measuring coil primary voltage

[chadr]

I have an ms1 extra with vb921 charging a single coil

I am seeing coil charge times, based on scope current probe reading of the coil negative wire, that are a couple ms longer than what may be considered normal. I'd like to confirm the voltage available to the coil. Would it be best to put the scope leads across the coil directly or between coil ground wire and engine ground? I realize there may be voltage of a few hundred volts across the primary when measures across the coil. When measuring between coil ground and engine ground that will at least tell me if the wiring back the ecu and the vb921 is making a solid ground connection, right?

Is there a better way to test the connection? Perhaps disconnect the coil 12v wire and trigger the vb921 so I could measure the resistance to ground from the coil negative wire?

[Matt Cramer]

It's usually best to test the voltage on the coil positive terminal with a multimeter, as their inputs are a bit more heavily filtered.

[chadr]

The coil voltage is almost 14v when measured against battery ground with the engine running. I am thinking the wire size and distance from ecu and connection through the relay board are contributing to higher charge times. I'm running an msd 8222 coil now and it takes 4ms to nearly reach the peak current. With a bip373 and maybe larger wiring or more direct wiring for the ground I'm guessing I can cut that time a little. I may try a jumper directly from the megasquirt ecu to the coil to see if that lowers the charge time.