Idle Stepper Motor Tester

I built one for life testing IAC's a few years ago with the bipolar stepper driver board from Weeder Technologies, http://www.weedtech.com. I use an old 12vdc plug in wall transformer since I'm stationary having the card mounted to a desktop. The software with the card is GUI based and easy to command or write a test sequence.

Note from DJ
Thanks for the information regarding the product available from Weeder Technologies.

I am sorry about the delay in responding to your post.

Electronic technology is not my strong point so it is going to take some research time on my part to see exactly how to make this information work for me. And of course exactly how I can make it work.

But thank you very much for responding.

Knowing that it's a stepper motor, I'd imagine it would be actuated by pulsed 12VDC. A short pulse (~50-100ms) would probably be enough to kick out the next step, I haven't checked. This is easy enough to do - look up circuits based on the 555 timer chip (using that, you could probably build the circuit you need for manual control for a couple of bucks.)

However, the FSM cautions that the IAC should only be tested in situ, as it's possible to overextend the pintle and wreck the thing. This would mean:
- Let the engine idle, and note variation in idle speed as the stepper pintle is retracted into the motor, or the engine stumbling and stalling as it is extended.
- An external jig can (and probably should) be built to test the IAC off-vehicle, which would allow for visual verification of the IAC pintle movement. The external jig would allow the motor to be mounted and dismounted quickly, and would have a positive stop to limit pintle extension. Measurements for this jig can be taken from the throttle body, with only a little work. The design of the thing can be left to the interested - but I'm already envisioning a couple of possible ways to set the thing up - and those are just what I think the easy ways are. Probably at least a dozen possible configurations...

EDIT - if an external stepper is built, know that you need to come up with two separate output channels - the IAC has four terminals, two to extend, two to retract. I'd imagine that both use pulsed +12VDC - so it's a simple matter of either duplicating the pulse timer circuit (a simple one-shot output from the 555 will do,) or building the circuit and using a switch to shift between output channels (SPDT in the +12VDC line with both grounds constantly live, or DPDT to switch both the +12VDC and earth lines.)

It would be difficult to program accel and decel curves with the 555 as well as reversing pintle direction after a short pause. The IAC uses more power than the 555 can switch so MOSFET's, IGBT's, etc may need to be employed. Overload protection is also neccesary if the IAC under test is fried and wants to take the circuit with it. As far as the stop limiter, I use a momentary switch that runs into another Weeder board ( WTDIN-M digital input module) The switch contacts the pintle at extension and compares to command counts. I can run a sequence of 1,000 commanded cycles and not physicaly monitor the IAC. The switch clicking and motor going back and forth is quite annoying after a few hours. Great topic and good luck to anyone wanting to build a driver circuit.

For you EE challenged folks, a stepper does not simply move one position with one pulse unless you have electronics behind it. It has two coils and they are driven in what is called quadrature. In other words, you have a special sequence you have to energize and de-energize these coils to make it move one direction, and a different sequence to drive it the other direction.

old_man said:

For you EE challenged folks, a stepper does not simply move one position with one pulse unless you have electronics behind it. It has two coils and they are driven in what is called quadrature. In other words, you have a special sequence you have to energize and de-energize these coils to make it move one direction, and a different sequence to drive it the other direction.

Click to expand...

Ah.

So, how would one go about constructing a simple circuit to drive and test a stepper motor? I'm still thinking it would be pulsed DC - but apparently not as simple as a short square-wave pulse...

Should be pretty simple. The IAC (at least on OBD vehicles, and probably on RENIX 242s - I believe the RENIX 150 is a regular motor, but not sure) is a bipolar step motor, with 4 wires. Basically you want to build a separate H bridge driver for each coil, then set them up with a proper sequence driver to generate the proper signals. The easy way of doing this for a motor of this rating (guesstimating by size) would be to use an STMicroelectronics L297, an L298, and appropriate support components.

At that point, you'll be left with a circuit that only requires a direction signal (5V TTL, as I recall - it's been years since I used the L297/298) and a step pulse (also 5V TTL) to select which way to turn, and produce one step pulse at a time. A toggle switch and a pushbutton plus an LM555 or 74LS123 in one-shot/monostable multivibrator mode should suffice for these signals.

Check the datasheets for those two chips and you should find as much info as you will need, aside from exact specs on the drive current/voltage required by the IAC motor.

Note from DJ

Thanks for the additional information every little bit helps.

I was looking for a cheap and easy tester to exercise the Idle Steeper Motor to rule it out.

I and at least two good diagnostic repair shops have been chasing the high idle gremlin on my 88 4.0 since I bought it in 2005.
Both shops gave up. One after a week the second after a month in both cases they called and said DJ wegive up come and get this thing out of our site before we blow it up. But there will be no charge for our learning experience.

I had it go away for about four months once but of course it came back.

I do have one question what is the % setting for the throttle body at idle? If anyone has that information that one fact I have not been able to find.

Thanks old-man for your information I wasn’t sure if they were motors or coils inside the one I cut the back out of to see what was inside.

5-90 as far as the external jig goes how about the Idle Stepper Motor housing of a later model throttle body they are just bolt on.

I am currently looking at Ecomike’s post The RenX Files: High idle problems all 319 posts to see what I may have missed.

The only headache with using a throttle body for an off-vehicle test jig is that you can't see the pintle working - the point of testing it off-vehicle is so you can directly observe the motion of the actuator.

You could cut away the throttle body to gain a view, I was going to draw up a couple of simple block jigs once I pull out a throttle body to take some measurements. Should be fairly simple to machine from a block of whatever - aluminum, plastic, or whatever. It need not be strong.

The RENIX IAC is a standard GM part - the Chrysler IAC is probably similar to GM parts of similar vintage as well, I'm not sure.

The Mopar IAC's middle pins are one winding and the outboard pins are the other. GM uses the left pins for one winding and the right pins for the other. Otherwise, function is identical. I use a translucent project box with the IAC mounted on the outside of the enclosure and the pintle operating inside. The switch is mounted opposite wall as the IAC. If the box is too wide, you add a chunk of material between the pintle and switch to prevent over extension. This way you can test IAC's with differing lenght pintles.

Note from DJ
Newer Idle Control Valve Housing bolt on type photos in this link
http://www.cherokeeforum.com/f2/iac-idle-air-control-valve-wont-come-apart-68464/
See post #4by xjheep92 for photos of bolt on IAC housing

This would be a good place to start

http://www.ebay.com/itm/Stepper-mot...740?pt=LH_DefaultDomain_0&hash=item2a20be199c

Then connect it to this

http://www.ebay.com/itm/CNC-Router-...277?pt=LH_DefaultDomain_0&hash=item20c8f5e795