How do you ajust a/f in a 7747 ecm

[bbcjosh]

I was wondering if someone could tell me how you ajust the a/f map in this ecm. I am used to more modern tuning setups where you ajust the a/f table of rpm/kpa to the target a/f and then tune the VE table to meet that target. I see no direct way of tuning the a/f that the O2 sensors are striving for when in closed loop, only pe, ae, temp and other adders or offsets. Do you just change the VE to get the target a/f that you want, keeping track of the a/f yourself? If so what happened when the O2 takes over, or do you just trick it into constant open loop operation. Any help will be appreciated.

[lionelhutz]

That ECM basically runs at stoichiometric while it is in closed loop. All the OBDI GM ECM/PCM's use narrowband O2 sensors to close the fueling loop so they all basically run at stoichiometric while in closed loop.

You would have to run open loop and use the VE table to tune to the AFR you want if you don't want stoichiometric.

[bbcjosh]

I was afraid that was true, I guess that's why you just buy a stand alone if you don't want a challenge.

[dave w]

Tuning the A/F on an older OBD1 system is a CHORE!:mad1: OBD1 uses the Block Learn Multiplier (BLM) method to correct fuel A/F.

You can adjust the OBD1 BLM averages for 128 ( + / - 5 or 123 ~ 133) with TunerPro RT.

Look at post #7 on this tread ( http://www.gearhead-efi.com/Fuel-Injection/showthread.php?304-1227747-ECM-Information-42 ) for a VE Fixer spreadsheet to correct the BLM averages.

Copy the BLM running averages from the TunerPro RT History table and paste the BLM running averages into the spreadsheet. Copy the VE table from the .bin file and paste the VE table into the spreadsheet. The spreadsheet does the math, then paste the New VE table back into the .bin file. You will need to interpolate some of the BLM averages, figuring an engine under light load / low rpm needs less fuel than an engine under heavy load / high rpm.

If you post a TunerPro RT data log and the .bin file you used to record the data log, I can post back my results for you to compare your results with.:thumbsup:

dave w

[lionelhutz]

Well, if you want closed loop and the ability to pick your AFR then you need a PCM that uses a wideband O2. Open loop and tuning the VE table to get the AFR you want does work.

[dave w]

Well, if you want closed loop and the ability to pick your AFR then you need a PCM that uses a wideband O2. Open loop and tuning the VE table to get the AFR you want does work.I've been able to use wideband data to tune the AFR's with the second generation TBI PCM (93 ~ 95). See attached screen shot.

dave w