Join Date: Sep 2016
Location: Not in Boise but nearby.
I think what he is asking is does it matter where the switch is set while loading the new tunes? The answer is no.
The tuner overwrites the existing information in the ECM with the information that applies to the tune. First off, the new information contains a set of instructions on what to do with a particular piece of data. For example, the data might be the intake manifold pressure. The instructions will tell the computer to convert the pressure into a number and do several things with that number like adjust the injection amount and timing. How much it adjusts things is related to several tables full of numbers. Those tables are also part of the tune.
Most tunes actually keep the original information and call it "Stock". There is room in the memory of the ECM to hold more than one set of parameters. Different tables of parameters correspond to different tunes. What the switch does is tell the instructions where to go for the parameters it needs. Think of the switch as being like an index in a reference book. The index tells where to go for the maps and charts you are looking for. The switch is not involved in writing the info, only in selecting which set to use.
It is possible to edit the tunes that are already in the ECM rather than to completely overwrite them. It is also possible to overwrite one tune at a time. You can even change a single number in the vast tables. You need the right kind of tuner to do that but it can be done. Mostly what happens though is that the tune is rewritten on a desktop computer and then written in one pass to the ECM memory.
Last edited by mizterwizard; 06-24-2019 at 08:08 PM.