toofast928

If the AFM is maxed out at 3000 RPM (door at max swing), then how does the the controller figure injector pulse after the AFM is maxed out?
RPM is one input.........

John Speake

RPM is the only one..... temp 2 sensor also affects the fuel pulse length.

What are you wanting to do ?

Daniel Dudley

O2 sensor is there to fine tune the signal. Ljet signal can be made to read stronger by taking the cover off and manually moving the pickup on the potentiometer. Ljet "tuners'' usually upgrade the AFM by swapping for one from an engine with a power output similar to that they hope to achieve. TPS also tells engine to go to wide open throttle setting. I am not an expert, so feel free to correct me. As I have always understood it, the engine only gets a gross signal from the air meter and corrects it with the O2 sensor. Mass Air Sensors, the so called hot wire, were the upgrade to allow finer air flow metering with less air restriction. There are companys that will build you a conversion, and they are fairly popular with the 944s, because they are said to increase low end torque and top end power.

toofast928

I'm trying to understand how the fuel pulse is adjusted after the AFM is at the end of it's sweep. That occurs at 3000 RPM WOT. So from 3000 RPM to 6000 RPM what inputs are their to tell the injectors more pulse? Their's no MAP for speed density, Right?
RPM is one input,
02 can't be an input because the full throttle switch is activated and tells the ECM to ignore o2 input. Air temp is a modifier.

So how does the ECM know measure the air after AFM is maxed?

toofast928

by MAP I mean L Jet is an analog controller. Is RPM at WOT a linear logic?

Scott M.

there may be some info on this site:

http://frwilk.com/

John Speake

Quote" "So how does the ECM know measure the air after AFM is maxed?

A: it can't measure airflow, so it just injects the same amount of fuel PER RPM as it does at 3000rpm.

GlenL

Check that FRWilk stuff. The 944's "DME" system is the same as a 928's "L-jet" system.

The important detail is that the brain has three modes: idle, mid-range and wide-open. These are signalled by switches in the throttle. Only in mid-range does the brain consider the AFM door position. Idle and WOT fuel needs are figured out from RPM.

toofast928

Thanks everyone. So John if I measure the injector duty cycle, I should see same duty cycle at 3000 RPM as at 6000 RPM?

John Speake

Yes, I believe so. I have the Bosch L-Jetronic book here... it doesn't indicate another ways the fuelling is modified after AFM max out. In fact it is rather quiet on the subject :-)

Regards

GlenL

No. The duty cycle, as a percentage, should be higher at 6000rpm compared to 3000rpm. Each charge will need the same amount of gas and there are twice as many firings. The on time, as a period, should be the same.

John Speake

Yes Glen you are correct ! My mistake :-(

Daniel Dudley

So are you saying that full open flap would signal WOT even if the WOT switch was not activated?

mark kibort

Guys, had some of the same questions, especially when the "experts" said that the AFM was "not well suited to " perfomance mods such as headers, cams, fuel regs, etc etc`. hmmmm, i went from 175 to 293hp to the wheels !
as we did the mods, i monitored the voltage out of the AFM. it went up to about 8.4 volts with all the mods, whereas before most of the mods, it was in the low 7volt range. As i understand it , it goes to open loop, but still getting a voltage signal based on the flap maxing out or getting near it. as it goes near max deflection, it still richens the mixture by increaseing duration. also, we did duration measurements, and it was in the 90% range for 293hp for stock US L jet injectors.also keep in mind, the Ljet system has fuel enrichment by 15% at 75% throttle (microswitch) without it, it goes way lean after 3500rpm WOT. (yes we tested that too)

MK

Rich9928p

The main measured variables are the RPM and amount of air entering the engine. With a fully opened air flow meter vane and the full throttle switch closed, more fuel is injected.

Bosch indicates two extreme operating conditions:

1) the amount of air flow measurer signal remains the same (vane fully opened) and the RPM increases (as in our case) at rising number of revolutions,

2) less air is sucked in per piston stroke due to filling losses so the fuel injection rate increase is less than the rate of RPM increase.

A special component in the L-Jetronic controller provides this control function, it is called the division control multivibrator (DSM).