It's a 4 wire stepper motor without feedback (see bottom right of diagram). The ECM just sends pulses to "targeted" counts. This is for my 88 but s/b the same for an 89.
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Thank you.
I'm trying to envision if / how a stepper motor might respond uncharacteristically if, say, one or both of the two "phases" was weak...
- bad / weak driver on the ECU board
- ground problem (IAC passes all current through the ECU, so same problems might exist on "power" side, see next)
- power problem on the ECU (oxidized trace on the PCB, other current paths that may have been compromised)
- wire problem (oxidation / high resistance on one or more of the four wires)
- internal problem to the IAC (I think OP said it was new)
I was hoping there was position feedback.
The ECU's where... behind the glove box, passenger's side? Where's it grounded? What's the condition of its ground and power leads?
OP, have you tried disconnecting the ECU leads and inspecting the condition of the contacts? If look good, that's good. Still, "working" the connectors by removing / re-inserting them can "fix" poor connections; it's something that's easy to try.
Fact is, I don't yet have a sound position on the matter... any of these could be a problem, but I can't say whether any of them would cause the uncharacteristic behavior. I need to think about it more.