I asked Matt at DIY AutoTune if MS has a sense circuit. His answer is "NO", but a little confusing:

Quote
The Megasquirt does not determine coil saturation time - that certainly
would be handy, but we don't have a good way to do this. I think what's
going on is that the Megasquirt changes the coil dwell as a function of
battery voltage because the coil takes different amounts of time to
charge at different voltages. When the engine starts and the voltage
increases, the Megasquirt drops the dwell because the coil charges
faster. You can adjust the battery voltage compensation.



Apparently, MS does use the battery voltage value before the alternator kicks in to raise dwell as a coil charge compensation. After it reads the alternator-supplied voltage, it reduces dwell to a compensated value. This amounts to the same thing that Charlie was saying..... so I need to find the coil inductance and adjust voltage compensation to make the dwell time match the coil requirement.

So, I just bought this LC meter for $35 (inc. S&H):

[Linked Image]

Who wants to bet that after I make the calculation I find that the present 3.1ms setting is correct? <img src="/forums/images/graemlins/lol.gif" alt="" />

Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum