on my 00 gt vert the IAC went(no suprise from a ford IAC). When i bought the car a few months back it would make a slight bucking under light acceleration. I thought that it being a 5 spd and me being a noob at it that it was me. Well in the past 2 days the bucking has gotten extremely worse. At first i thought the fuel pump was going and decided to check TSBs...nothing. Ran a fuel pressure test and even the WDS test and nothing came up. Even did a self test for DTC and nothing came up. Well i was at a red light going to a friends house and the idle was fluctuating crazy and IAC popped to mind. Banged on it with a metal socket wrench and found that to be bad cuz the idle corrected itself...but like 2 hours later the bucking came back. My question is how long does it take for an IAC to go bad? could the cold NJ weather have affected it worse? Was it my noobie driving or an IAC? Would there have been a way to prevent the IAC from going bad when i first realized the problem?
Ford IAC's aren't very good from what I have seen. I've seen them go bad in just a couple days. You may want to try cleaning yours first to see if it solves the problem before buying a new one. Good luck. Erik D. '94 white lightning
The IAC is a "duty cycle" solenoid (that is to say the PCM controls it's position by opening and closing the circuit very rapidly)..... the springs are very weak and it doesn't take much for them to 'hang'.... apparently, an IAC can go 'bad' in as little as two hours. You can try removing it and cleaning it with carb or brake cleaner - this often helps.
This type of IAC is used on Ford trucks but not the Mustang. The IAC on the Mustang recieves an AC signal, not quite sinusoidal, and the PCM floats the ground to change the position of the valve.
Sorry... no AC but it is pulsating DC with the ratio of "on" time to "off" time being varied to control the position of the valve spool. IAC is ground side switched by the PCM (semiconductors cannot control an AC circuit, though they can rectify AC current to DC). Like I said... the IAC is a duty cycle solenoid and my source for that statement is Fords Technical training - I believe we covered it in "Advanced Electronics"..... Viewed with an oscilliscope you will see a square wave with a duty cycle somewhere between 0% (KOEO) up to about 55%. The principal and function is used across both car and truck platforms.
Here's an excerpt from the 2003 PC/ED manual "The PCM determines the desired idle speed or bypass air and signals the IAC valve assembly through a specified duty cycle. The IAC valve responds by positioning the IAC valve to control the amount of bypassed air. The PCM monitors engine rpm and increases or decreases the IAC duty cycle in order to achieve the desired rpm."
Given that some IACs have clamping diodes built in to them, makes the AC voltage idea even more curious....
Jim.... this is commonly called PWM, or Pulse Width Modulation, where a semiconductor switches rapidly to simulate an AC source or an analog voltage. By changing the duration of the on pulse (hence the name pulse width), the average voltage is changed. When coupled with a capacitor, this can be filtered out to be a true DC, but typically, and from what it sounds like in this application, it's a pulating DC. I suppose you could also call this an AC signal with a DC bias...
No... pulse width is different. Fuel injectors are pulse width modulated..... their on time varies irrespective of their off time - With duty cycle the on time and off time add together to make one cycle of predetermined timespan - hence duty cycle being measured in percent and pulse width being measured in milliseconds.
I am well aware of the principles behind PWM and there are not many places on the vehicle where PWM is a PCM output. Now.... for duty cycle solenoids...... the time it takes for one complete on/off cycle to complete does not vary. If one duty cycle lasts for (this is NOT the actual number)
100 milliseconds and the on time of the solenoid is 40 milliseconds - then the solenoid will be off for 60 milliseconds and we will have a 40% duty cycle. It can be easy for the neophyte to confuse PWM and duty cycle but they are 'different' because of the fixed time span associated with duty cycle applications.
Typically, pulsating DC is a fixed square wave.... on time equals off time and generally used in circuits with a transformer. Duty cycle and pulse width signals appear to be pulsating DC in that they are also a square wave but their on time will vary from ther off time - by calling them pulsating DC, you are committing a faux pas that will confuse your thinking. Additionally, we cannot call them AC -again, it will add to your confusion. AC is alternating - it goes above zero volts and it goes below zero volts (I know that might be hard for some to grasp....
