No PWM during cranking?

Philip Lochner · Post by **Philip Lochner** » Tue Jul 11, 2006 12:30 pm

Testing the MS-2 I built on the bench with purely resistive loads (560Ohm) on the injector outputs, it would seem that MS-2 does not activate PWM for LoZ injectors during cranking. I had both opening time and PWM time threshold at 1.3ms, PWM current limit at 30% and period at 66us with a cranking pw of 7ms and I could see no PWM. Only when RPM exceeds cranking rpm does PWM activate.

1) Is this correct?
2) Should it be so?

Mike_Robert · Post by **Mike_Robert** » Tue Jul 11, 2006 12:57 pm

I'm pretty sure it's that way by design. I seem to recall that the logic for this design decision was that it would be very unlikely to hurt anything (overheat injector, etc) at cranking rpm and this scheme does make certain that the injectors will open if the cranking voltage is marginal.

-Mike

Bernard Fife · Post by **Bernard Fife** » Tue Jul 11, 2006 1:03 pm

Mike,

Yes, that's right. The duty cycle is very low, so there's no chance of overheating the injectors, and the voltage drop while cranking can be quite signficant, often to less than 10 Volts. Eliminating the the PWM during cranking allows the running PWM% to be lowered quite a bit, and overall makes the injectors run much cooler.

Al made this change in the late stages of the MS-I code, I recall, and it has been carried forward into everything since then.

Lance.

Philip Lochner · Post by **Philip Lochner** » Wed Jul 12, 2006 3:41 am

Lance wrote:
Yes, that's right.

Excellent! Thanks for the confirmation. It helps for that first time you turn the key after having wired up the injectors and the adrenalin is running in overboost.... If I destroy this V12, I might as well throw the car away.

Philip Lochner · Post by **Philip Lochner** » Fri Jul 14, 2006 1:43 am

Mike_Robert wrote:I'm pretty sure it's that way [no PWM during cranking] by design.

I realised last night that I WILL therefore have to install resistors (or use the existing). If there is no PWM during cranking, with 6 x 2.5Ohm inj per bank, that could total 24A (@ 10 V) during cranking. MS limits current to 14A.

This might explain why instead of hearing the regular tick tick tick when cranking, I heard "crrrrrr". I am assuming this is due to current limit kicking in and out.