Low idle with A/C

[Mastiff]

I noticed that my idle goes very low when I'm in gear (auto) with the A/C on, like under 500. It's not rough, just very low. Is this how a 7747 is supposed to work? I would think it would open up the IAC to get it back up to target, like 700 RPM.

The reason I noticed is because my voltage was dropping down to 12 volts or so when idling. Should my alternator be able to make 14 volts at 500 RPM? I think this is my original alternator from 1983...

 

[eagle mark]

Need to hook the sense wire to the A/C compressor. When it kicks on it will raise the RPM to be stable. Can change to whatever you want in chip.

I did one once an added 200 RPM when it sensed winch, this keep the charging system going max while winching.

 

[Mastiff]

Yeah, I don't have the A/C hookup in my harness. I guess I just don't understand why the ECM wouldn't do what it needed to do to get to the normal target RPM anyway. It seems to me that it should do that and then maybe up it even more with A/C (according to the settings). Why does it just sit there and let the RPM drop so low?

 

[eagle mark]

Just not enough IAC to compesate for the draw from the A/C. Same reason newer ECMs have a power steering sensor.

It's in the EFI Library in my link, get with the program!

Here I'll spoon feed you! B8 Dark Green goes to A/C power side

 

[eagle mark]

And here!

 

[Mastiff]

Just not enough IAC to compesate for the draw from the A/C. Same reason newer ECMs have a power steering sensor.

It's in the EFI Library in my link, get with the program!

Here I'll spoon feed you! B8 Dark Green goes to A/C power side

Thanks. I'll have to trace my wires around to find that relay. I'm confused though, if I don't have enough IAC range to cover the A/C load, then what good could the extra signal do? I'm not saying I don't need to run the wire, I'd just like to understand. Right now the ECM is just sitting there and letting the RPM drop when the A/C is on. Maybe there is some weird logic that it doesn't care that it's not meeting its idle target. If it does care and can't fix it because the IAC is wide open already, telling it the A/C is on wouldn't make a difference. Do you understand what I mean?

 

[eagle mark]

You don't need the clutch control relay part on your rig, that is a differant one, look at the top one only. One wire!

I guess GM designed the ECM that way? the IAC is just not enough to raise idle with A/C load that's why they built in the A/C raise idle circut. Stock it's only 25 RPM. I usually change it to 100 because I leave my Suburban running with A/C on when I go in the store and the dog or ol lady is in the rig. Have to be careful at a stop light like that as the a/c will kick on and the truck will try to roll forward if your not hard enough on brake.

 

[Mastiff]

I guess GM designed the ECM that way? the IAC is just not enough to raise idle with A/C load that's why they built in the A/C raise idle circut.

If the IAC can't do it, how does it do it when the A/C signal comes in? I thought IAC was the only way the ECM could raise the idle. Thanks.

 

[eagle mark]

---Quote (Originally by Mastiff)---
Why won't the ECM at least try to achieve the standard RPM set point without the A/C signal?
---End Quote---
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.
-------------------------------------------------------------------------

I would never doubt what RBob says but I have tried that adjustment and for some reason it did not compensate. I think what he is saying is the A/C value adds 25 RPM to desired value and because of that IAC can compenstae a stable idle.

 

[Mastiff]

I tried to clarify my question on the other forum. The upshot is, why is there a few hundred RPM difference between where it idles in park and in drive with A/C on? This isn't the dead band. The ECM is not even trying to raise the idle to the 700 RPM target when the A/C is on, or it has some software set limit to the amount of IAC. The IAC can't be physically maxed out or else telling it the A/C is on would make no difference.

And yeah, I looked at the wrong picture. I'll see how hard it would be to tap the line to the compressor and get it to pin B8. Shouldn't be too bad I don't think.

Do you have the park/neutral hooked up? I wonder how much difference that makes.

 

[eagle mark]

Do you have the park/neutral hooked up? I wonder how much difference that makes.

Smoother transition into gear!

I really don't know how to answer your other question but yes there are dozens of settings for IAC in that Super $42 I told you about. Maybe they could be tweaked. But it's to easy to run one wire...

 

[Mastiff]

Yup. I read something that might be helpful on the DynamicEFI site regarding my terrible acceleration performance when cool. I have larger injectors ("cop car") and I tweaked my BPW to compensate, but:

Affect on AE of Changing Injector Flow Rate:

Changing the injector flow rate directly affects the volume of AE fuel. This is because AE is pulse width based and is not affected by the BPC value.

The best way to compensate for a change in injector flow is to use the ratio of the old injector flow divided by the injector new flow rate. Using that ratio compensate the two AE PW tables (MAP & TPS). As an example, say that the 61 #/hr injectors are replaced with 80.5 #/hr injectors at 18 psi. The new flow rate is then 94.7 #/hr.Creating the ratio:

61 / 94.7 = 0.64

Go into your calibration editor and open the two AE PW tables. Multiply each entry by 0.64 (toolbox!). Save tables, save calibration (BIN). Done.

This could be my problem. But do you understand where he gets 94.7? It must be to do with the 18 psi...

I'm highly tempted to just buy some normal 5.7 injectors so my bin can be almost totally factory. Injectors can be pretty pricey though.

 

[eagle mark]

Math is not my strongpoint... but 5.7L injectors are not expensive at pull and Save. Cop Car injectors are. I'll trade you?

 

[Mastiff]

I lucked out. I started poking around the harness and I actually found an unconnected wire tucked in there that showed continuity back to B8. I didn't even remember doing that back when I installed this stuff. I didn't use A/C when I lived in Iowa. Anyway, so now all I need to do is find the easiest spot to tap into the compressor wire and I'll be in business.

 

[eagle mark]

So does it work in Tucson yet! Even hooked up you nned some tuning so I know you'll be in there adjusting it...

EDIT NON TECH: Funny story, I loved in Yuma AZ, retired but drove a three car carrier for a junk yard there once or twice a week to pick up cars at insurance yards. Left one morning early to make Tucson and Back one day with three cars. One has to have one good axle and tires as it is the last car and tows like a regular tow truck, long day.

It's sunrise and I am 50 miles from Tucson and as the sun comes up things loot weird? Road still dry, little more sun and weirder, road wet, little more sun and I say OH $HIT!!! It's snow and slow down to 40, then 30, then there was snow on the roads as well. Lasted till oh 9:AM. What a sight!

 

[Mastiff]

All set. I haven't played with the chip yet but it's a lot better. I think 50 RPM boost might do better than the 25 it has but I don't have my chip burning stuff set up right now.

I did try playing with the AE tables to help my bogging issue (using the ostrich) but it's too warm to get a good test. I'll have to wait until later in the year.

Unrelated thing, but my TBI setup has two relays associated with it. What are they for? I wired them up a long time ago but can't remember. One must be fuel pump, but what's the other one?

 

[eagle mark]

The other one probably does the rest of the EFI. One is definately fuel pump.