Hi Guys, I'm knocking off my XK8 issues one by one and today it's the turn of the Cruise Control Failsafe Engine Mode. I have read the forums and found threads regarding the brake switch and so that was the first thing I checked. With the aid of my 10 yr old daughter who crawled under the dash, I removed and tested the two switches on the bench with an ohm meter and they both operate perfectly , one normally open and one normally closed. So the problem is somewhere else.

Here's what happens...... I switch on the cruise master switch, it lights up OK. I then drive at 40mph and press the '+set' button and the cruise control switches in and works for about 10 seconds, and then the failsafe engine mode with an amber light comes on. The cruise control master switch LED goes out but the car drives normally. The warning goes away after the car is switched off; left for a few minutes and switched on again.

SO it looks like it tries to work but then is getting disabled by something shortly after setting. I was sure this was a brake switch issue. Does the cruise get a signal from the TPS or from a speed sensor somewhere ? Any help would be appreciated. I have ordered an OBD2 scanner ( Innova ) so that night give some more clues when I have it.

 

As far as I'm aware, the cruise control uses speed data from the ABS module which determines the vehicle speed from the four wheel speed sensors. If a problem occurs with a wheel speed sensor or the ABS module, then the ABS and the cruise control both stop operating.
I'm sure you will get more detailed information on the matter from more knowledgeable members but hopefully this will help you get started.
Andy.

 

I don't know what year your car is, and that is important. If you check the bar at the top of the page you will see USER CP. Go there and you can put your car info with your signature. Also, if you have not visited the new member area, please do so when you have time:

New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum

There is input from the transmission speed sensor, the instrument pack, and of course the ECU and TCM, as well,as the ABS module. A generic code reader will not be able to read some of the stored codes.
The early models have a vacuum controlled system for the cruise, with three solenoids located in the fender well. There is a dome of about 2.5" with a vacuum line mounted above the throttle body on those cars. One of the solenoids could be faulty.
Do you have the electrical diagrams and JTIS21 (factory manual) downloads yet? They are available through the sticky's on the forum page, but I believe you will need 10 posts before you can open pdf's.

 

Thanks for the replies, I will go to the new member area today. The XK8 is MY2001 and is UK spec, imported to Oz last year. I have downloaded JTIS21 and also electrical diagrams so will take a closer look.

I am an electrical / electronics engineer ( working in radio/tv/audio )and also just finished restoring a Lotus Esprit Turbo, so not much worries me when it comes to fixing things. A point in the right direction always helps, so thankyou for your input.

I have found the vacuum line that goes from the back of the engine through a hole in the front wing/fender and then a return vac line going the throttle body diaphragm. That all looks to be in good orde, no leaks. How do I get to the valves under the fender/wing, I can't see any access holes ?

I took the car for a drive and the cruise control came on and held the speed for about 2 minutes and then the failsafe engine mode light came on and the cruise disabled again. It almost works, the vac/diaphragm seems OK as it holds speed for a while. My OBD2 scanner just reports a CAN buss error, and then the XK8 displays a whole heap of errors on the LCD, traction control, gearbox, stability etc etc as if the OB2 scanner has made it unhappy. If I turn the Ignition off and on again clears the displayed faults and car runs OK.

 

Surprised nobody has mentioned the brake pedal switch. It is often the culprit when the cruise gets disabled randomly.

 

Matt - he checked those to start.
However I remember you saying about the timing - can't find your post but one has to open before the other closes or vice versa. Can you help with that?

 

Something doesn't make sense here. Only the '97-'98 XK8 has any vacuum control of the cruise control. The supercharged also had vacuum operation for 1999. After that, nothing used vacuum for throttle control of any kind.
I think you need to have the codes read by something more than a cheap reader. Like others, I suspect the brake switch, and think you may find fault code P1571 in the ECM. Some readers can't read corporate codes that start P1xxx. I know you've tested it, but sometimes ohmmeter testing can mislead you. The current applied by a meter can be more than just the signal circuit that the ECM applies. It may 'switch' the higher current just fine, but not the lower current from the ECM. I was bit by that years ago.......never again. If I want to test the brake switch I do it by voltage drop when in operation, and by fault code.

