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ECU Basics

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Ecu basics

The intent of this is to introduce (and help people understand for themselves) how an ECU works, mostly in order to help people decide whether or not they “need a tune”, or figure out “what a tune does”, etc... I will try to keep it short and simple, but I have learned a lot about this platform over the years, and this is my first attempt at writing it all down. I'm also going to keep it Focus-specific so I dont have to go into the other kinds of systems out there. In fact, a few of these things may be Zetec-specfic, so I apologize to duratec/spi owners when this happens and hope you or other experienced tuners will correct me where needed. So in general, keep in mind there are a few different ways things are done on different cars. In particular, the 08+ cars have things are quite different... Anyway, here goes nuttin.

The ECU, like any computer, has inputs and outputs. It takes information from the inputs, looks in its memory to see what to do with those inputs, and then sends commands to its outputs telling them what to do. I think it would be good to start off by listing the major inputs and outputs and a description of what they do, along with a few “trade secrets” to keep in mind about each one.


Mass Airflow Sensor (MAF) This is the most important part of the entire operation. If the ECU does not know how much air is coming into the engine, it is next to impossible to get the correct information to its outputs. The Focus MAF is a hot wire sensor. Using a Wheatstone Bridge circuit, the MAF measures the resistance of a wire in the path of the intake airflow. This wire changes resistance as its temperature changes, and its temperature depends on the velocity of air going past it as a result of convective cooling. At the end of all this scientific mumbo jumbo, the MAF outputs a voltage (0-5 VDC) that is representative of a certain amount (mass) of air going past it. You can log this in either volts or counts (0-1024). Counts are more useful on the Zetec (non-SVT) Focus since the MAF transfer function is done in counts.

inlet air temperature (IAT) aka air charge temperature (ACT) Intake temps are important because they affect the autoignition point of your air/fuel mixture. Basically, this means you have to change your spark timing based on intake temp. The intake temp is also used by the MAF to figure out air density and the convective cooling stuff I mentioned earlier. On the Focus, the IAT sensor is part of the MAF, so it is advantageous to run a “blow-through” configuration on turbo/Procharger/VF supercharger cars, or relocate the IAT on JRSC or PWSC cars.

crank position sensor This is a “Hall Effect” sensor that uses a magnetic pickup to read a steel pattern on the flywheel or crank pulley. For Hall sensors, there is usually one spot missing on the otherwise regular pattern that signifies TDC or some other known point of rotation. The ECU uses this to know where the crankshaft is in degrees.

throttle position sensor (TPS) This is hooked up to the throttle itself and uses a potentiometer to change its output voltage as the throttle opens/closes. In automatic trans cars, it is used in shift logic so the ECU knows when to tell the trans to downshift or upshift based on driver demand. In all Zetec Focuses, throttle position is used to control the open/closed loop transition and also the idle/part throttle transition. It can be logged in volts, counts, and counts relative to closed. The most useful of these is counts relative to closed, since many of the ECU's tables use that value.

fuel rail delta pressure sensor Fuel pressure is important because your injectors are calibrated to run at 40psi. There are compensation tables for fuel flow at other pressures, so they will still run fine at other pressures. You can log this value as “pressure drop across injectors”. The reason it is called this is you need to know the pressure difference between the manifold and the fuel rail. If your manifold is at 20inHg (idle), you only need 30psi of rail pressure, but if you are only at 5inHg (typical for WOT), you need around 37psi of rail pressure. The same concept applies to boosted cars. At 10psi of boost, you need 50psi of rail pressure. Boosted Focuses run into a problem here. The sensor does not read over atmospheric pressure! So you need to command 50psi in the tune.

coolant temperature (ECT) Your engine's overall temperature affects the knock threshold, so the ECU needs to know this value. Additionally, it needs to know when to turn on the fans. The coolant temp also affects cold start functions including the first closed loop transition. On the regular Zetec Focus, there is a sensor that reads the metal temperature on the head, then the ECU uses a transfer function to infer coolant temp.

oxygen sensor This sensor outputs a voltage that represents the oxygen content in the exhaust gas, from which the ECU can infer the intake air/fuel ratio. You can log fuel trim (STFT and LTFT) to see what this sensor says. In closed loop, STFT will show the actual fuel trim value. In open loop, this sensor is not used, so the STFT simply reports what the base fuel table says. In closed loop, STFT on the Focus is opposite of lambda. A reading over 1.0 means the sensor is reading rich, and under 1.0 is lean. On the Focus this is a narrowband sensor. It is only good at reading afr near 1.0 lambda. As a result, you can easily tune the closed loop range of your MAF using STFT. While tuning, it is a good idea to turn off adaptive learning so you don't have to worry about LTFT. In fact, most people just leave adaptive off since your tune should be pretty good when you're done (“good” is within 5%, aka bouncing somewhere between .95 and 1.05).

