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What does a MAF "Control" on the PCM

B

Beans

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Someone educated me.

I understand it sends signals, but why are mafs calibrated for injector sizes and specific vehicles then?
 
Thanks, that's mostly the stuff i understand.

I'm just going to make a example so don't eat my head, im not actually doing it.... just as a example. to understand more.

So, lets say for example, you put a Mustang MAF on a contour, that said maf is "calibrated" for 39lb injectors I guess it would be "calibrated" for a v8 whatever liter as well. How is it calibrated? Will the ECU think it's getting more fuel or air than it should be? How can that be... Isnt that the ECU's job to manage the air fuel? Doesn't a maf just read the amount of air then send a signal back? "Ok i'm getting XXX amount of air, i'm sending 4 volts so the ECU understands how much air is coming in the engine"
 
well without personally knowing too much, off the bat, I'd have to say the problem(or a potential problem at the very least) lies in the air to voltage ratio on the MAF. If you don't know the response characteristics of the MAFs, X amount of air in one maf could send 4 volts, while X amounts of air on the other maf could send 5 volts. But the ECU is expecting the initial air/voltage signal of X/4V so it will be screwy. That's my take on general unknown electronics swapping anyway.
 
But what about when people boost their car and use the lightning MAF? Is the ECU tuned around the MAF? So it basically adjusts to what your saying?

X air in 90mm Maf = 3v
X air in 70mm maf = 4v

But

2 volts = Z Air in 90mm maf
2 volts = Y Air in 70mm maf

???
 
MAFs arent calibrated to a given injector. they are calibrated so that a given volume of air cools of the resistor a given amount (changing its resistance). the difference in resistance results in a different signal voltage seen by the PCM. the PCM is tuned to interpret a given voltage from the MAF as a set amount of air. the PCM can then use other signals (such as the O2 sensors, coolant temp sensor, air temp sensor, etc) and the calculated airflow to determine how long to hold open the injectors (which the PCM has to be told their size so it knows how much fuel they inject for a given duty cycle/pulse width).
 
To expand on what striker said a little bit:

Every MAF has a map associated with it, this MAP has voltage on one axis and airflow on the other axis, and so any given airflow through the MAF results in a particular voltage output to the PCM. The PCM stores a copy of the map for the MAF it thinks is installed in the car, so it looks at the voltage from the MAF, and the map, and determines how much air it thinks is coming in. Based on that input, and from some other sensors, it decides how much fuel to spray and when to spray it out of the injectors. The PCM also has been told what size the injectors are supposed to be as well. All this is basically the same as what striker said above.
Now, in the aftermarket world, people like to change thier injectors and thier MAF to different sizes than what the PCM thinks it should be using. The right way to deal with this, is to recalibrate the PCM so that it knows exactly what MAF and injectors it really has, so it can make the right decisions about fuel delivery. However, some people take shortcuts by tweaking the MAF output voltage to trick the PCM into changing the fuel delivery to compensate for larger injectors, rather than change it properly in the PCM. Basically, if you install larger injectors, by a certain % over the original injectors, then you would decrease the output voltage of the MAF to be equivalent to a decrease in flow by the same % so that the differences balance out. It works as a bandaid in many cases (sometimes not at all though) but its a hack way of doing it.
 
Does it truly matter where the change is made? I mean, on one hand you have the PCM, which interprets the voltage. On the other hand you have the MAF, which is sending the voltage. If you modify the MAF voltage such that it will match the fuel delivery calculated by the PCM, isn't that just as effective as altering the way the PCM interprets the voltage?
 
zorrex said:
Does it truly matter where the change is made? I mean, on one hand you have the PCM, which interprets the voltage. On the other hand you have the MAF, which is sending the voltage. If you modify the MAF voltage such that it will match the fuel delivery calculated by the PCM, isn't that just as effective as altering the way the PCM interprets the voltage?
Click to expand...

Yes it does matter. The PCM makes its decision based primarily on the amount of air coming in, but not solely on the MAF signal, it is modified by other parameters, and the amount of modification varies depending on what the actual amount of air coming in is. This is one of the primary reasons why aftermarket MAFs and lots of aftermarket "tuned" cars have serious driveability issues. Again, sometimes it all works ok, and sometimes its a mess, ie a bandaid solution, it'll get you by in an emergency, but it just kind of covers the problem. And bad tuning won't heal itself like a cut on your arm will...
 
OK that makes sense. I guess it wouldn't be logical for a different size MAF to be hacked for it to register "correct" voltages for the PCM anyway.
 
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