My MAF Swap. Write-up included

[ryan_george]

Finished my MAF swap a little while ago, and thought you guys would get a kick out of it.

Tools Required:



Just your basic tools for wiring. Soldering iron, solder, knife/wire cutters, multimeter, electrical tape, and the most common tool.... the Bentley.

Hardware:

'98 M3 MAF, made by Siemens
SMT6 piggyback computer

This is mostly a stock engine, but I wanted a project anyways.

SMT6 Wiring:

It was pretty simple to wire up the SMT6, assuming you've done some basic soldering before. I had to solder a total of 7 wires to hook up the SMT6. I'll detail those here:

Pull apart the glove box and get access to all the ECU wires.



Find a good source of switched 12v power for the SMT6. There are a few leading to the ECU, but they were a pretty light gauge and I didn't want to draw too much power from the ECU. I went to my stereo and grabbed the power and ground from there.



Red is switched power, black is ground. Tap into these and you're set. I ran a set of wires from here to the glove box to feed my SMT6 with some juice. Connect the power and ground to the Red and Black wires on the SMT6 respectively. It's a good idea to plug your laptop in now, turn on the ignition, and see if you can communicate with the SMT6. If you're still with me, proceed...

Next, you'll need to get some inputs and outputs for your SMT6. The basic ones are:

Load signal (AFM)
RPM signal

I checked the connector at the AFM, and found out what color wire was the signal wire. In my case, it was the yellow/gray wire. Dig into your ECU harness and find this wire. Cut this wire, and connect the AFM side to both the Brown and Blue wire on the SMT6. Now, connect the ECU side of this wire to the Violet wire on the SMT6. Doing this lets you take control of the AFM signal. On the SMT6, the Blue wire represents the Air flow input. The Brown wire represents the Load Signal. On these early Motronic units, the same signal is used for both Load and Air flow. This is why both Blue and Brown are connected to the same Yellow/Gray wire.

You should be able to turn the car on, and see the load input on the laptop. If you can’t, go back and check the wiring…

The next wire you’ll want to find is the Crank sensor wire. This is a black wire. Now you’re gonna tell me, “There’s so many damn black wires in there! Which one?!”

Lucky for you, it’s located at pin # 47 on the ECU harness. Unfortunately, this wire is covered in shielding almost until it reaches the ECU connector. What I did was pull the ECU connector apart, and run a small length of wire out to the SMT6. This made the wiring a bit cleaner. Remove the screw holding the connector together. Using a long piece of wire (coat hanger), push through this screw hole, and the connector should slowly pop out. Find this black wire at pin #47.





There are now 2 options:
A) Tap into the wire, get an RPM signal for reference only.
B) Tap into the wire and take control of it.

At first I though, hell, I might as well take control over it. If you want to take that route, follow this:

A) Cut this wire and splice in a length of 2 wires (speaker wires, etc). This is pretty much just to extend both ends out of the connector. Drill a small hole in the black casing to pass these wires through. Remember which wire was from the ECU and which was from the sensor. Attach the sensor end to the Yellow wire on the SMT6. Attach the ECU side of the wire to the Pink wire on the SMT6. You should now be able to start up your car, see an RPM, and be able to modify it.

After doing this, I noticed that my car took a few more cranks to finally get started. After living with this for about a day, I said screw it. It was simply too much. Although you may not have this problem, I decided NOT to modify the RPM, but just simply read it.

B) A quick fix from the previous step was just to remove the Pink wire completely, and have both ends of the speaker wire connected to the Yellow wire on the SMT6. If you decide to go this route from the start, it’s a better idea to just tap into the black wire at pin #47 and run a wire to the Yellow. This would make it a bit cleaner.

Anyways, this is what I was left with after the wiring:



You can now clean everything up in the glove box and get ready to move into the engine bay.



I tucked the SMT6 above the ECU, and left the communication cable hanging down for easy access.

In summary, this is what should be wired:

AFM signal (yellow/gray) to Brown and Blue on SMT6.
Violet from SMT6 to AFM signal (yellow/gray) on ECU.
Yellow from SMT6 to black wire (pin #47) on ECU
OR
Crank sensor (black pin #47) to Yellow from SMT6, and Pink on SMT6 to pin #47 on ECU.

MAF Wiring:

I went to my local BMW salvage shop and picked up a connector for my MAF.



After doing some research, I found out the following for this MAF:

Pin 1 - Ground
Pin 2 - Signal
Pin 3 - 12v switched power
Pin 4 - Nothing

Luckily, I had an old spare AFM with a temperature sensor, and some other goodies attached.

