Re: [ST] RS post-idle report

[Barry Croft <barrycroft@xxxxxxxxxxx>]

I'm glad Marc asked what a stepper motor was. Cos I didn't know either, so I went and looked it up! It's an electric motor with a cunning arrangement of magnets so it can rotate a fixed number of degrees then stop, in a series of steps. They're used in home printers and fax machines and stuff to control the paper precisely, but also commonly used in fuelling systems, in the Triumph's case to control the air flow by controlling how far open a bypass is. When the throttle is closed it moves a tiny plunger in discrete steps to control the idle. I haven't actually found precise details of Triumph's system, but on other vehicles I've seen steppers move in anything up to 200 steps from fully closed to fully open. And two things can go wrong mechanically: the O rings sealing the plunger can leak (so air is passing even when closed), or the plunger can get dirty and stick partly open. I think my problem was the latter, since the symptoms were right - when the plunger sticks, the ECU 
 *thinks*
 that it's fully closed even when it isn't, and tries (but fails) to compensate by changing the number of steps open it tells the plunger to be at, but also meanwhile adjusts the throttle butterflies, mixture, timing etc. According to my job sheet the mechanic put in new seals AND cleaned the plunger, so I can't tell which it was - but I can at least tell that the mechanic knew what he was doing. That was Taylors in Taunton for anyone local, a good shop.

 

Barry

2000 RS

		
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