Good luck!

 

The VIN and Engine number, plus documents suggest that the vehicle was built early in 2001 and first registered in Sept 2001. Maybe I am looking at the wrong thing, doesn't the Cruise Control use the large black diaphragm on top of the throttle body to control speed ?

The photo attached isn't my engine but it's identical at the throttle body so I have included it for clarity.

The Innova 3100 OBD2 scanner claims to be able to read Jaguar P1 codes so I will have another go with it when I get chance, as I don't think I had it setup correctly and report back with a code if I can get it. i will also do more road tests with and without traction control on etc to see if that makes a difference.

I am 100% sure that the brake switches are OK, I have tested them in situ and on the bench and they both change over OK measured with a Fluke low current DVM. The switch asembley also looks new, not a sign of any wear or marks on it which suggests to me that the previous owner may have been down this route before and had a new switch fitted.

Thanks all for you input, keep 'em coming.

 

1997-1998 XK8s use the AJ26 Engine which uses the Vacuum Cruise Control Servo shown in your Picture.

1999-2002 XK8s use the AJ27 Engine.

AJ27 Engines use the ECM and Throttle Body Motor To provide Cruise Control. The Throttle Body does not have the Vacuum Servo,

 

Must admit if you have a 2001 car I would of thought you would have a AJ27 engine which has a different throttle body to what you show, you even have the twin over flow system on the header tank which was discontinued after 98 I believe.

But you do have the earlier vacuum system, these are in the right hand wheel arch, there are a couple of solenoids which are known to fail however this does not normally bring up a code

 

When I was having problems with cc I did get codes for failure of all 3 solenoids on one occasion, but only once. Mine would work for a week or two, then fail, with no codes on the Autoenginuity. I finally decided the problem was in CAN bus circuit and cleaned all the connectors from the instrument pack, transmission switches, BPM, etc. Everything I could find in the wiring diagrams that appeared to have anything to do with cruise control. I have not had a problem for several months now.
So what is that vacuum switch on top of the throttle body of your 2001? It looks just like my '97.

 

Here's some more info. Engine number NB0012150454 suggests it was built in December 2000 and married with the car a few months later in April 2001, best guess from the VIN NA16984.

The photo in my previou sreply was a generic pic from t'internet, so here's some photos of the actual engine in my car. Cruise control vac connections can be seen, one of the T connector at the back of the engine, and one of the Throttle body diaphragm. How can I tell if this is AJ26 or AJ27 ? If it was built in Dec 2000 it would have been an AJ27 surely ? Engine number also suggests steel liners too as it's well after the Nikasil numbers stopped.

Remember this is a UK delivered car, not USA or ROW - would that make a difference ? I'm confused.

 

VIN range 001001-A24195 is plastic timing chain tensioner country - have you looked at them?

 

I bought the new metal tensioners and gasket kit last week, as soon as there's a good weather day ( not too hot or wet ) I will be doing that job, certainly within the next few weeks. I have about 6 different versions of how to do the job, the zip/cable tie method seems like a good start.

I'd like to get the cruise control sorted soon though, it's niggling me. Also the door seals as per my other thread today.

 

Spudd.....

You are right to be confused, something stinks here. Where EXACTLY did you get that engine number?? Is it possible for you to get pics of the ECM label and the row of ECM connector pins with it unplugged??

 

It's getting interesting, it's not fixing my cruise control issue but interesting none the less. Where is The engine number stamped on the block of the AJ26 and AJ27 ? The car was imported last year into Australia and had a very intensive inspection for Australian compliance approval and customs clearance. The paperwork from the import process confirms the VIN and Engine number, and the details tie up with its UK registration documents so I doubt they are wrong, but I would like to look for myself.

If it turns out the car has had an engine swap somewhere in it's life there's not much i can do about it I guess. It runs very quitely and smoothly ( at the moment ) so I'm not overly worried about it.