I pulled off the black connector on the old AFM and wired the appropriate terminals to the new MAF harness. I did this so that I would have a “quick connection” to install/remove the MAF as I please. I did this so I wouldn’t damage the existing engine harness.

I wired the wire corresponding to Pin 2 on the MAF, to the connection corresponding to the Yellow/Gray wire on the engine harness. Likewise with the ground.

Now, the AFM used a 5v power source, but the MAF requires a 12v. I tapped into a switched 12v source from one of the relays (the Main one I believe).



I also removed the temperature sensor from my old AFM and installed it in my intake pipe.





I took the sensor and wired it to my MAF harness with plugs.

To make swapping from AFM and MAF a breeze, I made sure that all of the connections were plugs that could easily be removed. This is my final AFM to MAF harness:



Note the quick connection for the temperature sensor and power.

At this point, I was ready to install the MAF. I went to my local hardware/plumbing store and picked up some rubber reducers.

This is a 3”-2” reducer with a 2” abs coupler. This combination worked perfectly with the intake boot.



Everything installed:





Tuning:

Tuning is the next thing… Unfortunately, I was low on funds and couldn’t afford a wideband sensor. These are necessary to properly tune. I’ve got a rough map setup that seems to be fine.

I should have started with this, but it’s not as interesting (but equally important). Like I said, I have no wideband O2 sensor, so this is the method I followed:

With the SMT6 installed and your existing AFM, you need to do quite a bit of logging to get an idea of how the AFM load outputs are related to your driving.

Turn on the logging and start to subject your car to different loads and rpm. You want to get as much data as possible, so try a bit of everything. Make sure that you do a few full throttle runs all the way to redline.

Get comfortable with playing back the data files and try to understand what is happening.

Now it’s a good time to get tuning.

Let your car idle and notice the voltage that the AFM is sending. In my case, my AFM was outputting about 1.05v. Rev the engine and hold it at several rpms to read what voltage you have (1000, 1500, 2000, etc). The more data you can collect the better.

Now install that MAF.

Crank up the car, and it should start and idle a bit rough. Check what output the MAF is sending. You need to use the SMT6 to alter the output to what the AFM was giving. In my case, the MAF was giving about 0.75v at idle. If you enter the number “1” into a map site, it will add 0.04 volts to whatever it’s receiving. So, to find the correct number to put into the map, take the difference of the 2 voltages (1.05 - 0.75 = 0.30), and divide it by 0.04 (0.30 / 0.04 = 7.5). Enter this number in every map site where you are idling, and it should return the idle to normal. Continue doing this for as many sites as you can, and you will have a good map to run with.

My numbers varied from about 7 to 15 throughout the whole map.

This method will take a long time to get it perfect, so that is why I recommend buying a wideband O2 sensor to make life easier.

Hope you enjoyed it!

[Snarcher!]

Wtf Is The Benefit

[ryan_george]

For now just an increase in throttle response. Once I can obtain a wideband, I'll be able to tune for performance.
But mostly it's just another project, something to get my hands dirty.

[turningermanese]

Wtf Is The Benefit

It's f*cking cool, that's the benefit

[DirtDog]

how much was the MAF and piggyback

[ryan_george]

MAF was about $50, and smt6 was $270. Both were from fellow board members.

[vashts1985]

all i gotta say is someone for once made PROPER usage of a cone filter
and the computer ahhhhhh.....(cant say retrofit ) upgrade, yeah that will work. im assuming even after the fine tuning you will only see a very small amount of HP increase without other mods?

[ryan_george]

all i gotta say is someone for once made PROPER usage of a cone filter
and the computer ahhhhhh.....(cant say retrofit ) upgrade, yeah that will work. im assuming even after the fine tuning you will only see a very small amount of HP increase without other mods?

No need to poke fun at my poor excuse for a heat-shield...

And yes, probably not a lot of hp increase. But ah well.

[Isaracing]

omg thats what I paid for megasquirt and the o2 sensor.

[ryan_george]

omg thats what I paid for megasquirt and the o2 sensor.

Is that good or bad?

[Isaracing]

Is that good or bad?

Well megasquirt is a standalone FMU and gets rid of the AFM just the same. And its tunable. Just saying, but props for what you did.

[vashts1985]

it wasnt a jest, it was a compliment.

[Joshh]

Nice write up!

[draculia]

And its tunable.

as is the smt6, fuel and spark. its a piggyback though, so it has limitations. its still one of the best piggybacks out there though.

you can also toss the afm altogether and run a map, like the megasquirt.