Weston

Can anyone tell me where the ignition coil's ground wire is coming from? Is the DME pulsing it or something to control the timing?

Here's why I ask...

I had my '83 944 idling in the garage today, and when I shut the driver's door, the engine stumbled and then died. It would not restart within the 5 minutes or so that I tried it. I also put the key in the ignition position and used a multimeter to check if the ignition coil was getting power, but it wasn't. The weird thing here is that the positive wire to the coil was hot, but it was the ground that was dead. And to make things stranger, when I disconnected the battery, the coil's ground started showing continuity with the chasis... I re-connect the battery (key still in the ignition) and that goes away. I know that the coil's positive wire is switched, but I was under the assumption that the ground wire was simply a connection to the chassis somewhere. But then again, that would make me wonder how the DME controls spark timing, as the coil's positive wire seems to be constant.

Thanks for any info.

fast951

Pin #1 from the DME is connected to the coil. The DME provides the signal to the coil by grounding pin 1.

Pin #1 does not stay grounded all of the time.

tyro

So John,

Pin 1 is floating with respect to ground when a given cylinder is near TDC (fire)?

fast951

Pin 1 goes to ground when it's time to fire the coil, then it releases.

tyro

I need to read my Bosch engine management book

I must have been thinking of it in reverse; the DME is providing a +12 pulse from pin 1, right, while the other side of the coil is at ground?

SoCal Driver

Coil gets it's power from the ignition switch. Straight from the battery; no fuse. The DME grounds and releases the coil (green wire) to fire it. The DME gets it's operating current from the first set in the DME relay that are closed by the same circuit/current that goes to the coil. The injectors get their power from the first contacts in the DME relay also. There have been problems with the large transistor coming unsoldered in the DME and causing intermittent spark. Search here for info.

lleroyb

I wonder if the door jarred the DME relay?

As far as the coil goes, I think the spark is produced when the 12v side of the coil is de-energized as in points opening. I belive it is the magnetic field colapsing that actually makes the high voltage in the secondary.

Lou

SoCal Driver

The DME controls the grounding of the coil. The 12 volts is constant. Points in a dist do the same thing -- ground then release the coil's primary windings.