Another VSS / Pk/Nt Question - Third Generation F-Body Message Boards

SB406 · 04-09-2007, 03:08 PM

I've been searching, but I can't seem to find a straight answer to my question. No need to re-explain if somebody has a link to another topic.

I'm putting an 89 TBI 1227747 harness on a 350/ th350 in a '79 K5 Blazer. The vehicle is for OFF ROAD ONLY and will never see pavement. Generally, speeds will be pretty low.
For simplicity, I'm planning to NOT hook up a VSS and turn off the flag in the bin. I will not be running AIR or EGR and all of that junk will be eliminated in the bin.

Now the question:
To prevent a crappy idle problem in gear (no VSS), can I just "tell" the computer that the truck is in Park all of the time? I'd accomplish this by faking out the wires normally hooked to the PK/Neutral switch. The ECM is only going to look at the VSS when it thinks the truck is in gear, right? Will I run into any performace issues since the ECM won't be anticipating an engine load? This is an off-roading application so speeds are generally very low.

RBob · 04-09-2007, 06:53 PM

On the '7747 the idle speed is only in closed loop control when the prk/neut switch shows 'drive'. IOW, leave the prk/neut wire to the ECM open (pin B10, Org/Blk).

For good info on the '7747 IAC control, the Tuning Guide Book sticky (above), under 8063/8746/7747 ECM's, is an article on IAC. Most important is the last post to that thread.

RBob.

SB406 · 04-10-2007, 09:24 AM

Thanks!
That's exactly what I wanted to know. I'll re-read that sticky too.

SB406 · 12-23-2009, 11:48 AM

2 1/2 years later....
Part 2 of this question...
New Rock Crawler, but same concept as above. How does a Manual transmission bin deal with the VSS? Does it always watch it? The auto bin doesn't look for the VSS until it sees 12v on pin B10. But in a stick vehicle, what happens with this wire? Is it open or connected to 12V? The diagram here isn't real clear-
ftp://ftp.diy-efi.org/pub/gmecm/manu...47/747ecm5.jpg

I've been reading the sticky that RBob pointed me to above, but according to post #5, it seems that a stick bin just always treats things like the truck is in gear (from an IAC standpoint anyway).:

Quote:

Originally Posted by Grumpy

0x5EF: Minimum TPS% required for throttle follower IAC steps.
0x5F0: Percent of throttle follower in park/neutral (for auto only).
0x5F1: Percent of throttle follower not in park/neutral (also manual).
0x5F2: Maximum throttle follower in steps.

As the throttle opens, the IAC is retracted. As the throttle closes,
the IAC will extend. The speed at which the IAC extends following the throttle is controlled by the filter coefficients at locations:
0x5E4 - 0x5E6, dependent upon the VSS.

The underlying story is this- I tuned my Offroad Blazer just like discussed in post 1 (It's more of a tube buggy rock crawler now, but that's beside the point). We're now tuning a 4.3L for a friend's offroad truck, using a 1227747 ECM. He suggested avoiding the (lack of) VSS issues by starting with a bin from a manual transmission truck. He also "heard" that using an auto bin without a VSS (and pin B10 open) caused performance issues because the ECM won't let the engine run like it would in gear. That seems like BS to me, but I figured it's worth a shot.
Any insight? Thanks,
Mike