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Yes, when you said 200 rpm, the A/C did came to my mind as that's a number I both recall and observe, but I'm not sure it agrees with your symptoms.
"But why are there TWO pins from the A/C to the ECU? ... I tried to trace them, could see where they (14: GN and 15: GR-R) entered the wiring loom, but didn't see them exit. Logically, there would be a connector left hanging somewhere from the loom."
Good questions. You and Art are looking at the A/C on the ECU, so I thought I'd look at it from the vehicle wiring perspective in case that triggers additional thoughts.
My theory is that pin 15 says the A/C compressor is about to kick in and pin 14 says it has kicked in, built on the following analyis:
First off, I note that when A/C is first engaged, idle is raised by 200 rpm only momentarily for a second or two in preparation to handle the initial inertia when the compressor kicks in. Once the compressor is running, maintaining idle under a minimal compressor load is no different than maintaining idle under any other load, such as an auto trans idling in gear. On the other side of the coin, switching off the compressor barely makes a 50 rpm hiccup in idle when the clutch releases so I rather suspect the ECU isn't doing a lot other than the usual matching IAC operation to CPS rpm to maintain the target idle speed.
Looking at the green manual wiring diagrams I have showing LH 2.4 with a B200F engine, they show a bit more detail for the ECU wiring than what you may be seeing on page 54 in that 240 Service Manual for LH 2.4 with a B230F. I couldn't find a green manual on-line for your 240, but the principles of operation I see in my 700/900 manuals of that vintage should be very similar for 240 LH 2.4 connections.
Pin 14 is from the A/C high pressure switch (a 12 volt source from the A/C delay relay connected to the dash switch) which also lets the ECU know when the compressor is actually engaged (or supposed to be engaged). That same signal goes from the high pressure switch to the compressor, hence why test E15 for pin 14 in the service manual says 0-5 Ohms between pin 14 and the A/C compressor to suggest they can both see the high pressure switch. If you don't have A/C, I quite expect pin 14 should simply be left open, not grounded. I expect the GN wire in your harness should go as a single wire to a main connector block in the engine compartment, where it should terminate and be available for dealer installation of A/C.
Pin 15 is directly from the A/C controller, which in the 240 is the console rheostat to indicate when A/C is being called for and perhaps even to what degree. It's not clear to me if this is being used as a variable resistance or more of an on/off type of signal. Why would it want to know how much A/C is being used, the compressor load is either on or off, not variable. What pin 15 might do though, is allow the ECU to get ahead of the delay relay and know the compressor will soon be kicking in, giving it a chance to raise the idle ahead of time. Now as much as test E15 says you should see approx 1K Ohms when A/C is off (10 Ohms when it's on), in practical terms for engine operation, the difference between 1K and infinite ohms seen by the ECU may make little or no difference. I expect the GR-R wire in your harness will go to a main connector block under the dash and terminate, waiting to have the dealer install an A/C switch. I'm very much guessing that without A/C, pin 15 can be left open, but certainly not grounded. In a non-A/C car, I somehow doubt you'll find a 1K resistor in a wiring harness or even in the ECU, that's too fussy for a dealer installable option. If pin 15 does actually care about variable resistance, then I imagine it does so for the purpose of altering fuel trim, not idle.
Not too sure how much of that helps you or how much of that I got slightly muddled, but you asked why two A/C connections on the ECU so I tried to do a bit of digging.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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