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  #91  
Old 10-17-2018, 01:46 AM
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It's just a variable position electronic actuator that pushes or pulls two pins in the carb body that control how rich or lean the engine runs.
Holy shit, that is awesome! On paper, assuming everything is working properly, it should have almost comparable performance and emissions to an equivalent fuel injected engine. Well... you know, kind of... not really, but it's still awesome. I'll grant, I don't know a whole lot about old carburated cars, but I never knew that mixture adjustment was a thing, at least not on stock cars. And not on an AMC. That's pretty advanced tech for AMC back in the day.

And it's further reason why it's so upsetting to me when people disable these systems. That's literally something that's good and helpful! Good for fuel economy, the engine, AND the environment!
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  #92  
Old 10-17-2018, 08:46 PM
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I think my plan after the car is on the road and I am positive the ECU is properly serviced I will throw the car on an old AirCare dyno to see that it says, then do the nutter bypass and try again. I am starting to now get curious exactly which way the emissions, performance and economy goes. According to the official documentation the ECU/ECM/CEC (I'm running out of things to call it but for all purposes I am referring to the emissions computer) keeps the air/fuel ratio at 14.7:1 with a physical either/or adjustment for high-altitude driving but no fine tuning outside of using a wideband O2 sensor and adjusting components not monitored by the computer, else it just tries to compensate for the changes.

My goal today was to inspect and test the stepper motor. It's held onto the BBD with four torx screws. The only warning is that while the pins are loosely retained by a plastic disc on the motor assembly there is a spring that can fall out and easily get lost. If it is not on the center pin in the photo below, make sure it's still hanging out of its recess in the carb body.



When the key is in and turned on the ECU will initialize the stepper motor by advancing the pins out to a full-closed position and then pulling them back. That is what happened on my unit so it's safe to say the ECU, the stepper motor wiring and the stepper motor are functioning. The motor is made up of two windings with taps at each end and in the middle. Refer to the wiring diagram on the last page. End-to-end resistance should be around 140-150 ohms with each half of a winding being half of the total value and no continuity to the exterior casting. The readings should be identical when comparing one winding to the other. It all tests good.
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  #93  
Old 10-17-2018, 10:06 PM
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I suppose the book must have a procedure for adjusting the carb... there must be a 'neutral' or 'test' mode, some method of setting the stepper motor to whatever position it's supposed to be in for making adjustments. Hell, it might be as simple as unplugging it while the ignition is off.
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  #94  
Old 10-18-2018, 08:47 PM
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When on in open-loop there are several fixed positions the pins can be in depending on several on/off sensors. The manual states how to enable each mode for testing and that various carb adjustments can only be performed under certain ones. If you cannot reach the specified mode there's 20 pages of logic flowcharting to test each component in the CEC individually. I guess this is why people hate the BBD electronic carbs so much. If the emissions system is not working the adjustment instructions are useless and you are forced to fudge some things to get it close enough, or just do the nutter bypass and get another carb because computers are hard.




I think my major strength here is that I'm approaching this from the opposite direction. Whereas most skilled mechanics of this era vehicle (and most jeep people) really don't know much about electronic emission systems my background is early computer theory and digital electronics so this is all making total sense to me as opposed to how the carburetor in general works.

Again, I want to point out that the AMC repair manual is amazing. It's twice the thickness of your average Haynes manual and is pretty much identical to that of the average phonebook. It's like reading a tome.


Last edited by MIPS; 10-18-2018 at 11:07 PM.
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  #95  
Old 10-19-2018, 02:28 AM
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And this is why I despised working on computer controlled cars with carburetors.
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  #96  
Old 10-19-2018, 09:25 AM
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nvm

Last edited by init4fun; 10-21-2018 at 08:32 AM.
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  #97  
Old 10-19-2018, 09:44 AM
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nvm

Last edited by init4fun; 10-21-2018 at 08:32 AM.
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  #98  
Old 10-19-2018, 04:44 PM
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Quote:
Originally Posted by MIPS View Post
I think my major strength here is that I'm approaching this from the opposite direction. Whereas most skilled mechanics of this era vehicle (and most jeep people) really don't know much about electronic emission systems my background is early computer theory and digital electronics so this is all making total sense to me as opposed to how the carburetor in general works.
I would be taking a similar approach. It's pretty safe to say my knowledge of those things isn't as extensive, but still.

Around '96 there was a new tenant in my old building who knew how to work on EFI, which I think ticked off the super who could fix just about anything else on a vehicle. However, the super serviced arcade machines regularly and often worked on TVs and other electronics, so go figure.

Last edited by Jon A.; 10-19-2018 at 04:47 PM.
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  #99  
Old 10-20-2018, 12:07 PM
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Finished cleaning and verifying some cabling and slapped the stepper back in for a test. Success.

https://www.youtube.com/watch?v=b_BOvlLOrP0

The car sounds WAY better now at idle and you can smell it's not running as rich. Still not perfect but I think that might just be the O2 sensor needs to be replaced.
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  #100  
Old 10-20-2018, 10:57 PM
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Sweet!

O2 sensor, but also perhaps those vacuum switches. You might want to get a vacuum gauge and test those switches to see that they open/close at correct vacuum (or at all). Nice that it says right on the wiring diagram how much vacuum they switch at. Vacuum gauge is a good thing to have for a carburated car, anyway.

Last edited by MadMan; 10-20-2018 at 11:24 PM.
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  #101  
Old 10-21-2018, 09:25 PM
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Great to see somebody fix one right instead of trashing ever little bit of emissions control. This is what made most 70's-80's vehicles so intimidating to me, that sea of vacuum lines and "doo-dads".
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