The cruise control on my 1997 Vanden Plas 4.0L works great on a level highway; however, when going downhill, the car speeds up and the cruise control cannot hold the set speed.

Comments and recommendations would be appreciated.

Thanks,
Joe

 

You did not say how downhill the downhill is, but the cruise control cannot set the brake!

 

Hi DevilDog,

I agree with Ross that it is perfectly normal for most cars to accelerate when going downhill in cruise mode, especially if the angle of declination is significant. The next time this occurs, it might be worth observing the engine speed on the tach to see if it remains at a steady idle or if it increases with your road speed. It might also be worth checking to see if there is any difference in the behavior in Sport mode versus Normal mode.

If your car is accelerating when going down even minor declines, and if this behavior seems different than in the past, it just might be worth changing your transmission fluid, assuming it has been a while. I know, I'm totally crazy.

Here's my theory: In cruise control mode, the Transmission Control Module (TCM) uses a special gear map to reduce the frequency of gearshifts as well as gear "hunting." I do not know for certain, but it seems possible that the cruise control strategy includes keeping the torque converter locked up more often than usual. If this is the case, it may be that when the ECM/TCM sense conditions that may indicate coasting down a hill (no load on engine, engine at idle, road speed above a certain threshold), the TCM may keep the torque converter locked up in order to benefit from engine braking (even in an overdrive gear there is presumably some degree of engine braking).

If your torque converter is not fully locking, perhaps you are getting reduced or zero engine braking....

When I changed the trans fluid in our '93 earlier this year, aside from the smoother gearshifts, I noticed a distinct improvement in engine braking when I would downshift for significant declines, and when I have the car in Sport mode it holds lower gears longer when I lift the throttle to decelerate or corner. Our '93 has the ZF 4HP24 electronically-controlled gearbox, which may be the same model that is in your X300.

I can't guarantee that changing your transmission fluid will make any difference at all. What I'm proposing is just a hair-brained theory. But unless someone else can explain why it isn't possible, it might be worth a fluid and filter change just to see if it makes a difference.

Cheers,

Don

 

In many of my other cars/SUVs (Honda, Honda CRV, Dodge Durango, etc.) the cruise control always held its set speed doing up and down hills. Mountains no, but small hills and declines, yes. My Jag will not hold the set speed unless on level ground. Even the smallest grade, it will "cruise" past the set speed. It doesn't accelerate, just the weight of the car going down the grade goes past the set speed. All of my other vehicles hold the set speed driving in the same places. See I have neuropathy (numb feet) and extensively use cruise control. Hope this helps in me defining the problem.
Thanks

 

those other cars were not as heavy OR had much greater wind profile ie drag. With a slick body and high weight of these x300 they are quite "slippery". I think Don's idea about changing the fluid would be a good place to start. I know mine holds quite well but the 4L80E might do a better job engine braking than the ZF...

 

Thanks Don....and Chattanooga_XJR. Will check the fluid and filter. It might have been awhile since they were changed.

 

Update....my cruise control now works perfectly. Had it in the shop for some electrical work because of a check engine light . After the work was completed, to my surprise, the cruise control works like it should. It holds the set speed up and down large hills with no problem just like I thought it should have. Thanks everyone for your help.

 

Glad you got it mended and even moreso you posted back! Any idea what codes you had that they fixed?


Mine behaved somewhat similarly, but it wasn't so mysterious as it did the same thing when driven without cruise control - so was obvious the throttle wasn't coming back to idle.


https://www.jaguarforums.com/forum/x...carman-111454/

 

Do not know the code per se, but he replaced an O2 sensor and he found a fuse blown and two wires crossed that were shorting out each other. Fixing all fixed the check engine light and the cruise.