt light, stereo turned back on, etc. I managed to make it do it once more, just for repeatability's sake. Same scenario. However, soon after that I hit a moderate bump on the road and the batt light flashed for a moment there, but I didn't notice lights dimming when it happened, and the radio didn't cut out as it did under braking.

Above where I mention batt light/error message, both came on together each time, as I assume they're supposed to.

I don't /think/ it's the ABS eating up/shorting the power, unless it uses more power under hard braking. I haven't tried unplugging the ABS module and repeating the scenario yet. However, I had tripped the ABS a lot driving around in some snow, never had any power cutout when the ABS was engaging in the snow.

I've checked and cleaned the alt connection, and both ends of the engine to chassis ground strap. I've also checked the false bulkhead connector, battery ground strap, and battery positive strap for voltage drop per the Electrical Troubleshooting pdf. Everything checked out fine there. The only things I can really think of would be the battery sliding around and pulling a wire loose, or a severe short only happening under hard load... Any thoughts for what else to look for? Maybe my V mounts getting old and letting the engine loose under very hard braking?

 

Hi nilanium,

At some point Jaguar issued a TSB about stalling when slowing to stop or make a turn, due to low transmission fluid. My assumption is that the torque converter does not disengage, bogging down the engine and causing it to stall or come close to stalling. So it would be worth checking your fluid level just to be sure it's at the proper level.

Cheers,

Don

 

Thanks, I'll take a look at that when I get a chance. Think I saw an appropriate dipstick online for cheap, so it couldn't hurt to order that. I guess that would be easy to eliminate as a cause as well, by shifting into neutral before hard braking, and seeing if I get the same symptoms.

 

I got a dipstick and a chance to drive it around, and what do you know, the fluid is very low. After about a half hour of normal mixed hwy/city driving, the dipstick barely registered fluid at the 25C measurement range (where it should have been in the 80c range). I'll have to pick up some fluid and see if that changes anything. A fluid/filter change is in mind for the future, but a little later...

I'm starting to think the batt light over bumps and the power loss are unrelated - when driving yesterday, I had the batt light come up a couple times over bumps, but without any dimming or power loss. I'm guessing that could be caused by a loose connector at the gauge cluster. I've had issues with half the backlights on it dimming sometimes due to a weak connector, since the batt light wire connects to the gauges from the alt, it could be the same problem.

Along with that, I was driving yesterday in the rain, and I managed to replicate the power dimming, without driving like a madman, simply by driving at 30-35mph and emergency stopping hard enough to trip the ABS. I tried the same thing while in neutral, and same symptoms. Along with the radio cutting out/restarting, I noticed the transmission acted a bit more "grabby" when putting in just a little bit of throttle (sport mode was off), almost like the way it acts when the battery's been removed and the shift mapping is still adapting to my driving. Could the low fluid prevent the transmission from completely "declutching" even in neutral?

 

Topping up the fluid is definitely going to help. I'm sure you know this since you drove the car for half an hour before checking the fluid, but for the sake of others, the fluid really does need to be topped up while hot to get the most accurate fill. After your drive, park on a level surface, depress and hold the brake pedal and move the gear selector lever through all the gears, pausing at each position on the J-gate at least 3 or 4 seconds for the gear to fully engage. Return the lever to Park, set the hand brake and check the transmission fluid and top up to exactly the HOT or MAX mark. Do not overfill!

I think you're right that there is a loose connector somewhere - but the instrument cluster isn't the only possible place. For example, if the battery terminals, battery ground strap, engine ground strap or false bulkhead connector are corroded or loose, or if the battery mounting bracket is allowing the battery to move around when going over bumps or during hard braking, could something like that cause your symptoms?

I don't know if it may be helpful, but Rob (avt007) created a very good Electrical Troubleshooting guide for the X308 that might be worth consulting:

X308 Electrical Troubleshooting

Cheers,

Don

 

I found a section from the 722.6 manual which stated the same thing, running through the gears while parked, trans warm, after each fluid fill, then rechecking fluid level before adding more. I'll attach the PDF.

The dimming under braking does feel very much like a loose connector somewhere. I did a quick run through of the electrical guide when my alt was acting up, but it's probably worth running through everything, yanking on whatever could possibly be coming loose. Engine ground strap and alt-side connections all received a lot of attention when I swapped the alt out.

The battery being loose/moving was the very first thing to come to mind the first time it happened, but I've since rechecked battery terminals and the battery hold strap. I almost overtightened it until I saw that the strap was bowing upwards, so I backed off a bit there. It's still pretty solid, leaning hard into the battery didn't seem to move it at all.

 

*bump*

this is still happening. Dash lights up little a Christmas tree and radio cuts out under hard braking. Crank pulley seems to be fixed. I added a good amount of fluid to the transmission, maybe 2/3L got it to 3/4 between the 80c and 25c ranges on the dipstick (I didn't want to overfill and the trans pan measured around 65c). Trans surges just slightly when gently decelerating to a stop, most noticeable in sport mode, but I think that's a leaky connector(see my p1797 thread). Shifting to N before hard braking doesn't affect the power loss.

At this point I feel like the likely remaining causes are either a short in the ABS system or maybe that starter to bulkhead positive cable shorting? I'm going to try disabling ABS and seeing if that affects anything... does ABS take more power under hard braking on dry road than it does in say snow? I triggered the ABS a ton in snow at low speeds without issues... also does the trac button disable abs or do I need to pull a fuse? The bulkhead to starter positive cable had seemed good when I crawled around underneath but I don't know...

 

Have you had a look at the condition of the Mega fuse connections? they're on the bulkhead low down facing the battery. If the battery travels forward under heavy braking they can damage these parts.

 

The other thought - unlikely, but worn subframe bushes/engine mounts can lead to the engine moving under hard braking, maybe the engine harness is getting stretched or bad earth so removing and cleaning each main power end, front false bulkhead, and the short link lead between the front inner wing fuse boxes, engine ground, remove each end and ensure good ground to all posts. Oh and inspect the lower front subframe bushes.