Hunting cruise control on 2006 E280cdi

OilyBob

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Hi,
Just wondering if anyone would have an idea what may be up with my fathers car...

He recently had two new front tyres fitted and I noticed shortly after that when hi has the cruise control on the car was hunting around.

I have now had a chance to have a drive myself and noted that he had not reset the fl'sat tire indication system as given by a warning message upon start up, so I have done the procedure and reset this, I thought that may have cured the problem.... no still the same.

When it is in cruise control or if you have a speed limit selected and apply throttle the car is noted to hunt in power, when driven otherwise there is no fault at all in the engine so i am thinking it must a speed input to the cruise control which is incorrect, it literaly feels like it suddenly things it is going to quickly so drops the power and then shortly after re-applys the power to increace speed.
It almost feels like it is getting two speed input signals which are maybe 2 or 3 mph out and switching between the two.
I dont think that it is a coincidence that this occured only after the front wheels are off, is the speed sensed from the ABS sensor's?

Intrestingly the speedometer is very steady in reading and only reacts if the 'hunting' stopps and the car settles at a steady speed, but I guess this is 'damped' a bit more than the signal to the ECU.

I have it booked in at a MB dealership for him next week but just wonder if anyone has any ideas before then(It also has glow plug issues but i think that this is the control unit going south, it has had this for a while and was booked in for this fault orgionally).

Open to suggestions

Rob
 

television

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I would say the wrong size tyres fitted.
 
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OilyBob

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Correct....

Before I saw your reply I was trawling through threads relating to cruise control and at the end of one was mentioned incorect tyre size....

So I went out for a look as it currently with me and yes the front dont match the back, on the front is 245/45 R17 95Y and the partly worn ones on the rear are 245/40.

Looking through the car manual the correct size has been put on the front of the car which means it has been running the last few years on a set with a rather low profile... Apparently the last lot where all changed after an unfortunate run of punctures.

So Ill be ringing up for a set of rear tyres tommorow....

Either which way they will be a little bit cheaper than the ones for my SL where 2 months back.

Thanks for your suggestion, the poor cars brain was probably in a tizzy over that, luckly the car has only done a hundred miles or so since they others where fitted.

rob
 

television

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That is enough to cause your problems and you will get a much better ride from the 245/45 tyres
 


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