I bought a 1999 XK8 recently with only 85K miles on it, and all systems functioning well. I've only put about 200 miles on it since purchase, and everything was great until a few days ago. Pulling into a parking spot with foot on the brake gently just to slow to a stop, suddenly the engine went full throttle with no gas pedal input on my part. I quickly took it out of gear and into park, and the engine continued at full throttle until I shut it off; it didn't redline because I got to the ign. switch in time. Of course, my first thought was that something had jammed up the gas pedal-NO. There was nothing obstructing it, and it moved freely as always. Upon re-starting, the car ran fine with no further issues that day, but I now am leery of driving it again. Earlier in the day before that happened, upon starting for the first time the 'ABS' and 'Traction Control' lights had come on and stayed on until restarted later in the the day.
I do know that the brake pedal switch(es?) are in need of repair because the light switch tends to stay on if I don't gently lift up on the pedal a little with my foot. I've found several others online who have had exact same throttle occurrences, many of whom ended up in serious 'accidents' because of it, but have not found ANY answer to the problem. I know there are several switches and sensors involved in throttle response, as well as cruise control and ABS all tied in. I also have not found any reference to this problem in this forum, so I am desperately asking if anyone knows what caused this on earlier cars. I understand that most models have experienced it in the late 90s to mid-2000s. This car has been well taken care of, including chns./tensioners and trans. service, so I don't think it is from neglect. I have had an XJR '98 for 7 yrs. and never had this issue, so am completely baffled and now afraid to drive it.
Please help if possible; I really miss driving the XK.

 

This is likely just an adjustment. The "trigger" on this switch has a slide/ratchet mechanism that can be extended so the trigger reaches the bar off the brake pedal stem. It's going to be a pain to reach and adjust, but no need for a new part.

Best of luck, keep us posted.

 

Please provide data on them as they are hard to find.

 

I would suggest you get those brake pedal switches repaired as one of them shuts off the cruise control and the cruise control is the only thing that can override the physical position of the throttle pedal. Although the throttle is electronic, it has a mechanical guard that prevents the throttle opening without input from the pedal itself, unless cruise control is on...

 

Since you were pulling into a parking spot, we can rule out cruise control as the culprit. Cruise control is not available below 17.5 MPH, and it is disengaged with your foot on the brake pedal. "It didn't redline" could be because of the mechanical guard in the throttle assembly, or your quick reflexes, we'll never know. The electronic throttle has a DC motor and a few springs, and they work in opposite directions so if something failed, there may not be enough force to oppose a spring and it can do undesirable things. Consider this: The Jaguar Technical Guide states on page 29, "The input shaft spring and the mechanical guard spring oppose movement in the throttle open direction (move throttle toward closed), and provide the "feel" of the accelerator pedal. The throttle valve spring and the drive gear spring oppose movement in the throttle closed direction (move throttle toward open)." I added the parenthesis for clarity. It goes on, "The DC motor drive gears turn the throttle valve in the closed direction, the throttle valve spring turns the throttle valve in the open direction..." Here is where I think you need to focus. From what I'm reading in the Technical Guide, the accelerator pedal input does not have a hard, full time mechanical connection to the throttle shaft (this isn't your father's Oldsmobile), it just moves the input shaft to provide an input to the ECM through sensors. The input shaft also moves the mechanical guard. The input (movement) from the accelerator pedal (input shaft) causes a misalignment of sensors between the input shaft and throttle shaft, which the ECM reads and corrects by commanding the DC motor to move the throttle shaft. When the movement of the throttle shaft causes the sensors to realign, the ECM uses this feedback as proper throttle position and it commands the DC motor to stop. That is the closed loop operation. Therefore, I don't think the ECM would fault the DC motor if it fails, as the ECM is not looking directly at the operation of the DC motor but instead at sensor position, and misalignment of the sensors is a normal transient condition. So what could give an uncommanded opening of the throttle? If the DC motor failed to operate momentarily, you lost the force to oppose the throttle valve spring, which would then open the throttle valve as much as can be allowed by the position of the mechanical guard [which I believe is positioned by the accelerator pedal (input shaft) position]. Especially if this is a momentary or intermittent problem, I'm thinking that the DC motor or its command from the ECM is a good place to start. DC motor beginning to fail? Parts often begin to fail intermittently before they completely die. Loose or failing wire connection? Maybe. Also, the thermostatic air valve bypasses the throttle plate so that could mimic an open throttle but it's only supposed to open at very cold engine coolant temperatures...if it's working properly. But you were pulling into a parking space so the coolant should have been warm, at least. Again, I would start at the DC motor on the throttle assembly. I hope this helps.

