Cruise control - speed surge?

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ss201

MB Enthusiast
Joined
Aug 29, 2009
Messages
1,078
Location
Near Lancaster UK
Car
190E 2.6 Sportline, W203 320 CDi SE Estate
I drove my 320 CDI on the motorway yesterday. Set the cruise control for 70mph. Cruise control kicked in, but instead of maintaining a constant smooth speed (as it has always done), it sped up and slowed down by approximately 5-6 kph. I could feel the car surging and slowing, confirmed by watching the digital speedo display (in kph) varying between 107 and 113kph. When altering the speed up or down with the cruise control stalk the car continued to surge/slow, irrespective of the selected speed.

Can anyone advise what might be causing this, please - and preferably offer a solution?

Thanks.
 
Probably one of the reluctor rings on the wheel hub is cracked or rusted way.
 
I think I may have caused the problem myself.....

I recently had two front tyres fitted and went for Michelin 225 50 17s, instead of the recommended 225 45 17 (which are fitted on the rear).....on the basis that they would provide a slightly more supple ride than the Uniroyals I replaced (which they do - the ride quality has improved noticeably). My indie suggested that the difference in tyre size twixt front & rear may have confused the cruise control. I think he's right, as this is the first time I can remember using cruise since the tyres were replaced.

If that's the case, spot the Chump!
 

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