Function of AFM Temperature Sensor 85.5 2.5l NA

[ibtisam.jawad]

I was reading through the following link and it says that the AFM has an air temperature sensor that changes resistance based on air temperature.

Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com

How is this resistance reported to the DME? How does the DME use it? Is it used in conjunction with voltage to determine how much fuel to meter? Or does it have no effect on that?

I know that there is another DME temperature sensor that tells the DME which mixture to use.

If the DME does not use the resistance for fuel metering, it would be simple to program a home-made MAF.

Also, on another note, how is the AFM door actuated initially? Does the DME provide a signal to the AFM based on throttle position, or is it based on air-pressure difference? If its the former, then home-based programming becomes trickier.

I'm thinking about making an MAF kit that uses the stock chip, and answers to the above questions would be very helpful in helping me decide on a course of action.

I'm thinking in the $200-$300 range. I know I can buy miller MAF for a $100 more but it's not about that. It's about building something on your own and see if it works.

I will appreciate all your help on this.

Regards,

Jawad

[Rogue_Ant]

The air temperature sensor (AIT) is part of a voltage divider circuit... changes in resistance change the voltage - which the DME reads through an ADC (analog to digitial converter).

The DME then takes this info, and adjust fueling, warm-start, ect.

The other temperature sensor the DME uses is coolant. Again in a similar fashion, and again the DME uses this sensor to adjust fueling, cold-start, timing, ect.

I'm not sure I understand your question "how is the AFM door actuated initially"...? The DME uses the AFM door position at all times (other than cranking when rpms are < 160). The DME never knows exact throttle position, rather, it only knows throttle "states". The three possible states are: idle, part-throttle, and WOT.

If you just want to get something "working", then that isn't too hard. However, to put together a kit that is proper is a different story...

[ibtisam.jawad]

Thank you, Joshua. You've provided me with valuable information. I did read more information since my earlier post and look through some bins, and understand that what you're saying is very accurate.

My initial idea was that I will put some kind of a signal conditioner between MAF and the stock DME. The signal conditioner's job would be to translate output voltage from MAF for given mass airflow rate into equivalent voltage for the AFM. So if for a give rate MAF outputs 1 volt and the AFM outputs 2 volts for an equivalent measure, the conditioner would convert 1 volt into 2. This, I was hoping, would obviate the need to adjust values in the chip.

Turns out that buying/building a programmable signal conditioner is way harder and expensive than buying eproms and burning them. Tuning the DME values is also relatively easier (since it can be done via software) than changing one voltage curve into another through some circuit.

I was hoping to put together a singal conditioner initially. But the more I learn, the more I'm leaning toward tuning the chip.

[Rogue_Ant]

No problem - glad to help