93-94 truck VAF to MAF conversion project

semlin

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semlin

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My W.A.G. is that VCC is your supply voltage (also for the TPS sensor?), and E2 is ground.

could be but VCC is labelled as a TPS terminal in the summary. Here is how it plays out on the TPS.
tps.jpg
 
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I just read through the 93 VAF sensor on TIS again and this could be a little more than what we think.

Basically the MAF unit outputs a voltage proportional to the air flow.

The VAF unit is basically a variable resistor that drops the signal voltage down by reducing the resistance as air flow increases.

So the circuit would need to be able to do that.

EricE, your up!

Symantics. The output of the VAF is a voltage. The way the VAF produces the voltage is by varing a resistance...but the output is still a voltage that the ECU reads.

The other pins concern me a little but I'll know more once I get my truck back and can play around with those wires a bit to see how the V on them behaves.
 

NaterGator

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The VC/VCC thing had me stumped as well. The IAT is a slam dunk, it matches perfectly to the MAF system.
VC/VCC looks like it completes the circuit to a 'swept' resistor. In other words from that schematic (and I've never seen a VAF so this is all by the schematics) it looks like the airflow causes that paddle-wedge to push up and sweep a contact over the length of the resistor, which feeds the ECM data.

The VC/VCC contacts look like they are there to give the ECM an overall reference of the resistor's current resistance since it would potentially fluctuate with changes in temperature.
 
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Here is something that is confusing.
Take a look at the manual page in post #17. At the very bottom of the photo it shows the last two pins being E2 and VS.
Now take a look at the manual page in post #18. At the very top of the photo is shows the last 4 pins (in order) being VC, E2, VS, THA. Same orientation, same plug, different pin assignments?

Are those pics covering the same truck, are they from the same book?
 
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never mind. Can I delete this? I didn't see how.
 

semlin

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nate, thanks. what you said is very helpful but i disagree with your conclusion :D and I end up very confused.

first off, the IAT sensor (Intake air temperature) is intended to monitor temperature to allow the ECU to adjust the VAF signal so the VC would be redunant if that's what it is. I think it is the power source and E2 is a common ground or vice versa.

looking at it, it seems the VS voltage will vary from 4.5v down to as low as 0.2v based on the point of contact by the paddle contact along a resistor depending on paddle position.

the VC/VCC to E2 voltage goes through the full resistor. If you connect the two ECM terminals and thereby jump that resistor you get a voltage of 4.5 to 5.5 volts. that is the max input voltage going into the resistor which then I guess actually drops the voltage down to at least 0.2v (based on the minimum VS voltage measurements).

so either the VCC or the E2 is the power source and the other is a ground. Since the VS voltage is measured across E2, VCC must be the power unless VS is another ground. since E1 is a ground, I'm going with E2 as the other ground even though the diagram does not clearly show it as a ground. That way VS receives whatever voltage is left from VC after the varying portion of the resistor remaining between the moving contact point for VS and VC is done with it.

That makes VC/VCC the raw power input for the VAF. If correct, it is also the power input for the TPS. If incorrect, it is the exact reverse.

and it also means the full voltage signal range for the VAF is the allowable voltage between the two extreme measuring plate positions with the ignition on. That is, from 0.2-0.5 to 3.5 to 4.5 volts open to closed. how much of that the ECU reads I do not know.

now if all this is correct I should be able to confirm it by checking resistance when the measuring plate is open or closed. Voltage from VS to E2 is highest when the plate is closed or at idle. Resistance should thus be lowest in this position, and it is.

BUT... when you look at the ECM reader page above for measuring resistance with the VAF removed, for some reason the at rest E2 to VC resistance is lower than the E2 to VS resistance. e.g., max 400 vs. 600 ohms. I don't understand that at all. Surely the resistance on the full length resistor should be greater than the resistance of a section of it?
 

NaterGator

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No, I know and I see what you are saying, but that may not be the case. There may be a contact point where the swipe completely bypasses the resistor and it may have an inherent "base" resistance of 200Ω... hence it sweeps from 200Ω (bypass) to 600Ω (200Ω inherent resistance plus 400Ω from the full resistor)

Sorry, I'm skimming the post and pdfs fairly quickly because I'm 'hit and runing' the forums over the next few days because of all my finals, but I think you are on the right path.

The best bet is to get in there with a fluke (or similar quality) multimeter and start getting your own readings. I dont know if you mentioned if you've got one or not, but a spare VAF to tear apart would obviously make this a matter of time, not guessing ;)

EDIT: The line "HINT: Resistance will change in a wave pattern as the plate opens" is not really a good sign. That makes me thing there is circuitry and or the scale is nonlinear.
 
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The circuit is drawn up and I get the truck back today. Once I get time I’ll take some readings off my VAF to verify the 5-0V behavior.
Current plan for circuit.
VAFtoMAF2.JPG
 
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I know it will likely be called "unnecessary" . . . but is the vaf on the early ifzfe the same or at least similar to the one on my 3fe? Believe it or not mine is running great and it might be a useful upgrade if it would work.
 
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The PCB is designed now.
1.25"x1.25"
VAFtoMAF2_PCB.JPG
 
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I ended up giving my modified MAF housing to someone so my stock MAF won't be available until the production units come in, which will be in about a week.

Sorry about that but I'm trying to juggle a few things right now.

You gave away the one your were running? :doh:
hehe, no worries.
 

semlin

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eric it looks like you have been very busy while i have been away. i wish i understood it. is that a linear voltage inverter? why does it need power?

do you think bench testing one vaf is best? judging by the fsm there is at least .5 volt of allowable fluctuation on an individual vaf at the same setting assuming it is working to spec. after 15 years i wonder if results may vary further. would it make more sense to take apart and figure out the circuit or to test multiple vafs and compare? not sure if the vaf can be taken apart and put back together.

anyway, send me a pm if you want me to repeat your tests on my vaf for verification.
 

landtank

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One thing on the PCB design, you might want an in and an out connector, The factory harness is a 4 wire connector to the VAF but the MAF is a 5 wire connector.

So one side of the board gets the 4 wires from the factory harness and the other side of the board has the 5 wire connector already on it for the MAF sensor.

The MAF sensor uses two grounds which I believe can be a common one.
 
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I just did some testing on my VAF.
Interesting results.

Mapping of the connector (not vaf side showed)
Engine off, Key turned to on.
pin 1,2,3,4,blank,5,6 (seen as you look at the connector)
Pin 1; 5V
Pin 2; floating, as shown above, varies with air flow when engine on.
Pin 3; ground
Pin 4; 5V
blank
pin 5; 12V
pin 6; ground



I measured voltage across the 2nd pin in from the right as you look at the VAF (2nd from the left on the connector).

Warm Engine, parked, no load.
Voltage with Engine off 3.93V
Idling at 700RPM; 2V
2K RPM; 1.31V
3K RPM; 1.04V
4K RPM; .84V

My initial thoughts of all this is that we may have to adjust the conversion of the MAF to between 0 an 4V.
I'll take a look at how I might do this and adjust the circuit design.
 
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