Hi all, my 2000 Grandeur (XG300) has done 74,500 klms and is in great condition generally. About 8mths ago, I had the input shaft speed sensor replaced because the trans. went into "limp home" mode and generated the related code. Recently, perhaps once or twice a week, the trans. will seem to get confused when under very light acceleration (taking off normally from a stop or accelerating from very low speed etc).

What happens is this: Moving off, it will change normally up to 3rd and then within the space of about .5 of second, the gear indicator in the dash will cycle quickly and randomly through the lower gears. This is accompanied by a hesitation of power transmission and usually, it will end up selecting 3rd or 4th and carry on normally. Occasionally though, at the end of this short period of confusion, it will slam back down into 1st gear with quite a jolt (almost got rear ended a couple of days ago!).

I've taken the car back to the trans. specialist and there are no trans. codes being generated. The only code he found was one relating to the MAF sensor (an issue I was already aware of).

Any ideas what could be happening here? I'm wondering about the throttle position sensor sending bad data to the trans....(but no codes?)

Thanks for any help you can give!

This post has been edited by Essex: Mar 4 2009, 08:52 PM

And throttle can be pushed by human rapidly as malfunctioning sensor 0 to 100% still there would not be no jolt or slam. If You take Your leg off the pedal rapidly it wouldnt go in first gear at this very moment. Its sounds to me its transmission control module, let Your dealer scan the car to search signs of malfunctioning tcm.
I had my tcm changed after the gearchange got chaotic, but I dont remember if there was an extra code for bad tcm.

Sometimes the sensors dont give out failure code as the problems can be complicated. I had tranny oil temp senor failure detetected only by the info that the oli temp was always 0C even after driving. So let Your dealer scan the car while driving- checking the figures of scanner at the very same time of the gearchanging.

Thanks for the reply Mihkel, I also think it might be the TCM. I hate when modern cars do this... playing up but not generating any codes. I guess all I can do will be to replace the most likely modules until the problem goes. This could get expensive!

Thats why You should let electrician or tranny specialist check it some more.

PS! Some codes come after sertain period (like 5 seconds) of giving out not-maching (uncorrect) values. It means, sensor could give a few wrong valus in a moment every now and then, but it will be enough to mess up TCM calcuations for choosing the right gear.

This post has been edited by Mihkel: Mar 13 2009, 07:28 AM

Case anyone interested, I had similar problem with my 2001 XG300, Problem fixed after replacing the input and output speed sensor. Did the output first which everything pointed to it as the cause, no change. Then the input sensor, problem fixed. Hope this helps someone else.