GM AFCR parameter question

[dex]

Should this ALDL value represent the A/F ratio the ecu is trying to achieve at all times (e.g. in both open and closed loop fuelling)? I ask because in my $74 ecu I have a flag that indicates closed loop operation and the HEGO voltages are oscillating about the switch point yet the AFCR value indicates rich

[robertisaar]

if by AFCR you're refering to something like Target AFR, then yes, that's what it's attempting to run at.

voltages osciallating around the switch point could be just due to how slow the ALDL stream is compared to how often the ECM actually reads the O2 sensor(either 40 or 80 times a second IIRC).

[dex]

I think AFCR is the target A/F ratio, the thing is that the values are in the low 13's which contradicts the idea that it's in closed loop mode.

[robertisaar]

depending on coolant temp and time since starting, i've had my target AFR in the 13s and 15s while in closed loop... it eventually blended into 14.7:1 though.

[dex]

Sounds like it's not in closed loop then. Maybe the CL flag is actually a flag to indicate conditions have been met to allow CL fuelling.

[robertisaar]

possibly. i never check to see if the BLM is locked at 128 or not during this though... i know the INT will move to where it wants though. so the ECM at least tracks what it thinks is the correct amount of fueling, it just might not act on it until the desired AFR is closer to stoich though...

[dex]

From memory, the last warm up datalog I looked at had BLM fixed at 128 and INT was changing.

On a slightly different note, why does no one convert BLM to a percentage and use it that way? (I'm a GM tuning newbie)

[robertisaar]

ok, it seems like my previous statement would hold true then... so essentially the closed loop flag means the ECM quits ignoring the O2, but it hasn't allowed the BLM to enable(kind of like when in PE mode...)

and by percentage: when i make my autotuners and histograms, i convert BLM and INT into using the range of 0-2. 128 is 1, so anything less than that will be a direct representation of exactly how far my fueling needs to be multiplied by to correct the rich condition. anything above shows how far i need to multiply to correct a lean condition...

using the equation of X/128, you'll get those numbers, but using X/1.28 would get you an actual percentage. in the end, you're getting the same numbers, so it's actually just a matter of preference.

[dex]

OK, it seems that AFCR is not the target A/R ratio as even when adjustments are made for INT the values come no where close to stoichiometric. Time for more rummaging through the code.

Looks like we're on the same page when it comes to displaying BLM; 0-2 just gives a decimal fraction so it's similar to a percentage depending on the number of decimal places.

Thanks for the responses