'747 idle control

[rr]

steve ravet wrote:

> Thanks for the explanation Bob!  In your case, if the engine got to 158
> and idle control starts at 133, then why didn't it control your idle?

The table shown below is cut from the hac. This is different from what
I am/was using.

> Is kickdown the same as "idle flare", where the engine revs up then
> slows down right after you start it?

This is an interesting area. When the engine is cranked the IAC is at the
park position ($5C2). Upon 'engine running' there is a time delay ($5C3)
that occurs, then the IAC moves to the position vs coolant steps ($5F5).

Then after another time delay ($5D7) the IAC is 'kicked down'. This
kick down is the number of steps at $5D8. This is subtracted from the
IAC position vs coolant steps value. It is a combination of these values
& the propensity of the engine to catch and run that contributes to the
idle flare.

>
> Also, (re-reading your idle strategy article), you descibe a process
> where after startup, the IAC is moved to a particular position based on
> coolant temp.  Once it gets there idle control takes over.  Is the table
> below the one that defines this initial position of the IAC?

Yes. The only thing that I'd like to add is that full idle control does
not happen until the position vs coolant - kickdown < 0. Until then
the IAC code will bring the idle up, however, not necessarily down.

I have emails from folks that have inquired about this 'high floating'
idle speed. I have also experienced it. Then after a bit of run-time
the engine would settle down into an idle. I could never figure it out
until now. Many times I'd drive the car from a cold start and not stop
for an idle until the engine was up to temp. Then I'd have no idle
problem and no data to go by.

>
> It sounds like now the process is idle flare, then IAC moves to it's
> initial position based on the table below.  There is no idle control
> until the coolant rises above the temp specified by $53f, at which point
> the only control is an upper limit specified by $5e1.

Yes, this is how it looks. (for our readers, $53f above is really $5F3).
My data is from a stick tranny that is always in drive. There appears to
be another upper limit at $5E2 when in prk/neut & hot.

Below the temp @ $5E1, there appears to be no upper limit to the
idle speed. The lower limit seems to be controlled. Along with stuff
like the stall saver being available.

>
> Full idle control isn't achieved until the "kickdown" value is larger
> then the position value specified in the table below, based on coolant
> temp.

Yep, this is the holy grail of full idle control. I think it's because it's
so non-obvious that I had such trouble figuring it out.

>
> Do you have any insight into why this algorithm is used?  If this table
> and the kickdown value are only used at startup why do they figure into
> when full idle control is enabled?

Being a bit light-hearted here, I have no idea why it's this way. Looking
at the '8746 hac, wow, it is completely different in many areas. Some
tables are the same, others don't exist.

On the '747 it looks like the kick down steps and IAC position vs coolant
are in control until 'they' turn it over to the full control routines. They being
the drivability guys. Now I 'yams one, lol.

BobR.

p.s. This is getting a bit long. I'd like to list the general quals required to
get to full idle control:

a. engine is running
b. vehicle is at 0 mph (zero! hardcoded)
c. tps% is less than val at $5C6 ( tps% * 0.390)
d. steps of "IAC position vs coolant - kickdown" < 0
e. in drive, or a stick tranny
f. idle control according to Desired idle vs coolant tbl ($60E).

>
> thanks,
> --steve
>
> rr wrote:
> >
> > Well, I finally figured it out! It started with a new
> > 195f thermostat that ended up being bad. Engine wouldn't
> > get over 158f (70c) in coolant temp. So I have 20 minutes
> > worth of low temp data.
> >
> > The kicker was that the idle would not drop below ~1100
> > rpm. This was even with the idle vs coolant at 838 rpm.
> > Hmmm, I've seen this before and now it's about time I
> > looked into it.
> >
> > The bottom line is that the kick down steps (@ $5D8)
> > must be one step greater then the IAC position vs coolant
> > table (@ $5F5) steps for the ECM to fully control the
> > idle speed.
> >
> > To better understand, here is a cut of the IAC posiiton
> > vs coolant table:
> >
> > *==============================================
> > * IAC MOTOR POSIT vs COOLANT
> > * (IN STEPS) ASDZ, 5.7L V8 TYPE $42 ECM
> > *
> > *
> > * TBL = STEPS
> > *==============================================
> > ORG $05F5 ;
> > ;      STEPS ;   Deg c Deg f
> > ;---------------------------------
> > LD5F5 FCB 24 ;   HOT   HOT
> > LD5F6 FCB 24 ;   136.0 277
> > LD5F7 FCB 24 ;   107.0 225
> > LD5F8 FCB 24 ;   91.0  196
> > LD5F9 FCB 24 ;   80.0  176
> > LD5FA FCB 24 ;   71.0  160
> > LD5FB FCB 24 ;   63.0  145
> > LD5FC FCB 24 ;   56.0  133
> > LD5FD FCB 40 ;   49.3  121
> > LD5FE FCB 50 ;   43.3  110
> > LD5FF FCB 65 ;   37.0  99
> > LD600 FCB 80 ;   30.5  87
> > LD601 FCB 90 ;   23.5  74
> > LD602 FCB 100 ;  15.5  60
> > LD603 FCB 110 ;  6.0   43
> > LD604 FCB 120 ;  -8.5  17
> > LD605 FCB 145 ;  COLD  COLD
> > *==============================================
> >
> > Note the steps of 24 between HOT and 56c/133f. These
> > values are one less than the kick down steps @ $5d8.
> > It is set to 25.
> >
> > What this means is that the ECM will take full control
> > of the idle only when the coolant reaches 56c/133f. Then
> > and only then will the ECM attempt to control the high
> > idle rpm. (Now, kick down also has to have occurred. This
> > is at $5d7, in seconds after eng  start. Usually not a
> > great period of time). This is when the ECM finally uses
> > the IAC desired idle vs. coolant table at $60E.
> >
> > Once the coolant is above the temp at $5F3 there is a high
> > limit. The idle speed will be reduced if too high. This
> > upper limit is at $5E1, in rpm / 12.5. Below that temp
> > there is no upper rpm limit.
> >
> > Until the coolant reaches the value that the IAC position
> > vs coolant steps are less than the kick down steps, the
> > ECM will only reduce the idle speed when it's above the
> > upper limit.
> >
> > So if you have changed the kick down steps or the IAC position
> > vs. coolant table, make sure the table shown above has the
> > proper steps: one less than the kick down steps, at the
> > coolant values desired. If not the idle will tend to 'hang'
> > high. Also note the upper idle limit at $5E1 that is used
> > until the cutover to full idle speed control.
> >
> > BobR.
> >
> --
> Steve Ravet
> steve.ravet at arm.com
> ARM,Inc.
> www.arm.com

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