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3 Wire ISC Idle Hunting Condition


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Hi All, 

I have a Toyota 3 wire ISC valve (OEM 4AG 20V) wired into aux 1/2 on a MonsoonX. 

It is wired in the following manner:

AUX 1 -> RSC (Rotary SW Close) 

B+ -> 12V

AUX 2 -> RSO (Rotary SW Open) 

The issue I am encountering is that the rpm begins to rise to 1500 from 950ish and idle hunt as the car warms up. 

I am able to mitigate the hunting and bring the car back to a steady 950-1k idle by going into the idle speed control settings and activating "Test Aux 1" which I am assuming closes the ISC valve. 

Please advise. 

 Base map is attached below.

4age 20v 1-17-23.pclx

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Sounds possibly like the valve is working in the wrong direction.  In the idle control setup, try changing the Aux 1 active state to high, this will reverse the logic.  If that doesnt help then please do a short PC log.  

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Thanks Adam, I'll give that a try. 

One more issue I am encountering is that it appears that there is a 2800 rpm rev limit on the tune above. I cannot find the option that shows it is enabled. 

I want to verify that it is in fact a safety feature turned on and not a mechanical ignition issue. 

Thanks again. Can post a log if needed. 

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If you can take a PC log of this happening and post it up it would be helpful to find any limiters set up.  I can confirm using a Toyota 3 wire ISC valve pinned to the outputs as you have does require active state to be high.  Also record a log while warming up for the ISC values.

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ECU turns off fuel & ignition whenever it loses the engine position due to unstable/unreliable trigger signals.
Could you again take trigger scopes, perhaps on idle and another around 2krpm?

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Yeah... there, for some reason your Trig 1 voltage is lower than other 4A-GE stock triggers I have encountered -- where your Trig 1 voltage peaks touch below the arming threshold.
Adjusting the trigger arming threshold (as per attached below) should remedy that, but try to adjust the gap/clearance of the NE pickup to the teeth. If I remember correctly minimum clearance as per repair manual should be 0.4mm.


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I took a look at the distributor a few days ago and encountered this. Pickups were fairly corroded. 

Cleaned up with fine Emory cloth and car fired up. 

It appears to be non adjustable though :/. 





Updating the arming threshold fixed the cutting issue. 

Will be locating another distributor to compare voltages.

Thank you for your support!


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Although your Trig 1 voltages seem low, they appear to be clean in all the scopes you have attached - so it will be fine.
...but yeah, it would be nice to have another dizzy just to compare and to have it as spare.

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Got the car tuned last night and made decent power for a 1.6 4ag :)

Ended up reducing the threshold a bit further up top and did not encounter any misfiring. 

Switching the ISC active state to high did in fact fix my idle issue as well. 

Thank you Adam, Koracing, and Essb00 for your support! :)



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