stumble when A/C kicks on

[RADustin]

Hi All,

As I 've been posting about- I have a swapped '49 chevy pickup with a cam/procharged ls1.

I've been working on the tune to get a more robust idle, but it seems no matter what I do everytime the A/C compressor kicks on the engine stumbles for a second before it settles back down with the added load.

I do not have the computer controlling the a/c at all-it doesn't have a clue the compressor is about to kick on.

(Question)-If I wire only into the a/c status wire(or similar), is this enough to have the computer anticipate the additional load and I can command a slightly higher idle via tune?

On future swaps I plan to use the correct GM pressure sensors and fully integrate the vintage air a/c with the GM ECU- but this truck has been 'done' for some time and I don't want to redo the a/c plumbing to wire it to the ECU fully. Just looking for a simple solution.

Thanks!

[RADustin]

Another problem.

When coming to a rolling stop- my MAP slowly climbs to baro, causing a loss of timing, eventually causing the engine to die.

I need to play around more with it to see if it matters if I slow down in gear or not.

Anyways- seems like a vac leak I guess?? I check brake booster and don't see anything weird. I logged it at idle and started stomping the brakes- nothing.

I haven't touched the idle air tables yet- just idle timing. My truck is ETC so no idle air control valve. It isn't throttle cracker or follower causing this- I don't think or I would see it in TP% I believe.

I attached a log. 2380 and 4760 the truck dies. Frame 914 it wan't to die but I blipped the throttle twice and got it to normalize.

What are you guys thinking?


18211


edit- I could add- Fuel pressure(manifold referenced) moves with MAP, so its legit pressure in intake. I do not think it is compressor surge because at frame 914 to 915 is the blowoff closing(2psi spring) and causes that sharp spike that another blip of the throttle cures. because RPM only falls during this 'boosted' time I know the blowoff wasn't closed before I snapped the throttle.

2nd edit- looks like idle air flow and throttle cracker could be the cause. does IAC work not show up on TP%?

[RADustin]

so- wondering what people's opinions are for this.

Going to do RAFIG again today with the a/c running. This will give me higher numbers and ensure my trims are always negative.

Hopefully this way the truck won't stumble so much and want to die at stop signs some times.

[240sxpooter]

What don't you have the ECU control the A/C?

[turbo_bu]

Not sure which OS you are using, but wondering if you might be able to tell the PCM that the A/C is turning on. There is a A/C Clutch Status input (C2 - pin 18 (LS1B)) that you could try even though you aren't commanding the A/C with the PCM. On the swaps that I have done, I let the PCM command the A/C, so it knows when it's turning it on.

[RADustin]

Not sure which OS you are using, but wondering if you might be able to tell the PCM that the A/C is turning on. There is a A/C Clutch Status input (C2 - pin 18 (LS1B)) that you could try even though you aren't commanding the A/C with the PCM. On the swaps that I have done, I let the PCM command the A/C, so it knows when it's turning it on.

that is my question. I'm not cracking the a/c lines to add the gm pressure switch- so wiring it all in correctly is out the question.

But I'm wondering if I add in the a/c status line if that is enough to tell the ecm to command more airflow.

In my future builds I will wire it 'correctly' but this truck has been on the road for years before it had a/c, then I drove it stock with a/c- and now modified. With the cam I have the idle is just more susceptible to the drain the compressor puts on it.

[240sxpooter]

that is my question. I'm not cracking the a/c lines to add the gm pressure switch- so wiring it all in correctly is out the question.

But I'm wondering if I add in the a/c status line if that is enough to tell the ecm to command more airflow.

In my future builds I will wire it 'correctly' but this truck has been on the road for years before it had a/c, then I drove it stock with a/c- and now modified. With the cam I have the idle is just more susceptible to the drain the compressor puts on it.

I would hook up the AC request to the ECU and add in the pressure switch signal to make the ECU happy. Even if the AC pressure sensor is not really reading true A/C system pressure you could trick it with a resistor making the ECU think the system pressure is in range. Then the A/C switch would just be a idle up feature.

[RADustin]

Is it that simple?

If was my understanding the ECU will want to see the pressure fluctuate such as the compressor kicking on and off?

I have RAFIG several times and got that good. When I first leave my house after adjusting the learning or idle flow tables I get some stumbles a/c off, but it seems to figure itself out and be fine after that. Still though, when the compressor kicks on the engine idle drops in half and it takes a split second for it to recover.

[turbo_bu]

yes, it should be that simple. There was another post quite a while back where they had used a dummy resistor to show A/C pressure to the PCM. As long as the pressure read is reasonable, (within the range of where the PCM would be commanding the A/C), then that should do it. Again, make sure and wire in the A/C status too, since that should be the signal the PCM needs to use the idle A/C tables.

[RADustin]

I actually feed my WB02 into the a/c pressure pins.

So- I could move this to the oil temp or something- or maybe it will work as is?

[RADustin]

do I need to put a time delay on the a/c compressor or can the clutch kick in and ecu get signal at same time?

edit- so I understand you typically send the ecu a 'request' and it fires the compressor(clutch relay control, feedback in a/c status). This would be a huge headache for me to wire- will just supplying the status wire +12v be enough to flip the ecu into a/c idle tables?

[joecar]

You can move WB to the EGR position input or to the Oil Pressure input.