A DIFFERENT TILT MOTOR SOLUTION

Attached is the full account of the investigation I did to arrive eventually at this solution to the well-known problem with the steering column tilt motor and its tendency to move only in small downward jumps from the fully raised position. For those wishing to skip that detailed story, here’s the vital bits!
As sometimes happens in researching the causes of this sort of problem, discovering the final simplicity of the proposed alteration had an element of luck. Furthermore, quite how and why the BPM allows the change to work remains an enigma to me, but I have run my XK8 with it for some weeks and miles to give me the confidence to stay with it. If any electronics experts can explain what is happening, or if there are any drawbacks unforeseen by me, I’d be pleased and grateful to hear about them!

But just before I start I’ll give the usual disclaimer as far as anyone else trying out anything I describe here on their own car. You follow my lead at your own risk!

The Fix! All that is required is to disconnect the wire that provides a nominal 5 volt supply from the Body Processor Module to the feedback potentiometer in the tilt motor/gearbox. Nothing more and nothing less. It really is that simple.

The photograph of the tilt motor and its wiring shows the Blue/White wire to disconnect arrowed. I did this by unsoldering it from the circuit, but wire cutters would be quick and easy or removing the wire from the arrowed 6 way White connector on the column is also feasible. As far as I can determine, that wire has always been Blue/White through all the model years.

Of course it’s not possible to prove that a tilt motor with this modification will never revert to its original intermittent stuttering as has often happened with various other proposed changes, but if some brave souls out there would be prepared to give it a try and thereby confirm or deny that my proposal is effective in more than just my XK8, a more definite result, whichever way, would be useful.

 

Are you saying the auto tilt works after this fix? Meaning you turn on the key and it comes out (reach motor) and goes down (tilt motor) as expected? The reverse with key off? And it has so far cured yours of the stop-in-mid-tilt syndrome?

If yes, then I may have to give this a go. Mine mostly acts up in cooler weather. Somewhere around 55F and it stops working correctly. 90F and above, it is flawless.

 

That is exactly what I'm saying. The whole system - Up/Down, memory recall, ignition switch In/Out - works as originally intended. As I pointed out in the attachment, the only oddity is that there is a pause of about 3 seconds after the wheel goes Up before it then goes In. I can live with that!

 

Presumably the BPM is detecting if the column is being obstructed or binding in some way and shuts the motor down. Removing the +ve from the potentiometer perhaps stops that happening curing the stutter. The downside is presumably that if the column did get jammed by something for real, such as part of your anatomy, the motor would keep on turning.

The memory function might just be a timer, or still be using the remaining two connections to the potentiometer.

 

Thanks for that. To check, I've just been out and using balks of wood against the seat, jammed the wheel movement in both tilt and reach functions (the reach function still has its feedback wiring as originally fitted).

There is no difference. in both directions the motor merely slows down to a stall; the power does not cut off to either one. This would also suggest that the potentiometer and its drive are unlikely to be able to stall the motor and cause the original problem.

The memory position equates to the feedback voltage present when the position is set. So the memory recall always moves the wheel until that voltage is achieved. That's why when the 5 v wire is disconnected and the feedback voltage range is reduced, the memory positions have to be reset from its original values.

 

The thing is I wouldn't expect the potentiometer to work at all with the +ve connection cut. It would normally work as a voltage divider with the Vout going to the A to D converter. As soon as you cut the Vin, the output is zero, assuming it's referenced to ground.

On the circuit digram both tilt and reach have a common Vin (which you have cut at one of the motors), but separate connections for the "ground" side, so perhaps there is more going on than just a simple potential divider.

I wonder if the potentiometer gets worn leading to erratic output?

 

Most likely the problem is the same that stops your seatback motor from working.
The concave washer inside the potentiometer gets bent out of shape.Some have pounded it flat and reported it works again, others find a mid point in the resistance value of the potentiometer before reinstalling.
It is not a matter of if it will fail but when.
I have yet met an Xk8/R that did not have one or both problems.

 

Re-reading the Electrical Guide, there are some weasel words in there next to the descriptions for the Input and Output labels: "These nine symbols are employed to assist the user in visualizing the ‘logic’ of circuits containing control modules".

I take this to mean that logically speaking, this is the function they perform, but the implementation might be quite different.

Speaking in general, making a voltage measurement that does not depend on the resistance of the wires in the circuit (that might change over time as a car ages, i.e. corrosion), the technique consists of generating a known voltage, and change it until the current in the potentiometer circuit becomes near zero. As the current is near zero, there is next to no voltage drop in the wires. This gives high precision voltage measurements.

In this case, I believe the cursor pin in the circuit is really a voltage output that is adjusted by the control module until the current falls below a threshold. That would explain why the tilt control still works (and why it changes after the mechanical movement is done).

