http://www.megamanual.com/ms2/tune.htm
Setting the PWM Criteria
To tune the PWM [pulse width modulation] values for your engine, you need to know what kind of injectors you have: low impedance or high impedance.
If you are running high-impedance injectors (greater than 10 Ohms), then set the:
PWM Time Threshold to 25.4 msec, and the
PWM Current Limit (%) to 100%.
In essence you are disabling the PWM mode. This allows full voltage to the injectors throughout the pulse widths.
For low-impedance injectors (less than 3 Ohms), you need to limit the current to avoid over heating the injectors. To do this, there is a period of time that you apply full battery voltage [peak] current, then switch over to a lower current-averaged [hold] current, i.e. peak and hold. Alternatively, you can add resistors in series with the injectors. See the Injectors and Fuel Supply section of this manual for more details.
To run low-impedance injectors with the PWM current limit mode, you need to set two parameters - the
"PWM Current Limit (%)" and the
"PWM Time Threshold (ms)" - both are on the
“Constants” page. The current limit % is the percent duty cycle when the current limit is invoked. The time threshold is the amount of time from when the injector is first opened until the current limit is activated.
Start with:
PWM Time Threshold = 1.0 millisecond, and
PWM Current Limit (%) =
30% on a V3 or V3.57 main board (if you have the active flyback circuit installed).
30% on a V2.2 main board (if you have the separate flyback board installed).
75% on a V2.2 main board (if you do not have the separate flyback board installed).
Once you get your engine idling, then first adjust the PWM duty cycle down in 1% increments until you notice a change in idle quality (be sure to hit the "send to ECU" button each time you change the value). This is the point where the current limit is too much and the injectors are not being held fully open.
Then move the value back up 3 - 5% (for example, if the idle falters at 45%, then put in a number of 48% to 50%).
Move on to adjusting the time threshold. Lower the time threshold by 0.1 milliseconds at a time until the idle quality deteriorates.
Then increase it 0.3 ms.
Now, repeat these steps (starting at #2). Adjust the duty cycle and time threshold alternately to get the optimum values for your set-up. You will converge on a set of numbers that work well for your set-up.
The injector opening time and PWM time threshold should be set to approximately the same value. So adjust the PWM time threshold, and the opening time together. However, once you start to tune the 'fine tune' the idle, if you have to revisit the PWM parameters, change only the PWM time threshold, but do not change the opening time.
The reason for this is that changing the opening time also changes the AFR, especially at low rpms, so it forces you to also retune the VE table. So unless they get to be different by more than about 0.5 milliseconds, leave the opening time alone after tuning the idle and cruise VEs (unless your are willing to retune those areas).
On the car, setting the PWM parameters is very easy to do and only takes a few minutes. At idle the overall injector pulse widths are small compared to their close time, so this will allow you to adjust the values. In other words, adjust the PWM current limit before taking the car out on the street where injector pulse widths become high, increasing the possibility of overheating your injectors (and blowing fuses).
You may find that you can idle at a very low PWM%, but the engine will stumble or cough if given any throttle. This can be due to a too low PWM% especially if your injectors are large and the idle pulse width is approximately equal to the PWM Time Threshold. In that case you may be idling almost entirely on the PWM Time Threshold. So rev the engine occasionally as you are tuning the PWM parameters, to ensure that you haven't gone too low.
Also, for some setups, an initial PWM Current Limit of 30% may be too low on a V3 main board (75% on a V2.2 main board without the flyback board), so they will need to increase this value - same for the time threshold. Use PWM time threshold values greater than about 1.5 to 1.7 milliseconds only with great caution – it is possible to burn out your injectors!
Note that if you are running the active flyback circuit and need PWM% of 40% or more to run properly (or 80% on a V2.2 with the standard flyback circuit but without the flyback board), your flyback circuit isn't working properly! Troubleshoot that circuit carefully to find out why.
