To determine both a fuel distribution map and an ignition timing map, two parameters are used, engine load and engine speed (rpm). The maps can be described as tables with engine speed rows and load columns. Both load and engine speed are divided up into segments. To help understand this more take a look at the diagram. Please note this is just an example of what a map looks like, the figures for timing in this map are not the actual figures from the ST185 ECU. To see the fuel and ignition map for the Japanese market ST185 click here.
O.K., so now we know what a map looks like, and we can easily see how the ECU determines fueling and ignition timing, but what happens if the engine speed and rpm fall between two segments? For example 4,250rpm and 11.25 psi. When this occurs the ECU carries out proportional calculations to arrive at the correct value. So using the above example, we see that the four figures involved would be 18.8, 24.3, 14.5 and 16.0. These figures are divided by four to give the proportional ignition timing of 18.4.
Another question that seems to come up a lot, is how an ECU that does not use a MAP (manifold absolute pressure) sensor can know the load on the engine. Well first off the ECU always knows how much air is entering the engine via the AFM (air flow meter), so it simply uses the volume of air entering the engine every one revolution to determine load.
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Please note that I am not a mechanic by trade, and that any information offered on this web page is free and without guarantee. Should you choose to perform any of the procedures listed on this site, you will be doing so of your own free will, and I will not be held responsible or liable for any damages that might occur from using information obtained here.