MAF Mod. Where goes the resistor?

Here's the wiring diagram on my Nissan MAF sensor.

Evidently, the voltage goes from around 0.8-4.0 volts from idle to 4000rpm.

Thereafter the engine runs in open loop.

Where should the resistor go, and what value should I use - if my aim is to lean out the mixture?

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Comments

MAF Mod. Where goes the resistor?

qsiguy's picture

You can't just put a resistor on it. You have to use a pot which is basically two resistors. If you just want to reduce the voltage the diagram below will work. About any pot will do it. 1k-25k. Doesn't really matter for this purpose. Use a linear pot. You could also install this circuit on the MAF input wire instead of the output.

A better way to do it is with an adjustable voltage regulator like the one below. This will allow you to reduce or increase the voltage going to the MAF/MAP sensor.

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MAF Mod. Where goes the resistor?

mpgmike's picture

The MAF is frequency based, and the LM317 doesn't work the way it does on a MAP. I was intrigued when I saw this thread, but it looks like I still may have some homework to do. If anybody knows, add me to the list of inquisitive minds.

Mike

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MAF Mod. Where goes the resistor?

manilow's picture

Thanks for the help, Qsiguy - I'll study those ideas.

Mike, I was a little puzzled when you said "frequency-based". I thought the MAF on the Nissan uses a linear voltage output to inform the ECU of the amount of airflow...

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MAF Mod. Where goes the resistor?

qsiguy's picture

I thought the same thing, can you elaborate Mike?

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MAF Mod. Where goes the resistor?

mpgmike's picture

No, maybe not. Most of the MAFs have a heating wire and a temp sensor that monitors the temp of the heating element. The amount of energy it takes to maintain a given temperature determines the amount of air flowing through the MAF. The ECU regulates this energy with a frequency. Typical frequencies have a fixed off time, so the on time varies with frequency change.

I'm not a Nissan guy, so I wasn't aware of the DC voltage output. Please accept my appology.

Mike

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Re: MAF Mod. Where goes the resistor?

E=mc2's picture

I'm going to be modding an '86 Nissan 720 pickup TBI MAF sensor that has 4 wires.

It is voltage...not frequency based...I think. It's the single hot wire type.

This was Nissan's first EFI on this truck.

The wires....

* 12V input

* ground

* output that is supposed to range at the ECU from 1.6v to 3.1v...I've measured the actual output at around 1.5-1.8 v to 2.6-2.8 v with 2 different MAF units.

* 4th wire...not sure

Just want to try a pot I guess either on the 12v input wire...or the output wire to the ECU. Wondering which wire would be best to use and what ohm range of pot to use.

I guess what I'm trying to do is slightly lower the voltage the ECU sees over all throttle openings to cause it to input less fuel relative to rpm?

Also going to put a pot on the CHT (water temp)...it ranges from 1v to 5v. Will use 2 pots inside the cab to adjust both while driving...plus an EFIE. Truck has no IAT that I know of.

I have two small pots that adjust from 0 to around 18K ohms...are these in the right range?

Here is a pic of the pots (trimmers) ready to mount in the dash...small, but guess they will work...turn pretty smooth and have a fairly accurate adjustment. Not sure if they will handle any real load...probably not.

The one on the left will be connected parallel with the 2 CHT wires and the right one in series in the output wire from the MAF.

note: have purchased some better pots that should be more reliable...though the trimmers might work

http://en.wikipedia.org/wiki/Voltage_divider

I guess this is what I'm trying to do with the MAF.

Some feedback would be great....am I on the right track? :shock:

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Re: MAF Mod. Where goes the resistor?

pimpsmurf's picture

I am currently doing this mod for performance gains, not MPG gains.

On my 2005 Nissan Sentra SE-R Spec-V, the ECM reads a single hot wire MAF and the ECM uses 0-5v. My car is turbocharged and the pressure in the pipe causes the hotwire, which is downstream of the turbo, to cool more effectively causing the MAF to max out very early. Instead of moving the MAF to the inlet side, which would give bad AIT readings, I opted to install a 15 turn 10kohm potentiometer to drop the voltage.

At around 3.8 to 4.2 MAF voltage, the ECM goes into "stupid mode". In stupid mode, the ECM modifies the intake cam angle (IVT) negatively, causing less overlap and closing the intake cam later. This causes a lower dynamic compression ratio than you would see otherwise. On top of this, it causes the ignition timing to be pulled fairly dramatically.

I said all of this to say this, for MPG gains, you want more timing (provided the fuel's octane can handle it) and high combustion efficiency via increased dynamic compression ratio. This mod will NOT lean out your fuel mixtures because the car will operate in closed loop, relying on the information from the o2 sensor.

Reducing your MAF voltage a good 20% or so will cause the car to attempt to operate with higher timing under light load because it thinks it is at higher elevations. However, if you are running too low octane fuel, the knock sensor will start complaining and the ECM will pull timing. Plus, your fuel trims are going to richen (because the MAF is seeing less air, and has to correct by adding more fuel per air. This will cause the ECM to DROP your timing. This is a bad thing.

Ideally, you want to increase your injector size a little, AND reduce MAF voltage. This will keep the trims closer (or even negative) which will maximize your timing and combustion efficiency.

Stock injector cc + 20% = new injectors
potentiometer tuned to 20% reduction in voltage

Now you have the issue of wanting to change the AFR the ECM is seeking.

You can use a wideband which can output a narrowband signal = current - X AFR. This will cause 16:1 to look like 15:1 to the ECM, or 14.7:1 to look like 13.7:1. Then the ECM attempts to pull fuel. All of this combined with the use of e85 (ultra high octane) will allow you to run the cheaper e85 gasoline and still only lose 5% of economy. E85 being somewhat better for the environment compared to dino juice, plus being able to run much more lean AFRs (in many cases 16:1-17.1 for idle and cruise.)

Currently I am running a very high ignition timing with e85 and 15.5:1 AFRs. I get very good economy because of the combustion efficiency and needing less throttle (ie, airflow) to maintain a 60mph cruise.

Just trying to help lend a hand.

-JNY

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Re: MAF Mod. Where goes the resistor?

Ryanrpm's picture

pimpsmurf wrote:

Reducing your MAF voltage a good 20% or so will cause the car to attempt to operate with higher timing under light load because it thinks it is at higher elevations. However, if you are running too low octane fuel, the knock sensor will start complaining and the ECM will pull timing. Plus, your fuel trims are going to richen (because the MAF is seeing less air, and has to correct by adding more fuel per air. This will cause the ECM to DROP your timing. This is a bad thing.

-JNY

This statement confused me.....

I was under the impression that if there is less air going into the engine, there will also be less air making it through to the O2 sensor, therefore, the ECM will add less fuel in order to reach stoich? This will in effect give you mpg gains, not make it worse?

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