I've noticed that my idle drops to 500 RPM or less when in gear (auto trans) with the A/C on. Is this normal? I would think the computer would open the IAC and get the idle back up. The thing can fast idle like crazy when cold, so I think the computer could do it if it wanted to. I don't really understand the logic the ECM uses to set the IAC.

By the way, anyone know if my alternator should be capable of still putting out 14 volts at 500 RPM? The thing that caught my eye on all this is that the volts are dropping to 12 or so when I'm at a light. The idle is not rough. Thanks.

This is a 7747 transplant by the way.

Make sure that the compressor clutch voltage or A/C request from the dash is sent to pin B8. That tells the ECM to bump the idle up. Note that there is also a calibration parameter for the bumped idle RPM increment.

> By the way, anyone know if my alternator should be capable of still putting
> out 14 volts at 500 RPM

Depends upon the alternator and pulley ratio.

> The thing can fast idle like crazy when cold, so I think the computer
> could do it if it wanted to.

IAC control logic for idle speed in the '7747 is not that good.

RBob.

Thanks for the reply. This is a transplant TBI and the A/C hookup is not in the harness. Shouldn't the ECM at least try to keep the RPM up without that signal? I can see using that signal to increase the set point, but I'd think it would keep the original idle regardless. I can certainly run that wire if need be.

Ha! Told you so and spoon fed ya on the other forum...

Small world. I asked my same question over there. I know I'll have to run the wire, but I don't understand exactly why yet. Why won't the ECM at least try to achieve the standard RPM set point without the A/C signal?

RBob told you.

I said the same thing but he put it more eloquently!

There is what is called the 'dead-band.' Which in other words means that there is an area of idle speed that is acceptable. Such as 550 RPM +- 37.5 RPM.

Note how the RPM value jumps +- 25 RPM, this is enough to satisfy the dead-band.

RBob.

Sorry to dwell on this, I'm just trying to understand. When the truck is in park/neutral it sits closer to 650-700 RPM. In gear (auto trans) it goes to something lower, then with A/C on, something even lower. What is the logic in the ECM? Simplistically, in the absence of any park/neutral or A/C signals, I would think it would still always work the IAC to try and keep the RPM in the target range. This must not be true though since the idle varies with load. Is there a max idle IAC or something else that is limiting it when it doesn't have the signal from the A/C? Thanks.

The park/neutral idle speed is typically higher then when in drive.

Recall that I mentioned that the '7747 idle control isn't that great. Hit the tuning guide book sticky on the DIY_PROM board here. In chapter 5, under the '8746/'7747/'8063 ECMs is an IAC logic write up.

Has a lot of info in it on the '7747 IAC control. Note that at the end of the write up is a eureka moment. It shows why when the engine isn't up to the proper temperature the idle control isn't really under control.

RBob.