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As I make repairs at work, I like to save old parts and tear into them as I have time for it. I know how they’re supposed to work but I always find it helpful to actually see how the internals are set up because it gives me a better mental picture of what a component is actually doing. Figured I’d share.
Although this IAC valve is outdated (anything with a throttle by wire system will use the throttle body plate motor as the IAC), we still have plenty of customers with trucks of this era so I get plenty of opportunity to diagnose them for various reasons. This particular valve came off a 1990 K3500 with a 5.7 and manual transmission, but any GM engine with TBI from this era will have an identical valve. I don’t know for sure what caused this one to fail - it just wasn’t working.
This type of IAC is a stepper motor - the ECM keeps track of its position using counts, with 0 being considered fully closed (seated) and 255 counts fully open (retracted). Any time the IAC has been replaced (or removed for TB service, such as cleaning), the position should be reset. It depends on the specific engine, but this is usually accomplished by cycling the ignition switch 1 or 2 times, or by driving at highway speeds for a few minutes after the engine has reached operating temp.
Normal operation is at key off, the ECM commands the pintle all the out and then backs it off a few counts where it will remain until the next ignition cycle. The ECM uses coolant temp, battery voltage, engine load (based on MAP sensor input) and RPM to determine the correct position (counts) for the IAC valve pintle.
Although I’m sure it’s possible to disassemble the valve without destroying it, I wasn’t concerned about re-assembly. The connector body is held in place with 3 pins, spaced on every other hex flat. I just tapped them slightly below the surface with a punch, then used the indent as a center for an 1/8” drill bit to remove them.
The set up isn’t much different than that of a starter motor or alternator, except that what would be considered the field coils aren’t mounted to the housing.
As you can see, there’s 2 separate coils - one used to extend and one used to retract. This is accomplished using magnetic induction. The armature is just a plastic sleeve with a magnet wrapped around it, with one end mounted in a bearing and the other is a slip fit inside another plastic sleeve. The pintle is threaded into the armature and has grooves on either side of the shaft that the plastic sleeve is molded around - this prevents the pintle from rotating. As current is passed through one of the coils, it creates a magnetic field that pulls on the magnet of the armature. This causes the armature to rotate, and the pintle is threaded in or out accordingly.
The quickest test for one of these is to remove the air cleaner and observe the valve as a helper cycles the ignition key - it can be seen by looking straight down the bore (flashlight helps). As I said, the valve fully seats and then retracts a few counts at key off, so if you don’t see it moving at key off, the valve or wiring has a fault.
GM considers the coils good if they have 40-80 ohms. I forgot to take a picture of the connector face, but if you’re looking at it with the tab UP, the layout is as follows:
[TAB]
B A
C D
The coils are separated A to B and C to D - any continuity other than these 2 pairs is reason to consider the valve faulty, as well as continuity between one of the coils and connector body (short to ground).
I hope this is useful to some of you. I have several old GM parts for many other models that I plan to post as I go.
I hadn’t thought of posting it on the General Tech board - I suppose I could have but I picked this one because this type of IAC valve is specific to these trucks. Later valves (1996-2002) look slightly different but still operate in the same way - starting around 2003, electric throttle bodies eliminated the need for a separate idle control valve.
I’ve got other components that would probably fit better in the General Tech board as they may have come off certain models but are still more generic in purpose - components such as a purge valve and HVAC door actuator.
August 16th, 2020, 2:00 PM
IAC Valve Guts - In Case You’re Curious