 

+1 throttle body problem. If you don't want to mess with it yourself you can contact ASI and send it to them for testing and repair if they think they can help.

Automotive Scientific Inc. Your #1 Source for ECU, TCU, ABS, EBCM, Climate Control and Speedometer Repairs

 

The same thing happened to me with a Daimler XJ6. I was travelling at about 40mph - I can't remember if I touched the brake - when suddenly the same thing happened. I switched off and cruised around a corner. I couldn't see anything wrong under the bonnet and so started it up. It ran normally and never had it happen again.

 

[QUOTE][QUOTE] The DC motor drive gears turn the throttle valve in the closed direction, the throttle valve spring turns the throttle valve in the open direction..
Disclaimer: I'm not saying the book is wrong. I'm just trying to understand the logic.

That is highly counter-intuitive. I had always thought it a design requirement that a throttle be commanded to mechanically close in the face of a mechanical (or electrical) failure.

Based on the text that assumes spring force overpowers an undriven motor, wouldn't our throttle valve go to WOT on shutdown? As we know, it doesn't.

 

https://forums.edmunds.com/discussio...n-acceleration

This seems to be the main listing of these occurrences I've found, although there are definitely others out there. Just doing a basic search for 'Jaguar_sudden_acceleration' turned up most of them. This happened to various Jaguar models, not just XKs. Apparently, Jaguar refused to acknowledge that the cars actually had a problem, simply because of the nature of the glitch: it left no record in the DTCs or anywhere else in the the memories of the computers. It all sounds very reminiscent of Ford's earlier problems with same symptoms in their cars, some even without cruise control fitted. It is always just blamed on driver error.

 

Hi, and thanks for your quick reply! I would certainly suspect that as well, except I'm puzzled by the fact that currently the brake switch at the pedal tends to stay 'on' unless I gently pull up on the pedal with my foot. That would lead me to think that the cruise control is also dis-engaged at that time. If the cruise is indeed the only thing that can override the pedal's physical position, I would be happy to cut all electrical power to the cruise control system in order to drive safely. I just don't know if I can pull a cruise control relay and/or fuses and not have it affect some other system.
From everything I've read of reported occurrences, there has been no definitive solution . One thing I'll be testing tomorrow is the charging/battery system, because I suspect a low charge rate or possibly bad battery since I'm getting a low cranking speed and dim/bright dash lights on starting. I imagine either of those could account for the acceleration glitch. Any more thoughts on that? I really appreciate the input. I was a factory Jaguar mechanic (before 'technician') for many years and ran my own shop, but got away from it when everything went electronic.

 

For those who have a hard time believing that unintended acceleration can actually happen without driver input, I would simply ask how many times your home or office computer has mysteriously shut down, re-booted, blue-screen of death-ed, etc. without you doing anything to cause it. Bear in mind that your pc or laptop has ONE cpu and probably one hard drive and certainly one motherboard. How many of these are in the average car these days, and aren't they ALL tied in and networked together? The chances of a glitch in a car's system are, in my opinion, far greater than in a pc or laptop, so please don't keep denying the possibility.

 

At least 2 of those are S-Types and consequently are false. The TB on an S-Type does not work in any way as described in this thread. It also has 2 different sensors and you get a limp mode if they disagree. (The accel pedal is similar, and it's a drive-by-wire car.) Trapped carpet or foot on wrong pedal or some such. Even better, the S-Type throws a code and leaves it logged for diagnosis with many driving parameters. Clearly those owners didn't know or look. (It's well documented, too - good place to start is the Electrical Guide.)

Sorry to say I don't know about this variant of the XK8 but some of the claims sound very unlikely. I suspect the documentation is wrong or misleading! Anyone able to be sure?

 

Sorry but that is ridiculous.

Software for cars is NOT thrown together by useless dweebs messing around at Mickey soft.

Both your & my country has big issues with car safety and car makers are forced to recall cars and modify them in cases such as these - as Toyota had to do. Have Jaguar? (They haven't for the S-Types, how about these?)

 

By 1999/AJ27 the TB had progressed to full authority (fully drive-by-wire) and cruise was electronic rather than vacuum/mechanically actuated.
The Jaguar Training Manual 881 explains the differences between the two. It's available here on Gus' site:
http://www.jagrepair.com/images/Trai...ual-9-5-01.pdf

I imagine that the complexity of the position sensors (twin outputs with an offset) is to minimise the risk of an erroneous (including WOT) command being issued, but I can't find fail-safe explained other than being managed by the ECM. No definitive either way, but I've seen enough wacky computer outputs to say that it's not impossible - even if the code isn't written by the folks in Seattle - although I would expect the ECM to be much more resilient and defensive since bad outcomes are potentially more serious than the odd blue screen.
Difficulty is that the cause may well be transient and therefore extremely difficult to identify.

