'747 idle control

[rr]

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.


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