Closing in on a problem

[dave w]

I don't think you have anything wired wrong. The Painless Wiring Instruction Manual I have, outlines two options for B10, permanently grounded for no emission / no VSS applications or a Park / Neutral switch. I was thinking you had B10 permanently grounded.

I've never measured B10 voltages. The factory wiring diagram shows when the transmission selector switch is in either Park / Neutral ~ ground is applied to B10, otherwise B10 is an open. Measuring an open circuit with a volt meter will typically reflect / show the source voltage.

ECM subroutines, like Deceleration Fuel Cutoff (DFCO) are a function of the VSS input. I don't know enough about the 1227747 code to know all the ECM subroutines related to VSS and idle. For all I know, there could be a timer that is activated when the VSS pulses go to Zero to start a subroutine for Idle.

Maybe you can post a .xdl file and the .bin file your using?

dave w

[Scrufdog]

Is this a B10 problem or do you just need the idle to stick to 725 in drive? Should be a setting for Idle in P/N and Idle in Gear.

[lionelhutz]

I don't believe there are separate park/in-gear idle rpm settings in a 7747 ECM. It uses a single table.

There is a %TPS to enable open loop idle setting. Having the %TPS with the throttle closed above this setting could be keeping the ECM out of closed loop idle control. There is also a speed setting that works the same way but I would have to believe the VSS you're using wouldn't give any speed pulses when you are stopped.

Why does your scanner change the idle rpm? Check what it does when connected. There is a resistor you can connect across a couple of ALDL pins to put the ECM into different troubleshooting modes but that resistor is not needed or desired for a 7747 ECM.

You really need to check that the engine is in closed loop idle control or not. you can even try adjusting the idle screw a small amount. The ECM should compensate by adjusting the IAC motor the opposite way to maintain idle. This works as long as you're within the operating range of the IAC motor.

[ony]

is the 4.3 you have a vortic?

[1great40]

Thank you for the responses. The engine is a basic pre balance shaft, pre Vortec 4.3

Lionel, can you please go into more detail about the %TPS stuff. Please spoon feed me here including explanations of 3 letter acronyms. :)

According to the scanner the engine does attain closed loop operation. The Actron scanner I have access to sets the engine RPM. I don't know why, it just does. That's what fooled me into thinking all was well when I installed the new PROM I got from Mark. He defaulted the diagnostic RPM to 725 as well. Without the scanner attached, even though the PROM is programmed for an idle of 725, I have a hot idle of a little over 600 and it drops to about 500 in drive, so the computer is doing squat to control the idle speed. Is it possible to have the engine operating closed loop but the idle operating open loop?

[ony]

do you have a moniter mode on your scanner? one scaner I have has a diagnoses mode which sets the ecm to were it wants and its got a ok to drive mode that just shows sensor data as you drive.

[1great40]

do you have a moniter mode on your scanner? one scaner I have has a diagnoses mode which sets the ecm to were it wants and its got a ok to drive mode that just shows sensor data as you drive.

I will have to check that out. With the the original prom, connecting the scanner would set the idle to 1000 rpm. With the prom from Mark, connecting the scanner sets the idle to 725. I will have to see if there's a mode that just lets me watch the data and not trick the idle.