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might helpclear that up) while our cars do not go below zero volts (and no, we wontmuddy the waters by talking about inductance just yet). Again.... if anyone has issues with this information, you'll have to take it up with the Ford engineers and instructors that teach us how their cars work and write the manuals we use daily.
The only reason I made any post is that I have a bit of electronics background, but I'm a bit rusty on some of it. I have been doing some PWM with microcontrollers and I find that the term duty cycle is used to describe how much on-time a PWM signal has. 50% duty cycle would mean that the pin is sourcing (+5V) half of the time and sinking (0V) the other half.
If the frequency of a PWM signal remains constant, the longer on-time means a shorter off time to still get the next on pulse in at the required frequency. Using 1Hz to simplify things, a 50% duty cycle PWM will cause the pin to be on for half a second, and then off for half. A 75% duty cycle PWM will cause the pin to be on for 3/4 second and then off for 1/4. I've seen this on a variety of PIC microcontrollers using an oscilloscope.
I know you have much more experience with automotive electronics than I do and have been through plenty of the books - I was just trying to clear this up for my own sanity.
Wouldn't the injectors need the same style signal? For a given RPM, they can only be off so long before they must fire again. The frequency would vary according to the RPM of the engine, but still they could not increase the length of the on pulse without decreasing the length of the off pulse, else the timing would quickly be thrown off.
Thanks for any insight you might have. I've always wondered the intracacies of these systems...
It's not actually ac since the floating ground keeps the (almost) sinusoidal signal all positive, but the Mustang IAC does not use a square wave or pulsing signal. Here's a page with an oscilliscope shot of the different wave forms...
I had to clean the one in my '89 GT twice in the 9 years I had it, same for the EGR valve. I never had a problem with the IAC in my '98 GT or my '00 GT but I only had them 2 & 3 years respectively.
Trying to quantify a pulse width modulated signal into a duty cycle would take a horrendous amount of computing power...... processor power (ergo time) that isn't needed. As it is, much of the processors time is spent computing spark and injector events (most, if not all of the COP applications have each coil driven directly by the PCM and most are now multi-sparking like MSD and such). Even then, the pulse width modulation used in the ignition and injector systems exacts a heavy toll in computer cycles. In order to reduce processor load, those outputs that are easily (and seemingly ideal) suited to duty cycle computations are do that way.
Each cycle (from the beginning of the "on" event to the end of the "off" event being the span of one cycle) is at a frequency "native" to the PCM.... no extra computations. All the PCM needs to do is vary the percent of on time during that one cycle. It's a lot like that old "a square is a rectangle but a rectangle is not a square" deal. The computations for PWM are much different from the computations for duty cycle and it is important to keep the differences in mind to make an accurate and timely diagnosis. Duty cycle computations are a cheap way out of performing unecessary PWM computations.
In other electronic applications, they toss duty cycle and PWM around somewhat interchangably, but there is a lot more "stability" in what's happening in your home stereo than could ever occur in an automobile (we have to admit that the modern car is a lot closer to the space shuttle than it is to a vcr). When we talk "duty cycle" and PWM in automotive applications, we need to understand that there is indeed a difference. Both Ford and my dealer principal have spent a lot of time and money drilling this difference into my thick skull.
The IAC "dithers" at a particular PARTIAL opening, because the duty cycle is altered. The fuel injector is either 'on' or 'off'..... no need for duty cycle computations. The heater fan since - nope, can't remember......... is duty cycle - turn it on and off fast with more off time than on is low speed..... turn it on and off with the off time being the same as the on time is medium and turn it all the way on is high. Duty cycle (Gad, I think I hit the magic explanation) is most easily accomplished using a constant frequency. Injectors need to be either on or off.... we control these by modifying the width of the injector pulse.Fuel pressure..... duty cycle.... turn the pump on and off in the right ratio and we can have high pressure or low pressure (notice the traditional fuel pressure regulator has been replaced, in most applications, by a fuel pressure transducer).
