Testing an IAC valve? (And playing with one for fun!)

[bill190]

I'm having fun with Idle Air Control Valves (IAC)...

What would happen if I applied 12 volts to one coil?

How do these work?

I know these are "stepper motors", but have no experience with them at all.

As with everything automotive, I'm having a hard time finding technical information which explains how they work. Do they get a "square wave" and many 12V peaks to make them move?

Do you send a 12V off/on/off signal to one coil, then a 12V off/on/off signal to the other coil and alternate to get the motor to "step"?

Reverse polarity to get it to go backwards?

I'm going to get one from a wrecking yard to play with, so if damaged, no harm done...

 

[MikeHalloran]

You want a supply that just makes it step when you energize one coil so you don't overheat it. A 5V lab supply might work.

Steppers step when the state of the coils changes. You have to go through a four-step sequence in order to drive them in a given direction. Reverse the sequence through the pattern to change direction. In this case, I think the steps are taken with each coil at each possible polarity, so you need eight transistors to drive four wires. Microprocessors make it easier. You might want to buy the $80-ish Basic Stamp education board at Radio Shack, or something similar, if you really want to get into this.

For giggles, you can do it with switches, or just by reversing a couple of 9V batteries.

The cute part about steppers is that when you want them to not move, you just don't change the state of the coils. You can even turn the power off (they'll take a half step).

I think an IAC takes a hundred or so full steps to go from full closed to full open. There's no transducer for position, and no zero switch. The trick to that is that before the engine start, the ECU sends a bunch of step sequences all in the same direction, more than a full stroke worth, so the motor goes to one end of its travel and bangs into a hard stop a few times.. or a lot of times.
Having done that, the ECU knows where the stepper is, and just counts up/down as it sends close/open sequences to adjust its position. The ECU never knows for sure if the stepper actually moved in response to the steps; it has to assume it did, unless it can infer something from something else, e.g. the engine doesn't start.

The speed of the stepper is not controlled by the voltage applied, only by the speed with which the coils change state, which in the case of the IAC, is controlled entirely by the ECU.

That should get you started. Good luck, and let us know how you progress.




Mike Halloran
Pembroke Pines, FL, USA

 

[dicer]

There are many types of control systems for IAC's.
PWM controled solenoids are in some FoMoCo's well at least the ones I'm familiar with, and other makes.

 

[bill190]

["You have to go through a four-step sequence in order to drive them in a given direction."]

OK, that get's me started! This is what I did not understand. I found a circuit diagram with 4 switches on it and did not understand what was going on. Now it makes more sense.

(I never get the basic stuff!)

Here are a couple of links if anyone is interested...

http://www.megamanual.com/ms2/IAC.htm

http://www.cs.uiowa.edu/~jones/step/circuits.html

 

[NormPeterson]

Might be something in here as well. I've got a CALMAP manual at home that might mention just how many steps are available, and a few saved files that should have some related data buried in them somewhere.

http://home.windstream.net/mcfly/DFI_EMIC.htm

Norm

 

[bill190]

Thanks for that CALMAP manual. Just the type of stuff I like to read.

 

[bill190]

Well I finally got an IAC to play with. I measured the resistance of the coils with an ohm meter and both windings were around 50 ohms.

And so far as testing these to see if they are operational or not, this is a good test.

Then I applied 12V to each coil and there was a small barely noticed bump of the pintle.

I could get the pintle to advance slightly by applying 12 volts to one winding, then reversing polarity on that winding.

Then to advance fully, I stripped 4 wires and stapled them along the side of a 2 x 4. Then 4 more stripped wires on the other side of the 2 x 4.

Then tried wiring these different ways to the two windings, so that if I touched two alligator clips connected to a 12V battery to each set of wires on the board, it would give the first winding +12 and negative. Then touching the next set of wires would give the first winding negative and +12. Then on to the next winding both polarities.

So I could go right down the board with my alligator clips and this would reverse polarity with each set of bare wires stapled to the board.

I was able to quickly advance the pintle in this manner. I did not go further with my testing such as going in reverse. Just wanted to see how it worked basically.

I saw the Basic Stamp board at Radio Shack and saw that it would be perfect for playing with a stepper motor! But I DID NOT BUY THIS! If I were to buy it, I would TOTALLY get side tracked playing with it and would not get any of my "to do list" things done! (When I have more spare time, I'll get one and then disappear from society for awhile.)

After I got the pintle to advance, I took apart the motor to see how it works inside. Looks like a regular electric motor inside pretty much, with a rotating shaft which is hollow. Inside the hollow shaft are threads. And the pintle shaft has matching threads on it. As the motor shaft rotates, it screws the pintle shaft in or out.

So far as testing these to see if they are working or not, I think a good simple test would be to measure the ohms on each coil to be sure they are not shorted or open. Then simply apply 12V both directions (once) on each coil to see that the pintle moves slightly. You can place a piece of masking tape lightly on the body of the motor so it lines up with a mark on the pintle (but not sticking to the pintle). Then you can see slight movements. Based on the way these work, if it moves a little, it should move all the way. (Not stuck and inside shaft is not prevented from moving.)

Then other than that, just a visual inspection of the pintle shaft area to be sure it is clean and would be able to fully retract.

 

[thruthefence]

If you really enjoy this sort of thing, acquire a non-operating Seeburg jukebox with the 'Tormat' memory, and return it to service.

 

[MikeHalloran]

One advantage of steppers that you may have noticed; no brushes.

There is also a strong permanent magnet inside, but it's magnetized after assembly, and loses its power (forever) when removed from the motor. I.e., it's not worth disassembling a stepper to get a nice magnet to play with, and it's futile to reassemble the stepper, because most folks don't have a way to re-energize the magnet in situ.



Mike Halloran
Pembroke Pines, FL, USA