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Old 05-21-2011, 09:28 PM   #1
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Default MS1 idle with A/C

I recently got the A/C reinstalled in my car for the summer, but I can't get the idle tuned right with ms1. I have searched and tried different setttings for idle speed, deadband range, hi and low, and fast recovery, but it still drops to ~600 rpm when the compressor kicks in.

Post up your good ms1 idle settings for a/c.
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Old 05-22-2011, 04:28 PM   #2
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Has everyone with a/c moved on to ms2 or ms3?
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Old 05-24-2011, 06:11 AM   #3
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I can't help you with MS1 specifics, but i am trying to solve these issues on my DIYPNP. You might pick up something useful on my thread.

https://www.miataturbo.net/megasquirt-18/diypnp-install-inital-tuning-56208/
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Old 05-24-2011, 08:12 AM   #4
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You have probably already read it, but here is another current thread on tuning idle:

https://www.miataturbo.net/megasquirt-18/timing-idle-requesting-some-help-57133/

I still have a MS1 with air conditioning. My idle without AC is great by following that thread. With AC on, I cheat with the gas pedal.

What does your timing look like?
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Old 05-24-2011, 08:20 AM   #5
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Hornetball puts the tuning around idle in great easy to understand context.
https://www.miataturbo.net/showthread.php?t=57133

I've read it several times, with it been able to have idle droop to about 700 rpm when idling at 900 rpm. I figure that's the sweet spot and not much better than that without a feed forward circuit (table switch to help with idle before the compressor kicks on).

With that i got the settings attached, they most likely will differ from your settings though.

Edit: Doh' already posted, damn I'm slow...
Attached Thumbnails
MS1 idle with A/C-ms1_idle.jpg  
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Old 05-26-2011, 07:14 PM   #6
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Yea, my idle valve was set with hornetball's method and 22 minimum dc. Before a/c I idled with pnp basemap settings for closed loop, 900 rpm, and timing at 17. It seems that fast recovery and upper limit are the most important with a/c. A fast recovery of 4 or 5 is the quickest I can use without oscillations. I tried 10 and 12 degrees at idle as recommended so it kicks timing up. I also tuned idle afrs to 13.8-14.0 and increased slow rpm to 1000. Any suggestions? I would like to get back to 900 rpm idle with decent recovery. I'm seriously considering ms2 for pid idle if I can't get this tuned better.

Here are some logs of a/c recovery with headlights on
with 10 degrees base timing:


same with 12 degrees:


..much better than the pnp basemap which never fully recovers.


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Old 05-26-2011, 10:06 PM   #7
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Your idle is reacting as it should, the log shows it neatly, idle DC stepping up as required.
Yeah any lower a step size than 7 on mine oscillates like a bitch and sometimes gets stuck into an oscillation loop that only disappears by changing the idle settings to warmup only or B & G on/off.

One thing i did notice, why it the idle frequency setting set to 62 (mine is too as i took it from the DIY base map)? That gives an effective iac frequency of 131Hz if the calculations are correct.

The NB IAC frequency is ~400Hz, the 94-97 basemap has that value as 27 (370 Hz)...

I think 130 Hz is quite low a value hence why the valve takes a big hit, no?

Does anyone know the IAC freq of the 1.6 models? (I don't have a scope and searching didn't bring up anything).
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Old 05-27-2011, 01:27 AM   #8
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Old 05-27-2011, 06:26 PM   #9
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I worked on the closed loop some more last night. Using your slow recovery time of 50 and a larger limit helped prevent overshoot when the compressor disengages. Also I changed idle timing back to the diypnp basemap and lowered the activation adder with tps threshold off. It recovers much better without any oscillations when coming to a stop even when the headlights on. This appears to about as good as it gets with ms1, so it's time for PID idle



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Old 05-27-2011, 10:31 PM   #10
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I've since changed my IAC frequency to a multiplier of 55, this made the idle a lot smoother and the frequency is around 180Hz.
It catches the dip a lot quicker and the oscilations are smaller.

But going through this thread makes me want more now

I've been meaning to try Justin's mod for ages, so i burned his s19 last night and I'm modding the a/c circuit to input into js4 now (using the voltge divider circuit)

described here:

https://www.miataturbo.net/megasquirt-18/purge-valve-solenoid-37364/
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Old 05-27-2011, 11:48 PM   #11
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Well that was a bust

Loaded the modded s19 and the idle stayed fixed, DC didn't move 1 point.

I had a look at the modded code and it says that EGT should react so I took a log as attached and as the a/c kicks in, EGT goes low.

Hope JuntinHomi sees this thread and is able to help, I'm not ruling out stupidness on my part, I have shitloads of it
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MS1 idle with A/C-acnoidleup.jpg  
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Old 05-29-2011, 12:18 AM   #12
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Yay i got it to work, but it isn't quite the robust improvement, the ramp up time to cold idle is slow, maybe 2 seconds, which is basically what the idle dip is.
It does help however when you put the clutch in on a coast (like coming to a stop) the idle dc is already up when the a/c is engaged so the drop there is hardly noticeable, but that can get tuned out with dashpot settle time.
oh the reason it wasn't engaging is that with tps unplugged the value was floating. Grounding the tps signal solved that issue.
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Old 06-03-2011, 06:15 PM   #13
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If the NA valve frequency is really 170hz then shouldn't the multiplier be ~58-59? I would definately try the modded code if I wasnt building an ms2. The latest tune recovers idle with a brief dip at max a/c and 90+ outside.
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