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Wayne, you said the IAC moves to full open when you push the throttle and it is fully closed at idle, correct? I am surprised that it apparently acts "backward". The DME is able to open and close it but the direction got reversed. I assume you have not done anything else to the car and this was not triggered by any other work or event, correct.

I tend to agree with your comment that wires do not get crossed by magic. Wost case check the loom with an ohm meter for a short or open. However, the output stage could be at fault though. It probably is a monolithic IC in thick fim technique.

Do you have another means to introduce more unmetered air into the engine at idle and observe the ICV in the process. Maybe there is another port that you can temporary open on the intake plenum. What I am trying to get at is to see whether you can find a operating point of the engine where the ICV does not rail to either end. If it just "switches" between fully closed and fully open this would support a fault in the driver stage.

BTW: Do you know if this is a stepper motor or a PWM-style IAC. The schematics are hard to read but it looks like a three-wire IAC with one pin going to GND.

Ingo
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Old 11-29-2007, 03:11 PM
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"The center pin in the ICV is in fact the ground and the outer two pins are the incoming signals from the ECM. The drive signal that comes from the ECM is a negative going 5 volts (-5V).

Basically, the valve opens-closes by the ECM intermittently (pulse modulation) switching -5V from one of the outer pins to the other."

Actually, neither statement is correct. The center pin is +12 from the relay.
The signals are out-of-phase (inverted on one pin) PWM signals (pull to ground).

"It probably is a monolithic IC in thick fim technique."

Actually not, it's the same design used on the 3.2 DME, i.e very simple as you'd expect.

More guesses, as in most of the previous posts?
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Last edited by Lorenfb; 11-29-2007 at 06:34 PM..
Old 11-29-2007, 06:22 PM
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You said you checked the TPS according to the diagram the micro switch and the potemeter are in the same part if you did a sweep of it plugged in with a voltmeter it might show good constant patter and if you did a contunity check of the idle switch it would show open or closed right? Ok but what if the switch internally had a short and was confusing the computer?I mean its possible that when the brush moves across the potiometer it might be making contact internally with the other side of the idle switch. It's far out there but.....

Also didn't you say something about this happening after getting gas? Does the 959 have Knock sensor control??

one more thing to try you probably no this but removing the IAC with the ignition on can fry the driver in the ECM. That being said I would leave it pluged in but the hoses dissconnected and conact another and jumper it in a way that would make it work and see if the car idles basically to trick the IAC in to working

I hope this makes sense it did in my head but that doesn't mean a hell of a lot...
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Last edited by 911quest; 11-29-2007 at 07:17 PM..
Old 11-29-2007, 07:07 PM
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Sorry, I was mistaken as noted by Loren (removed that procedure altogether) how the ICV actually works (not how it can be tested on the bench)

The electrical harness connector pin signal assignment for the idle speed control valve is as follows: With the ignition turned on, there should be 12V at the center pin (#4). The outer two pins (# 3 and #5) receive ground going signal from the ECM. The control pin assignment from the ECM (for 84-89 911 models) harness connects pin #34 to ICV pin #3 (closed position) and pin #33 to ICV pin # 5 (open position).
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Last edited by Jascha; 11-30-2007 at 03:37 AM..
Old 11-30-2007, 03:34 AM
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The TPS is working perfectly - I fully tested it across it's range. The car is at Tony Callas' place today - we were using to shoot the new cover for Bruce Anderson's 3rd edition of his 911 Performance Handbook. Tomorrow (after kid duty), I'm going to head over there and we're going to take another look at it. We got out their CO meter, and started at the basics to see where the mixture was at.

There's a remote chance I may have done something silly, and held down the wrong button when reading the codes on the computer and reset the idle / mixture on the whole car. If you hold down button #3 instead of #2 during the startup, you clear all the settings out. Holding down button #2 puts it into diagnostic mode. I remember showing someone the code reading feature, and I didn't have the manual handy, and was doing it from memory. We'll keep everyone posted.

As for the Idle Control Valve (ICV), I read up on the BOSCH factory Motronic "handbook" (the one with the yellow cover), and it says that the ICV at fully closed will still pass 18% air. So, fully closed may be the correct position, but something else may be wrong. I was assuming (without looking at the book) that fully closed meant no air was getting through. That was an incorrect assumption, I guess. I'm not sure why the ICV would be open at higher rpm, but the handbook does give some clues. It states that the ICV is used as a correction valve throughout the range, and is opened and closed according to the maps stored in the computer. It hinted that it works across the full spectrum of rpm, not just at idle. I'm not a Motronic expert by any means - I'm learning more and more how this particular system works. And the 959 system appears to be a bit different than most other Motronic systems too...

-Wayne

I don't what you are referring to when you say IAC?
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Old 11-30-2007, 03:55 AM
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Idle Air Control

Idle Control Valve

same beast just different way of saying it
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Old 11-30-2007, 05:38 AM
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It seems clear that the problem is in the control signal that gets grounded to open the valve. It isn't there. When the DME duty cycles the valve, only the "closed" signal is being received by the valve and it slams shut. When you apply throttle, the "closed" signal duty cycle goes to 0% and the valve floats open. So you either have a wiring fault from the DME to the "open" pin on the valve or a fault in the DME driver (MOSFET or whatever they're using in there) that prevents the signal from appearing on the DME pin.

Either case should be easy to diagnose.
Old 11-30-2007, 06:22 AM
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Wayne,

So if you caused the error, how does one 'reset' or reprogram the mixture?

Seems surprising it would be so easy to clear out mixture settings?

Tinker
Old 11-30-2007, 06:54 PM
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Hi,

I think your iac is working correctly. I haven't worked with the Bosch system on Porsches, but have quite a bit of experience with other systems. On most systems the iac opens when the engine is off idle to prevent stalling if the throttle is closed abruptly. Kind of like a decell valve on carburetors.

What may have happened is when you were using the service tool you inadvertently reset the idle set point in the ecu. On some systems the ecu looks at the throttle position sensor voltage and uses this to determine if the engine is at idle. These values are typically at 0.5 to 0.6 Volts. If the idle set voltage was set to low then the sensor will never get below that point and the ecu will not go in to idle mode.

If the system uses a throttle switch, then look in the service manual to see if the switch is open or closed at idle. Then unplug the switch and either short the circuit or leave it open to fake the ecu in to idle mode. If this works then your switch is bad.

I would really be surprised if you crank sensor (rpm sensor) is bad. If this goes bad then your fuel and spark control will be interrupted. The engine will miss and generally run rough.

Sorry for coming out of the blue, I read this board almost daily, I just never post much on it.

Pat
Old 11-30-2007, 07:36 PM
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Quote:
Originally Posted by Tinker View Post
Wayne,

So if you caused the error, how does one 'reset' or reprogram the mixture?

Seems surprising it would be so easy to clear out mixture settings?

Tinker
Quote:
Originally Posted by pmacmn View Post
What may have happened is when you were using the service tool you inadvertently reset the idle set point in the ecu. On some systems the ecu looks at the throttle position sensor voltage and uses this to determine if the engine is at idle. These values are typically at 0.5 to 0.6 Volts. If the idle set voltage was set to low then the sensor will never get below that point and the ecu will not go in to idle mode.


I think that this was the problem. All components checked out okay, and the computer was not registering any faults. Remember, this is now a non-O2 sensor car, so it's open loop with respect to mixture settings. I have more information in this thread:

[insert thread link here]

-Wayne
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Old 11-30-2007, 11:12 PM
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