How to adjust AC151 stepper motor/idle control? - Defender Source
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  #1  
Old September 18th, 2010, 03:16 AM
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Cool How to adjust AC151 stepper motor/idle control?

I got the replacement AC151 stepper motor/idle control valve, but the pintle seems locked - any tricks to get it to turn to adjust it?

Also, any adjustment tips would be appreciated. Seems it needs to be set to the micrometer to have it rest properly. Hmm.
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  #2  
Old September 18th, 2010, 03:58 AM
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Ian Gregory
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Adjustment?? Install it, plug it in and start the engine. If the idle is still off, visit a dealer or somebody with a an aftermarket LR diagnostic set up and have the base idle re-set (adaptive value cleared and the min and max position values set). It's a stepper motor so you shouldn't be able to move the plunger as far as I know. If it still doesn't work then it's either another faulty unit or you have an issue in the connection between it and the ECU or the ECU itself.
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Old September 18th, 2010, 03:04 PM
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I will try that on your advice, but the plunger on the stock one is adjustable, and the instructions that came with the AC151 replacement advise adjusting it to the length of the chamber it sits in.
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  #4  
Old September 19th, 2010, 01:14 AM
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Ian Gregory
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Hmm.. It's been about 7 or 8 years since I had to play with one of these and my memory is not the best but my recollection of the set up is as follows (jump in somebody if I get this arse about face):

The IACV has a stepper motor that operates the plunger via a worm type drive. Because it's a worm drive you shouldn't be able to move it fully unless you apply power to it. You may be able to push it in against the spring pressure but you shouldn't be able to pull it out. If it's already fully in then you wouldn't be able to move it. I don't remember any adjustment capability on either the stock units or the AC151 to adjust it's range on the IACV itself.

When you plug it and start the engine it attempts to control the idle by moving in or out to open or restrict a bypass airway in the throttle body. To do this the ECU needs to know where the plunger is in relation to it's range of travel. On different engines the plungers min and max positions will be different. Also on different engines the amount of travel (number of steps) needed to raise or lower the idle due to extra or loss of load will vary.

So in order to work properly the ECU 'learns' the # of steps over time needed to adjust for changes like the AC coming on or shifting in and out of park and so on. It stores and constantly updates these in it's adaptive memory.

When you remove the IACV to clean it or replace it then the ECU may lose 'touch' with the position of the plunger or if you replace it may not know it to begin with. It will then try to sync with the IACV by moving the plunger in and out to sense it's range by monitoring the effect on the engine speed. If it's successful then you won't know the difference, if it's not then you will get the strange idle issue where the engine may idle at up to 2 or 3k or so low it stalls.

In this situation the only way to fix it (if the IACV is working properly) is to run a procedure using Testbook called 'setting the base idle'. You can do it with 3rd party systems as well but you do it manually. Setting the base idle puts the ECU into learn mode and clears the adaptive memory. It sets up the min and max positions and checks the range of travel. It also requires you to check and or make adjustments using a bypass screw on the throttle body to set up the base idle. The ECU is then 'in sync' with the IACV and everything should work normally. Gradually the ECU then 'learns' over time the new movements of the IACV needed to maintain the correct idle speed under different load conditions.

This procedure is probably the 'adjustment' the instructions are referring to.

Does that help?

Thanks

Ian
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Old September 20th, 2010, 02:05 PM
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That worked, thanks Ian. Engine was revving crazy for 10 minutes but then sorted itself out.
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Old September 21st, 2010, 12:33 AM
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Ian Gregory
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Glad you got it sorted.
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