Just my 2c...

Brake lights staying on is most likely switch maladjustment as fmertz says, or something stopping the pedal returning fully upwards?

 

To 'JAGV8',
Please clarify what in particular you think is "ridiculous". Even the smartest computer developers can make a little mistake here or there, or overlook some unintended interaction between components. As far as Toyota is concerned, to my knowledge they NEVER admitted there was a problem. They just kept paying out on lawsuits and blaming it on floor mats, pushing wrong pedal, etc. They did the only thing they apparently could think of to resolve the problem: They added one more failsafe that , I believe, shuts off throttle when brake is depressed. That simply means that they never discovered the real problem OR decided it was cheaper to add the extra failsafe.
Having worked for several auto mfgrs., including Jaguar, Toyota, Nissan, Lotus, Jensen, etc., etc. I have seen how these corporations can frequently whittle it all down to 'the bottom line'. I still LOVE Jaguars, and have owned just about every model made from the 1950s to 90s. I just want to be able to drive the car I love without fearing for my life or the lives of anyone around me. That is why I'm searching for answers, and not just giving up and selling the 3 Jaguars I currently own. Unintended full-throttle acceleration REALLY HAPPENED to me and many others, and we need to find the answer.

 

Disclaimer: I'm not saying the book is wrong. I'm just trying to understand the logic.

That is highly counter-intuitive. I had always thought it a design requirement that a throttle be commanded to mechanically close in the face of a mechanical (or electrical) failure.

Based on the text that assumes spring force overpowers an undriven motor, wouldn't our throttle valve go to WOT on shutdown? As we know, it doesn't.

Yeah, it seems counter intuitive, but remember that something has to move the throttle toward open and it is this spring. And it's not the spring that you oppose when you step on the accelerator because that spring closes the input shaft and there is no direct positive mechanical connection between your foot and the throttle. There are two shafts, the input shaft and the throttle shaft. Sensors tell the ECM if these two shafts are out of sync. Your foot is moving a sensor on the input shaft which the ECM uses as information to then command the DC motor to position the throttle shaft to bring the two shafts into sync. Consider that the ECM is controlling throttle position through the DC motor, and shutting down fuel injectors and ignition, among other things upon engine shutdown. Throttle position is always limited by the mechanical guard (there is your fail safe), which is positioned by the input shaft, which is spring loaded to throttle closed until you step on the accelerator, so no, the throttle would not be wide open on shutdown unless you floored the accelerator.

 

These machines can fail, and often make no sense when they do. Not applicable to the OP's issue per se, but I once had a mildly terrifying ride in a early 1990's Ford Windstar van when the cruise control refused to disengage. Tapping the brake, or shifting into neutral did nothing except the rpm redlined + when in neutral. Even turning off the engine and restarting it did not disengage the WOT. Burned the brakes out coming to a stop (had to leave engine on or I had no steering or BRAKES. What a cluster F.

Z

 

Toyota had a specific problem in the design which Jaguar hasn't. Arguing Toyota did or didn't do something (and they were forced to do a recall so some of your claim is wrong) doesn't advance evidence against Jaguar, especially as it has a very different and this time redundant design.

Vital systems are coded so that vital things are kept simple and have higher control than the more complex parts. This is (sadly) not how Mickeysoft do things.

So for example, the 2 sensors for TPS or APP are read & compared in high priority control code and if wrong the car is put into limp mode. The more complex part to do with fuelling and rate of acceleration is disabled from doing things such as max acceleration.

It is ridiculous to compare vital parts of car software with the rubbish that Mickeysoft have conned people into buying by being a great advertising & marketing company. If only their software was as good.

 

There are two switches on the brake pedal, one for the cruise control, one for the brake lights, so its certainly worth fixing this issue.

You also need to work out which throttle body you have - the early vacuum cruise control type or the later electronic version as they work very differently. 1999 was a changeover year, so you need to check before researching further. You can post a picture of the throttle body on here and someone will tell you which you have got.

 

[QUOTE=mhminnich;2017530][QUOTE]
One would think that. But back in the 1970s Reliant fitted throttle springs where, if something snapped, the throttle flew open. I don't know how many, if any, other manufacturers did the same.