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  #1  
Old 01-05-2015, 02:43 PM
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Location: Matthews, NC
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Idle control valve

This is probably a strange question but one I really would like to know the answer too. I would really like the answer for a 500SEL but for any CIS with electronic idle control equipped car would be great.
When the accelerator is OFF idle, what does the IAC do? Does it stay where it was when last idling? Does it open up? Does it close up?
I have built a new digital idle control unit that works great, idles at 600RPM in or out if gear, A/C on or off, when warmed up, 800RPM when cold, 700RPM for a few seconds after a warm start up to give the fuel system time to stabilize. I just don't know what to tell it to do when driving down the road. It probably doesn't make any difference but inquiring minds want to know.

PaulM

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Old 01-06-2015, 10:02 AM
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I think it sets the idle at around 600 or so.

To make sure, connect an amp meter to one of the wires on the Idle Speed Control Valve (I separate the wires at the connector and use a clamp amp meter on one of the wires). Then you can have someone press on the accelerator slightly and see what the amperage is (mA).

Compare that with idle speed.

Note: DO NOT purchase an amp meter at an auto parts store! They still think a meter with test leads and a 10 amp maximum is plenty good! (May have been good for the model T, but not modern vehicles or measuring things around the home.)

Anyway you can get a clamp amp meter which measures AC, DC, may come with a temperature probe, and has a maximum capacity of 600 amps! All for under $80 if you shop around the internet. BE SURE it also says DC (for automotive work - AC home electrical work).

FYI - For other people reading this and going Huh??? There is an "idle" switch on the throttle valve. When your foot is off the accelerator, it tells the Idle Control Unit the engine should be at idle. Pressing slightly on the accelerator turns this switch off.
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Old 01-06-2015, 03:33 PM
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Thanks for the response. I don't have a working ICM to check. My idle is controlled by a home built controller using a PICAXE 14M2 processor. It uses PWM to control the ICV. I was just hoping someone had already checked the amps on one of these systems when the idle switch is open. My system works great at setting the idle speed but I don't know what it should do when it is not idling.

PaulM
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Old 01-06-2015, 04:21 PM
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pmc:

From reading through the descriptions in the FSM of the expected responses, and the test protocols, I've come to the following inferred conclusions:

1) The MB ICM controls current to the IAC via PWM, as does yours.
2) The current range is 0-1000mA. 0 is valve open, 1000 is valve closed.
3) At a road speed between 0-12 MPH the valve position is not clearly defined in the FSM.
4) Above 12 MPH the valve is closed to prevent "bucking" on closed throttle coast-down.
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Old 01-06-2015, 05:47 PM
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Frank:
Thanks That helps. Do you read it to mean that the valve is closed when the MPH is over 12, even when it is not in closed throttle coast-down or only when it IS in closed throttle coast-down? One minor problem with the above is the ICM doesn't have a speed input on my 84 500SEL. RPM is the only indication of speed. Right now, I have my ICV open when above 2000 RPM cruising. It slowly closes the valve if in coast down mode and below 1500 rpm. I have no problems with bucking doing it this way. So I guess I can close this book and worry about something else.

I have had this car for over 12 years and the last 4 or 5 months is the only time it has idled consistently.

Thanks again Frank and Elektri for the replies.

PaulM

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