Ok you wizards of XK8s. Right after I bought this beauty last October I'm driving down the road with my best girl (wife of course - second-best is the XK8). Suddenly she hits my shoulder and tells me to slow down. I check the speedometer and it says 85 MPH. Visually I can see that ain't so but I slow down anyway (don't want the missus REALLY mad), pull over and when stopped, the speedometer reads 35MPH. I figure I've got a bias in the reading and watch my speed until we get to our destination. The car sits for an hour before we get back in and standing still, the speedometer now reads 20MPH. One more time standing for another hour and we're down to 10MPH at a stand still. Finally after an overnight in the garage all is well. This happened one more time about two weeks later and then nothing through the Winter. This week I took both ladies to Las Vegas (only one had the top down) and everything was fine there. On the trip back, I noticed the opposite speedometer problem, reading too low, about 10MPH low (had a GPS in the car for the long trip). Coming to a stop seemed to start to correct the problem. By the time I got home all was OK. However, today, I go to work and on the way home again the speedometer is reading about 35MPH high! It's now sitting in the garage claiming it's going fast fast fast.

I know there are wheel speed sensors used for the ABS and traction/stability control but I don't know if they are used for the speedometer. Could this simply be an issue of a good cleaning being required? I also note that during the cooler weather (if there is such a thing here in Arizona) I had no problems.

Thoughts from the experts???

 

Woohoo!!! I just checked the weather in Vegas for the past week, and the high temperature never got out of the lower 60's! You go girl!


By the way, I don't think this has anything you do with your wheel speed sensors. If the wheel isn't rotating it can't be sending signals to whatever computer calculates the speed, therefore you wouldn't show any speed at all.

 

Sorry Sam you ol' horn-dog. It was my second-best girl with the top down.

 

I believe the speedometer is stepper motor driven. The speedometer motor is part of the 'major instrument pack' which is really a computer module also.

Do you have a OBDII scanner? I would recommend connecting that up to girl number 2 and set it to monitor the speed. It is probably a good idea to let girl number one drive while you monitor the OBDII speed readout. If the OBDII speed reads correctly, your problem is likely in the major instrument pack. If the OBDII speed is also bonkers, then there is some sensor malfunction or problem in another control module. I would venture a guess the major instrument pack fault is the more likely scenario if the car is otherwise running normally.

 

WhiteXKR - congrats for getting the precedence of the 'girls' correct. Ol' Sam just can't think with the right head sometimes!

Anyway, thanks for the good information and advice - I don't have an OBDII but now have an excuse to get one.

I suppose girl #1 will have to drive although she hasn't yet and seems hesitant to do so. The XK8 in non-Sport mode is more refined and docile than her supercharged Bonneville but I don't want to tell her that. If she finds out, I'm probably looking for a second XK8 as I don't really want to drive her Bonneville.

Hmm.. now how does that work? Girl #1 in girl #2 with me chasing them in girl #3. How many tops are down? Calm down Sam!

 

Oh my god... I could tell you guys some stories...

And I have the pictures and videos to prove it! Unfortunately for you guys, I have a stalker.

 

Are there any other 'odd', perhaps electrically related behavior? Any troublesome lights, any strange alarm behavior...you know, goofy stuff that isn't life threatening?

 

This girl has been behaving very well. No warnings or lights. Drives like a dream.

I did have a post a few months back, "Suddenly I Thought of Lucas" or something to that effect where I had a complete shutdown while driving. Motor, lights and everything electrical went off. Restarting the car returned all to normal and it hasn't happened again. Replies to the post indicated others had experienced the same thing but it doesn't seem to happen often enough to warrant diagnosis (especially as it isn't repeatable).

 

Strange behavior can often be cured by a hard reset.

you'd just have to reset your windows (if its a convertible) and reprogram your radio stations, but you could try a hard reset to clear the memory in the non-critical computer modules in the car...a reboot, so to speak. With the infrequent shutdown-while-driving, it may have confused the instrument pack...can't do any harm. Just pull the negative cable from the battery, reach it over to the positive, and give it a little touch for a few seconds, then reconnect. Report back if its still acting goofy.