knock sensor The bain of many DIY Focus tuners' existence. This sensor is essentially a piezoelectric microphone. If you search in the ECU chat forum you'll see a thread I made about listening to this sensor. Basically, the sensor outputs an AC signal and the ECU processes it to look for sounds in a certain frequency (tone) range. Detonation makes a very distinct sound, so in my experience, listening to it is much more effective than letting electronics do it for you.

battery voltage sensor This is important because some of the inputs and outputs (like fuel injectors) are affected by voltage.


fuel injectors You may have heard of injectors specified in terms of lb/hr. This is a bit of a misnomer, since they are more complex than just flowing one rate at all times. Injectors have a low slope, breakpoint, and high slope that are all programmed into the ECU. It uses these values to figure out how long it should tell the injectors to open (pulsewidth). You can google this stuff if you are interested.

coilpack The Zetec coilpack is a “wasted spark” system. The coil will fire 1&4 or 2&3 at the same time. This is why it is impossible to time your cams “180* out” of phase. The coil has three wires going to it. One is a 12V power source. The other two are switched grounds that complete the circuit to 1&4 or 2&3.

fuel pump driver module (FPDM) This is why WE DO NOT HAVE A FUEL PRESSURE REGULATOR. The ECU tells the FPDM how much voltage to send to the fuel pump in order to achieve the desired rail pressure (and therefore the desired psi across the injectors). The FPDM and rail delta sensor work together to keep you at 40psi of fuel pressure. No FPR needed.

idle air control valve (IAC or IACV) This valve controls how much air goes into your engine when the throttle is closed. If the ECU thinks valve cant perform that job for whatever reason, the spark will go into “idle feedback mode” and it will control idle via spark.

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So now that you know the communications aspect of a tune, it would make sense to explain some of the things in the ECU's memory that makes it all work. Here are some of the tables and calculations used by the ECU to get your engine to run correctly:

MAF transfer function (MTF) This is the heart and soul of your tune. If your MAF isn't properly modeled, the entire tune is going to be thrown off. The table in the tune has a list of either MAF volts or counts and you tell it how many pounds per minute of airflow are going through the MAF at the given volts. The ECU uses this function to figure out how much fuel to use. Therefore, if you change the diameter of the MAF housing and therefore the velocity of the air going through it, your MTF is going to be wrong. Using a bigger MAF housing without tuning will make your car run LEAN! Remember, in open loop, the o2 sensor is not used, so this lean condition cannot be mended by fuel trims! The MAF is easily affected by changes in airflow characteristics. Any time you change the intake or exhaust, the MTF will need some slight modification. However, in my experience, intake and exhaust mods do not necessitate a tune. When you do cams or a head swap, it will MAJORLY throw off the MTF, especially in the low range. In my experience, you NEED to tune when you get cams.

load aka volumetric efficiency Load is calculated using MAF air flow and rpm compared to the engine's full displacement. Basically, if you are at .8 load, you are only using 80% of your engine's displacement. The MTF is CRITICAL for getting this value correct. Since the vast majority of the other tables use load on its Y-axis, you NEED to have the MTF correct so your load is correct. base fuel table When you are in open loop, the ECU picks the afr it should use out of the base fuel table. This is another reason your MTF needs to be correct. It will infer the amount of fuel to use to achieve the afr in the base table based on the amount of air it reads from the MTF. You make this table the afr you WANT to see, then you change the MTF to get your wideband to match that afr.

fuel open loop TP This table controls your open loop transition. A big throttle body will affect what you want this set at since it will flow a lot more air at lower TP counts. When tuning, make sure you're not still in closed loop at a high load. Many tuners just set this table to 500 counts across the board and call it good. In my experience, there are better ways to deal with this, especially if you are the kind of driver that rolls into the throttle or lives in a hilly area. spark tables The EEC-V has three tables: Borderline Knock Table (BKT), Max Allowed Table, and Minimum spark for Best Torque table (MBT). BKT should be the spark at the knock threshold for 70F ACT and 200F ECT. MBT should be the spark at which you make the most torque regardless of knock (above this spark there is no additional torque to be made). Max allowed is simply the most spark you ever want to see. There are also a few adders (that can also subtract) based on ACT, ECT, afr, sudden load increase, and a few other things. The ECU will pick the lowest of the following three values for “final spark”: 1. BKT after all adders 2. MBT after ECT, emissions, and afr adders 3. max allowed.