Tick over stepper motor
Discussion
As a recent member, you need to do a search for stepper motor! There's a hell of a lot of threads about them, and their problems.
The bleed screw is used to set the idle speed but only with the stepper motor pipe blocked or compressed shut (set it to 600 rpm ish). With it open the stepper motor position controlled by the ECU is supposed to control the idle speed. It basically steps backwards and forwards to control the amount of air bled into the plenum with the throttle closed. It should motor back to the fully retracted position when you turn off the ignition and you should be able to hear this happen if you listen carefully.
It has no position feedback to the ECU however, so if it fails, the ECU doesn't know it has. That's why it starts from fully retracted every time so that there is a datum point to start from.
There are owners who've had stepper motor problems who have disconnected it when the engine is warm, therefore freezing it in a motored-out position. As I said, the ECU will not know this and will send pulses to it to try and make it motor in and out, fruitlessly. This will make it trickier to start when cold due to a lack of air but you would probably be able to compensate with the bleed screw. Others who have fiddled more will likely be along later!
The bleed screw is used to set the idle speed but only with the stepper motor pipe blocked or compressed shut (set it to 600 rpm ish). With it open the stepper motor position controlled by the ECU is supposed to control the idle speed. It basically steps backwards and forwards to control the amount of air bled into the plenum with the throttle closed. It should motor back to the fully retracted position when you turn off the ignition and you should be able to hear this happen if you listen carefully.
It has no position feedback to the ECU however, so if it fails, the ECU doesn't know it has. That's why it starts from fully retracted every time so that there is a datum point to start from.
There are owners who've had stepper motor problems who have disconnected it when the engine is warm, therefore freezing it in a motored-out position. As I said, the ECU will not know this and will send pulses to it to try and make it motor in and out, fruitlessly. This will make it trickier to start when cold due to a lack of air but you would probably be able to compensate with the bleed screw. Others who have fiddled more will likely be along later!
Edited by ianwayne on Saturday 8th June 11:57
The ECU uses the stepper to set the idle. It also lifts the idle as the engine starts from cold and gradually brings the idle down as the engine warms. It also holds the idle high as you overrun down to a junction and holds it high for a few seconds after you come to a stop. This is to prevent the engine stalling.
Normally does a good job so why do you want to bin it?
Steve
Normally does a good job so why do you want to bin it?
Steve
As the man says- but go back to the days of old and carbs- the throttle was opened when you pulled the choke out to keep the engine running when cold, so the stepper does this for you. Sure enough you can bypass it but either unplugging the stepper when the engine is warm, so it sticks partly open, or wind up the base idle to let in more air with the stepper blocked off, but you will spend the first 5 mins of engine start playing with the throttle to keep the engine running.
Barreti said:
The stepper is not only there for hot/cold tickover but also to keep the revs up a bit for when you are downshifting so you don't get the engine locking the back wheels.
A kind of crude mechanical heel/toe if you like.
I have managed to do just that - lock the rear wheels on downshifting ( for the last corner at Snetterton).A kind of crude mechanical heel/toe if you like.
Car is particularly prone to this changing down from third to second and starting to turn a tight corner.
Trust me - been there, done that.
Plenty of run off, so changed underwear and carried on.
Barreti said:
The stepper is not only there for hot/cold tickover but also to keep the revs up a bit for when you are downshifting so you don't get the engine locking the back wheels.
A kind of crude mechanical heel/toe if you like.
Are you sure of that? It seems a bit unlikely given that the ECU only controls idle when stationary (drives me nuts) and that the Lucas ECU is pretty basic.A kind of crude mechanical heel/toe if you like.
A good explanation of how the standard idle control is supposed to work is in here - see sections 19, 20 and 21:
http://ecumate.com/docs/Ecumate%20inst.pdf
http://ecumate.com/docs/Ecumate%20inst.pdf
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