Jump to content


Photo

MPi Stepper problem


  • Please log in to reply
2 replies to this topic

#1 tmsmini

tmsmini

    One Carb Or Two?

  • Members
  • PipPipPipPipPipPipPip
  • 706 posts
  • Local Club: MOASF

Posted 04 June 2007 - 07:39 PM

I think I created my own problem or at least aggravated it. I have an SP ACR4 and the spec for the stepper was high. It seemed like a straighforward process to adjust it within range. It was at 29 and I think range is 16-28. So I attempted to adjust it, but now I can no longer even adjust it within 28. The value seems to never go below 32. One time after revving it it seemed to drop to 0.

I am thinking that this indicates the stepper is starting to go bad. What are the symptoms of a bad stepper?

#2 tmsmini

tmsmini

    One Carb Or Two?

  • Members
  • PipPipPipPipPipPipPip
  • 706 posts
  • Local Club: MOASF

Posted 05 June 2007 - 03:16 AM

I tried adjusting it again tonight, but first I removed the connector and sprayed contact cleaner/lubricant into the plug.

Not sure if it made the difference, but I was able to adjust it to 23, which is within spec.

The symptoms with the incorrect stepper setting were uneven idle and uneven throttle control on decelleration. This sort of makes sense based on other information in the forum about not touching the idle stop adjustment.

#3 Sprocket

Sprocket

    Great on Injection faults

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 7,266 posts
  • Location: Warrington
  • Local Club: Manchester Minis

Posted 05 June 2007 - 12:07 PM

Ok stepper motor operation, i will split this off and pin it at the top for future refference as well
:D

The stepper moror consists of four windings. All the windings are commoned together at one end, this is the 12+ve supply, Brown/Pink wire. The other four ends of the windings connect to the ECU. The ECU then grounds these in a certain sequence to drive the motor in steps, each one of a predetermined angle of rotation. The max number of steps of the SPi motor is in the region of 180

When you turn off the engine, you should hear an audiable clicking, this is the ECU driving the stepper fully closed. It drives it for 180 steps and a few more to make sure its closed, in doing this it knows where it is, zero steps, and can then count the steps open. After it has driven the steppper full closed and before the ECU switches off, It then opens the stepper to a point determined by the engine temp, ready for the next start. Once the engine is started it motors in relation to the engine temp and idle load of the engine. Idle speed is not directly controled by the steppper.

I normaly set the stepper up to show about 30 steps at warm idle with no load on the engine, IE no electrical devices on.



Onto what might have been happening in your situation. If one of the windings has not been working correctly, it misses a step in every sequence, but the ECU does not know this, so, say the ECU drives the stepper 40 steps from closed, what would hapen in reality is the stepper would move 30 steps. The engine will not respond as it should so the ECU will drive it some more, resulting in the Service tool showing a high step count. the control will become unsteady.

I have found a couple of the steppers to have dry soldered joints on the small connection PCB on the motor terminals. Its a simple fix, remove the stepper motor from its housing an re solder. Aslo worth checking that the winding resistances are all the same, if not, bin it and find another.

Corossion on the plug terminals is also an other possible, as you have seen. Id check the solder joints just to be sure.




1 user(s) are reading this topic

0 members, 1 guests, 0 anonymous users