I hope that Horsepressured has come to the realization that there is nothing intrinsically special about the Mustang... it is indeed just another machine - a higher state of tune and configuration, but a machine none-the-less..... last time I looked, the Mustangs we sell still work on the old "suck-squeeze-bang-blow" principle and are bound by corporate profits to utilize as much in the way of corporate parts and technology as they possibly can. Even the 2003 Marauder "stalls after cold start but not for Jim" that is kicking my ass isn't special other than the DOHCs, traction control and sundry other electronic delights. It is still a machine and bound by corporate policy..... more shocking is that the newest Mustang uses the Windsor modular (IIRC)..... which, unfortunately is the same engine that the trucks see 8^(
Sorry man... you've misread the pattern. This is indeed a "square wave" but the rise time is very slow - please, please, please don't force me into an explanation of reluctance and inductance just before Xmas. It is NOT a floating ground.... it is a switched ground. I am not about to reiterate my reply to JS. If you prefer to believe your convictions, I will not attempt to alter them.
However, I will continue to reap the rewards of my chosen profession, working at a Gold Level Blue Oval dealer - The Mustang uses a duty cycle based pulsating signal...... either that or I suggest you call Ford and tell them that they don't understand what they built. Would you please get a grasp on simple electronics (including reluctance, inductance, the effects of magnetic flux across a winding, what I don't know about electronics could fill volumes - but I do know what I do know).
"Not actually AC" and "floating ground" tell of your need for information. You need to understand the system before you can make sweeping statements. Magic is based on perception.... unfortunately, facts is still based on facts.
I also have an electronics background, no need to explain anything to me. The IAC does not open and close period. it is held open to a specific point by the signal, lowering the average voltage closes it and raising the average voltage opens it. You know what, you sound like an over educated ass... You don't know what my electronics knowledge is, but the fact is the Mustang IAC does not use a square wave and it does not use PWM like a fuel injector. It is suplied a positive sinusidal signal by the PCM, not a square wave. Your starting to sound like every other Ford tech that only knows what the book in front of him says, which isn't always correct.
You're absolutely right... I have no idea of your electronics background. But the fact that you can't tell the difference between a ramped square wave and a sinusoidal wave speaks volumes. The fact that you have yet to take reluctance and inductance into the equation repeats the same speech. The PCM ground switches the IAC in a duty cycle..... disconnect the load and connect the scope directly across the connectors and you will see the square wave since you have now taken the effects of the magnetic flux out of the picture. We do remember what magnetic flux does, right?
In the end, I will let you form your own opinions...... you are quite correct that duty cycle adjusts the average voltage but you miss the point about the solenoid "dithering" to position. Additionally, pulse width modulation does NOT adjust the average voltage.Your assumption that the Mustang is somehow 'different' is erronious. Why, oh why, would Ford deviate from one protocol for one marque sold for low profit but high volume, yet return to standard protocol for low volume, high profit models? The several top images in your example were taken with either a real time lab scope or a digital scope with a very fast scan rate. The bottom example appears to be from a SnapOn Councillor or similar... very slow scan rate and more of a cartoon than an example of what's happening..... Fell into the same trap myself several years ago with a $40,000CA Sun MCA. It couldn't find a bad TPS but my $19.95 meter could..... Let's not forget that some models use a clamping diode and the pattern will be very different depending on the side of the diode that we base our measurements on.
I don't know what you are basing your information on, but if you want to call someone down I will suggest you take it up with Ford...... I am not the message, I am simply the messenger..... I can say that this overeducated ass fixes more cars than you can ever hope to open the hood on.....
As an added service, please let the NG know where you work.... that way they can avoid you like the plague....
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