Closing in on a problem

[1great40]

Hi Guys, I'm looking for a little help here. I have been dealing with Mark, he made me a new PROM for my 1227747 TBI computer.

I'm running a 4.3 Chevy V6 in my 40 Ford pickup and using a Painless Performance wiring harness. I have had a problem with the idle speed on this engine ever since I got it running. I'm sort of a FI newbie so bear with me.

I decided that an idle speed of 725 RPM would be about right for my engine. It felt smooth at this speed. The original idle was about 630 or so and It loaded down to about 500 in drive. The PROM I got had the idle speed set to 725. I installed the PROM and didn't really notice much of a difference in the idle speed. I hooked up an Actron scanner to the ALDL and started the truck and let it warm up. Sure enough the idle was 725 and it stayed at 725 in gear. Leaving the scanner connected, I drove the truck around a while. I was quite pleased. Later when I disconnected the scanner, the idle was about 725 in park but dropped down to around 500 again in gear.

To make a long story short, Mark had defaulted the idle speed in the diagnostic mode to 725 as well so when the scanner was hooked up, the engine ran at 725. When the engine was not in diagnostic mode, it was not under the control of the computer in drive. The engine was showing IAC counts of 15 at 725 RPM. I checked the Park/neutral switch wiring and the B-10 input to the computer was ground in park and 13.3 volts in drive just as it should be. I verified that the P/N wire did go to the B-10 terminal on the connector and that the connector pins were not bent on the ECU.

My thinking is that the ECU has taken a dump. The only other thing I'm wondering about is the AC request line. Does that have control over the idle? The AC is not yet charged so it will not run.

I don't mind buying another ECU but It would be great to really solve the problem before throwing parts at it. Any input would be appreciated.

[dave w]

I've always been challenged with getting idle speeds set correctly with a vehicle in drive, when the Park / Neutral input (B10) is grounded. With B10 grounded, the ECM thinks the vehicle is always in Park or Neutral. Adding a VSS is not a silver bullet fix all, quite often issues like / similar to the one your are experiencing are often resolved with a VSS. I only recommend grounding B10 for Off Road Only vehicles.

dave w

[1great40]

Thanks Dave. The setup I have is only grounding the B10 input when the vehicle is in park. Once it's out of park, that line is pulled up to battery voltage through an internal pull up resistor in the ECM. That's the way it was done at the factory and that's the way Painless implements it as well in their harness. The truck uses the factory VSS and it feeds a Dakota Digital SGI-5 Universal Signal Interface. I am using the 2000PPM output to fed the ECM. Please explain how the VSS interacts with the idle speed. Maybe I have something wrong with how I wired it.

[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.