guys, you're missing some things:
1. MAP sensor signal is an analog voltage that corresponds to measured Manifold Absolute Pressure. this is a 0-5v signal I believe. This signal is not clamped high, it is as continuously variable voltage. You want to pass this signal un-adulterated to the ECM, except that you want to impose a maximum ceiling.
2. series resistance won't work for two reasons: (1) The Cummins design is for a low output impedance device (the MAP sensor) to drive a high impedance device (the ECM). by the time you find the input impedance of the ECM (very small) in order to pick the right series resistance value, you'll end up with a very large resistor. Thus you will blow the original design to pieces, creating electronic instability and susceptability to noise. (2) If you put in a series resistance, all you'e done is voltage divided the MAP output (your additional resistor plus the input impedance of the ECM which appears as the other resistor in the voltage divider). If you do this, you will guarentee that the ECM never sees the correct boost level. thats bad.
3. a voltage divider is a bad idea for two reasons: (1) it guarentees that the ECM will always see boost levels below what they really are. This will have the effect of ham-stringing the ECM so that the engine will be in virtual limp mode (ECM won't fuel at high enough levels because you're telling it that boost is always low). You want to give the ECM
unprocessed signal (below the maximum) so that it fuels correctly. (2) a voltage divider is a really poor voltage limiter because it imposes the same scale factor across the entire signal range. For example, if you can acheive 35 lbs of boost and you want the ECM to read only 25, then you must put in a . 7 to 1 voltage divider. If you do that, when boost is really 22 lbs (perfectly normal) you will tell the ECM that boost is really 15. 4 lbs. and it will fuel accordingly.
4. putting a resistor in series with the supply voltage will cause everything dependant on that 5V supply to not work properly.
5. if your're commited to two components already (two resistors), make one of them a zener diode and all will be happy. the ECM will read actual boost levels (and fuel properly) until boost gets too high, and then the ECM will see only 25 lbs (or whatever the max is). don't be afraid of the lowly zener. it is your freind
