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Evening all,

I have a 2001 9-3 convertible; it's the second one I've owned and on this one the cruise control doesn't work as well as on the first one I had. Rather than hold a steady speed, it seems to constantly vary, as if it speeds up a little too much, slows down a little too much, speeds up again, slows down again, and so on. My first 9-3 (from memory - there was a nine-month gap between the cars) held its set speed very smoothly; you'd feel it pull more strongly up a gradient, and ease off coming down, but that was it.

So, does this mean my cruise control unit is on the way out? And, if it does, is this a separate part that can be easily and cheaply replaced, or is it just a function of the whole ECU thing which would probably cost an arm and a leg?

All help and suggestions gratefully received...
 

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It's a function of the ECU.

On a 9-3, the throttle is "by wire". You step on the accelerator, it pulls a control on the throttlebody, the ECU translates your request into how much fuel/air is needed, and it turns the throttle and varies the fuel amount as needed to get the torque you requested.

The cruise control is a similar request. You ask the ECU to hold a certain speed. It monitors the speed of the car and uses the above features to adjust the fuel/air to hold that speed. There is no separate cruise unit.

If it's doing what you say, I can only guess... I'd be inclined to think it's a general tune / run issue though. That is, the ECU is trying to do its job, but the engine is varying in performance and causing speed variations, which the ECU then tries to correct for, leading to what you see. I would start with new plugs (NGK stock) gapped to .9mm, swap in a known good DIC, replace all small vaccum lines, new air filter, fuel filter, run some injector cleaner through (Techron Fuel System Cleaner is the best). See where that gets you.
 

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hi,

on mine ( 2000 convertible auto) it is a separate unit plus a wire to the throttle body, when cruise was doing the same i checked where the wire went into cruise unit, as someone suggested the "cloth" that is inside the unit may be frayed, it was not so i just bought a replacement cruise control unit of flea bay for i think about £18 problem solved,

hope this helps
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