Something similar is described here:Constant Current Potentiometer

Could be this, too:Wheatstone bridge

 

Astromorg,

Thank you for this great feedback and for your wonderful discovery, I can't wait till we figure this tilt/reach circuit out. I've been battling this topic for over a year now. Mind if I ask a question to get centered with you and your findings?

Let me acknowledge that the "Auto" position circuit (both: memory buttons, and auto return to previous position after ignition is on) is different than the circuit to move the tilt/reach with the button on the steering wheel. I will call this later category "manual" for clarity here.

It seems that your description & discovery might center on the Auto / memory settings. Did your switch to manually tilt or reach work before or after your modification?

It would be wonderful if your finding allowed a previously non-functioning manual tilt circuit to be restored so I wanted to check in with you and get a handle on how this "manual" switch is affected if at all.

I ask because my car's auto / memory buttons circuit works just fine, but the manual circuit almost never operates. (FYI it is not a faulty switch) To me, proper positioning of the steering wheel is really crucial to comfort so I"ve been using my two memory buttons to control and adjust the steering wheel's tilt/reach position.

Please let us know if and how the manual swtich worked before and after this mod.

Thanks,

 

Before I started this project, I would get in the car, put the key in and the wheel would come out correctly on reach and would then start tilting, stutter one step and stop. Press Memory 1 and it might do nothing or it might stutter one more step. Continual press/release/press/release of the manual joystick would eventually get the wheel to about a third of the way down and then it operated correctly.

On key removal, reach and tilt would work properly to get back to fully Up/fully In.

Since I disconnected the 5 v supply, both tilt and reach operate correctly in EVERY way - Key In/Out, Memory recall from both 1 and 2 and the manual joystick. Only oddity is the 3 second delay in the reach motor cut-in as I said earlier.

 

astromorg,

I was waiting for a cool day to implement the solution to this problem supplied by Mish_Mish when I saw your solution and went immediately to the garage, dropped that panel, found the wire, and -- taking a leap of faith -- cut it. Problem solved.! It works great!

For me, this completes (I hope and believe) the short list of corrections for things that always seem to go wrong with our cars: headrest cables that require shortening, convertible-top hydraulic cables that need to be replaced with good ones that don't break, installation of a proper hands-off cell-phone (complete with iPod connection!) solution, and now, the pesky steering wheel positioner. Time to cross my fingers and settle in for the long haul!

Thanks!

mws out

 

My steering wheel has been working perfectly for the almost 7 years I have had the car. That is UNTIL I casually read this thread. I swear the next day (which was today), all of a sudden when I got in the car this morning the wheel came down just a bit and stopped. Then I got it to finally drop to the correct position by continuing to push the up/down adjuster on the side of the steering column. Same thing happened at the end of the day--the wheel dropped down in increments as I pushed the adjuster. Then I tried holding the adjuster to move the wheel all the way down and then all the way up several times and that seems to have (temporarily?) fixed the problem. I was thinking it could have been dirt on some contact or something. Or maybe reading this thread caused the problem

Doug

 

Mine does the same occasionally. Are you going to cut the wire and see if that cures it? If so please let us know how you get on.

 

Since using the switch at the side of the column to move the wheel fully up and fully down a number of times the problem seems to have gone away. The cause may have been not having used the car for a couple of days in a row--I am reluctant to do any cutting of wires.

With these cars I believe that most electrical gremlins and many other type gremlins are inversely proportional to how much the car is used. When I first bought the car back in 2008, the previous owner put on maybe 600 miles in the 18 months he owned the car. So soon after I took possession I had a number of weird false warning messages such as passenger airbag stuff and dsc warnings as well as getting the green monster.

Doug

 

That fixed it for me also, but only for a few weeks.

 

So what is really the underlying problem then? I know there are a variety of things that can go wrong with the steering column adjustment. I have seen posts where the in-out is the problem, posts where up-down is the problem, and posts about weird noises from the mechanism.

Do these all stem from the same thing or is the up-down issue totally separate from the in-out issue (this is starting to sound like a porno script).

Doug

 

I have no clue but the wire cut solution sounds good, I will try it in 6 months if the testers cars haven't gone up in flames.

 

I've had my 97 for twelve years, and always had the problem with the rake stutter. Recently I had to remove the lower column cover to get to the lighting switch which had a broken wire. After replacing the cover the auto wheel system has worked perfectly.

 

I ended up cutting the wire off a couple of week ago. The tilt has worked flawlessly since. It was erratic before that, only worked semi-reliably in hot weather, almost never in cold weather. A big thank you for the discovery.

Now, does the same wire need to be cut off for the reach motor? It is documented the same way in the schematics, so probably works the same way. Any thoughts?

 

As I've never had a problem with the reach system, I've left all its wiring complete and original. I haven't done any of the voltage checks I did with the tilt system, but would agree it looks the same.

I've never heard of the stutter problem affecting reach motors. It's usually the flexible drive shaft that gives up. Have you got a particular problem with reach or are you just looking ahead in case?