Moronic ignition question

[Raypratojr]

I have an '84 Carrera and an' 89 E30 BMW. Both run off of the Bosch Motronic DME. I've heard that the timing is set electronically by the DME. How does that work as there's a distributor? I can't make sense of it. Thanks

[mikesarge]

That's not a moronic question

There are several iterations of Bosch Motronic. Earlier versions with a distributor don't have electronic timing, after 1.7, they are set by the DME

https://en.wikipedia.org/wiki/Motronic

[merlinfe]

Your ‘84 Carreras distributor is fixed. There is no adjusting it. There are also no fly weights in it. Your timed spark events (advance and retard) are controlled by the ECU. Your E30 is the same (provided it has the stock M20 engine). Find yourself a decent set of literature on EFI. It’s worth the read for car people.

-Steve

[Peter M]

Originally Posted by merlinfe

….There are also no fly weights in it. .

-Steve

I think there is:
3.2 distributor rebuild

[Raypratojr]

Originally Posted by merlinfe

Your ‘84 Carreras distributor is fixed. There is no adjusting it. There are also no fly weights in it. Your timed spark events (advance and retard) are controlled by the ECU. Your E30 is the same (provided it has the stock M20 engine). Find yourself a decent set of literature on EFI. It’s worth the read for car people.

-Steve

Agreed, nothing is adjustable. There is advance weights and springs in the Porsche. Knowing that the coil is feeding the distributor and the distributor is mechanically spinning and distributing the spark, how can the ECU possibly be controlling the spark timing?

[Quicksilver]

Originally Posted by Peter M

I think there is:
3.2 distributor rebuild

There are fly weights in the distributor.
And they do nothing to adjust the ignition timing.

The distributor fly weights are there to keep the contact on the rotor pointed at the contacts in the distributor at higher RPMs when the engine is running more advance.

The coil fires when the DME triggers it. That is your timing. The DME looks at the sensor readings and uses that information to look up the data from a table on its ROM (the "chip"). That is 100% of how the timing is controlled in the Motronic 3.2 Carrera.

[yelcab1]

There is a TDC or RPM sensor at the flywheel. It tells the DME where Z1 is, and from there the DME figures out where top dead center is for each cylinder. Then it is just a matter of firing the ignition at TDC, before TDC, or After TDC and by how many degrees. There is no adjustment at the distributor. End.

[stlrj]

Think of it like coil on plug except with only one coil and a distributor. The DME does the rest.

[Raypratojr]

I didn't realize that the coil wasn't 'hot' all the time and that the DME triggers the coil for each cylinder. That explains how it controls spark. Combined with the large "contact" area of the rotor it allows for the DME to advance and retard timing

[Bill Verburg]

TDC is determined by the 60-2 tooth flywheel(2 teeth are missing and that marks TDC),There are 2 inductive sensors that monitor the teeth, 1 is a reference sensor to find TDC the other a speed sensor.

the fly weights in the distributor are for phasing, ie keeping the rotor aligned w/ the proper cylinder contact at all rpm

[GH85Carrera]

Yea, what Bill said.

You can hang your timing light on the wall for display only next to the dwell meter and oil can opener-spout like I have.