So back to th Cruise control issue...... I understand that there may be some vaccum valves mounted inside the wheel arch on the drivers side ( RHD ) but I can't see a cover/hatch under the wheel arch to gain access, does the whole wheel arch need to be removed ?

I have discounted the brake switch as being the culprit since testing it on the bench and in teh car by shorting out the cruise control part of the switch contacts to simulate driving with no brake applied. The Failsafe Engine mode comes up just the same a few seconds after engaging the cruise +set button. I still think the control system is looking for some kind of feedback signal to confirm that the car is actually holding its speed in cruise.

 

I had a look at JTIS for information on the different year models, and it shows 1998 as having an AJ26 engine with a vacuum throttle body ( Drawing number E32728 ) and then a 1999 model having an AJ27 engine, also with the vacuum throttle body ( drawing number E33957 ). The 2000 model shows an AJ27 engine with no vacuum throttle body, so it looks like there may have been a change over period where the old throttle body was fitted to an AJ27 engine.

The USA export models may have had a newer ECM and throttle body to meet Emissions and Smog tests which were not as stringent in Europe at that time. Maybe the UK models continued with the old style throttle body and ECM to use up stocks ? Anyway, I will get the engine number off the block tonight and it will tell a story i'm sure, I think it's on top of the engine near the thermostat housing based on other threads here and JTIS info.

 

Yes wheel arch liner has to come out

 

OK, so I looked inside the wheel arch and there are a couple of solenoid valves and what looks like a small vacuum tank, about 2" diamter and 4" long, i guess to hold a steady vaccum and remove any pulsations from the lines. The whole area is filthy so I will take it apart to clean and check everything this weekend.

Also in there are the remains of some water hose plumbing which I am guessing is part fo the cooling system, the 'header overflow' tank I presume. ( it's missing compeletely so I may have to fabricate something from a plastic bottle for now )

The plenum drain tube appears to discharge it's rain water all over the cruise control vacuum solenoids, and then it puddles in the bottom of the fender/wing adjacent to the wheel arch. I have started a new thread about that issue.

My theory is that the cruise control switches on, takes control and then runs out of vaccum so the Throttle body diapragm lets go and the cruise control puts up a warning and switches off. I might be totally wrong, but it's all I have to work on for now. I am also convinced now that sometime in th past this car has had it's engine removed and a 1997 engine fitted, which explains the vaccum cruise control. ECU has a number 079700 on a label, engine number is 971212xxxx. That might help in getting to the bottom of the cruise issue now, as I can look for the right information for that engine and ECU instead of an AJ27/2001.

 

Here's an update, I'm hoping somone might be able to help me out with where to look next.....

The Cruise Control is trying to work. If I get the car up to speed, say 40mph then switch on the Cruise Control and press SET+ switch, but keep my foot just slightly on the gas pedal the speed holds. My foot is barely pressing the pedal, just enough that it's not at rest. If I then increase the cuise speed in increments by pressing the SET+ again the cruise control speed increases and holds the speed, also I can feel the gas pedal get lighter as this happens. If I then tap the brake or press cancel, the cruise cancels and the car slows down until I put some more pressure on the gas pedal, which proves that my foot wasn't holding the speed, the CC was.

If I then just hold a small amount of pressure on the gas pedal, but not enough to accelerate and then press the CC Resume button, the speed increases back to the set speed - my foot is still only slighty on the gas pedal and I feel it go very light- I apply no pressure to increase speed, just enough to hold the throttle open slightly. I can cycle through brake, resume, cancel, resume and increase speed with the SET+ button all day long and the system works.

However, as soon as a take my foot completely off the gas pedal, Failsafe Engine Mode and cruise cancelled happens again. Pressing the brake pedal does not cause this fault at all - it's definately something to do with the gas pedal 'off' position

So from where I am sitting, either there is not enough vacuum to fully hold the throttle open and my foot is assisting slightly, or there's something in the throttle body that requires the gas pedal to be slightly off it's "bottom end stop" or the cruise won't work. Faulty switch in the throttle body maybe ?

Please, if you have any ideas yell out. Cheers.