Originally Posted by rick-l (Post 8250140)

Dwell angle in a points car set the charge in the coil and varied with rpm.

The DME sets the current to 8 amps for every spark so dwell time has nothing to do with the energy in the spark.

Originally Posted by scarceller (Post 8250213)

Rick, the DME has coil charge time (Dwell Time) I know this 100% for sure because I know were the Dwell map is and how it works. I actually tune the Dwell when using non stock coils I also slightly tuned it for the stock coil at lower RPMs since the stock table is a bit shy of fully charging the coil with the stock Map.

But you are also correct that the darlington transistor has over current protection as well but that's used as a fail safe in case the coil shorts out.

I scope the coil and alter the Dwell map to watch charge times and saturation. I can easily stop charging the coil far before 8AMPs if I wish, the DME has both things built in, a Dwell Map and over current protection but the Dwell map comes first in calculating when to start charging.

Originally Posted by rick-l (Post 8250221)

At idle is it hitting the current limit? This is the only place I looked at it.

Originally Posted by mysocal911 (Post 8250232)

No, or the coil driver would be over-heated, i.e. Power ~ = 8 amps X 13 volts - 8 amps X 8 amps X coil resistance.

Originally Posted by rick-l (Post 8250221)

At idle is it hitting the current limit? This is the only place I looked at it.

Originally Posted by rick-l (Post 8250246)

For 0 di/dt the power in the darlington would be 12*8 or 96 Watts which it won't do very long. I was under the impression from everything I've read and seen they contro the dwell to just hit the current limit.

Originally Posted by mysocal911 (Post 8249536)

1. Who said to use a LED test light?
2. When using an incandescent test light that test works perfectly.
3. What do mean try it? I've done that for years, i.e. grounding the test light. You need to try it!

"Reason is that when a coil first start to charge it has 12vdc across it as it charges the vdc starts to reduce to 0vdc once fully charged.
So when the DME commands the coil to start charging it grounds the coils '-' side and the coil has 12vdc across it and voltage starts to
decrease from 12 to 0 as it charges."

That statement is not correct. The coil does not saturate (zero volts across it), as the coil driver is switched off before saturation.
Besides once the coil driver turns off the coil voltage rises to about 350 volts on the primary which is more than enough voltage
to flash an incandescent test light.

A coil does not function like a resistor, although it does have a very low resistance, i.e. the voltage does not vary as the current
changes, only when it reaches saturation. Although it's series resistance does have a small effect on the voltage of the coil,
the rate of change of the coil current determines the coil voltage. Time to put an o-scope on the coil signal. The DME ECM
calculates the charge time (dwell time) of the coil driver.

Voltage (coil) = L (inductance) X d I (change of current) / d T (change of time)

Originally Posted by scarceller (Post 8250251)

I disassembled the entire Program code for the 89 chip and I know what the software does and how it works. I made no assumptions here as I found the Dwell map and the portion of code that calculates charge times and I know how it does it.

Originally Posted by porsche.racer (Post 8253455)

Update:

Definitively the no-start was caused by the DME.

Anyone have a good used DME (1987 911) for sale.

Originally Posted by porsche.racer (Post 8253607)

That's the plan! Sending it out to Sal for repair and upgrades.