Dwell goes to the roof while cranking

[Blazer79]

MSII v3.0 firmware recently updated to 2.905
Bought assembled from DIYAutoTune.
Chevy 350 with TPI setup.
Using original HEI dist. with 7 pin module.
Already replaced new module and new VR sensor in dist.
Maximum Dwell Duration is set to3.5ms

Is it normal to have the dwell go all the way to 7-9ms while cranking? Engine won't start. It sputters and tries to start but immediately shuts down. I know I still have to check timing, but didn't have anyone to crank while I adjusted with timing light. But I'm concerned because the dwell gauge blinks red every time I crank it. I've seen some screenshots of an option called "Cranking Dwell", but I can't find it on mine. All I have in TunerStudio is "Maximum Dwell Duration".

Right now I've been adjusting the R52 and R56 because I'm getting these peaks of 10,000rpms or so once in a while. But still hasn't fired.

Another question. Why does the megamanual says to configure the MS for Hall-Sensor if I'm using the VR sensor? Is it because of the module? Since it's configured for Hall-Sensor, do the R52 and R56 still have an effect on what it senses? Or have I been fooling myself thinking I was adjusting something?

Thanks!

[Matt Cramer]

That's probably your battery voltage compensation. Pretty normal. I've seen some insane dwell numbers on OEM ECUs while cranking.

The MS doesn't actually see the signal from the VR sensor; it sees a square wave from the HEI. If jumpered for Hall input, R52 and R56 have no effect.

[Bernard Fife]

Blazer,

In addition to the battery voltage compensation that Matt mentions, the dwell is also increased because the controller has a harder time predicting the crank position between tach pulses. This is because while cranking:
- there haven't been many crank signal to establish a trend,
- there is not much angular momentum in the rotating assembly at the low speeds of cranking, so the compression strokes affect the crank speed quite a lot,
- we want to be sure to start, so it is important to make sure all the ignition events actually produce a healthy spark.

Remember we have to start the dwell quite a bit before the spark, and guessing when the spark will need to be is most difficult while cranking. So the code tends to start the dwell earlier rather than later.

The increased dwell will not over heat the coil at cranking speeds. The duty cycle is still very low, and it is is the duty cycle (the overall percentage of time the coil is charging) that is important, not individual dwell event duration.

For example, if the dwell reaches it's upper limit at 5000 rpm (and at higher speeds there's not enough time for a full dwell), then at 250 cranking rpm the dwell may be doubled, but the time between sparks rises by 20 times, so the dwell is actual just 10% of the duty cycle it is when running at the max.

Lance.