RobHeinl

Does anyone know if the DME uses the reference and speed sensors to modify the spark?

My guess would be no since the timing is set by the distributor and spark advance is based off of load signal from the AFM but I don't know for sure if I'll effect spark by disconnecting the Reference and Speed sensors.

I guess I should mention DME no longer controls the fuel injectors.

jpk

I believe both are required. Speed sensor gives 1 pulse for every tooth on the flywheel, refrence sensor gives one pulse per revolution (at a known flywheel position) spark advance requires that the DME knows both - how fast it's turning and where in the revolution the engine is. On the 944, the distributor does not have an advance; ignition coil is driven bu the DME.

931guru

If you disconnect the flywheel sensors, you will get no spark, even if the injectors are otherwise controlled.

quinnfiske

The timing is determined by the map contained in the DME. The "chip" in other words. Take off the distributor cap and all you find is a rotor mounted on the camshaft. No mechanical advance of any kind. The DME looks at a number of parameters and finds a timing value from the map. If you want to change the timing you must revise this map and burn another chip. This is what all those aftermarket companies do.

RobHeinl

OK, thanks guys! I never knew exactly how the DME was controlling the spark, never really looked into it quite honestly. I just assumed it had base spark mapping based off of the distributor and made changes based on the load.

Thanks again!

RobHeinl

I've got another question for you guys: do you know if DME uses the throttle switches for spark? I have the ICV and injectors removed from DME and want to use the TPS wiring if DME is not using it for spark.

Thanks again, this is a huge help.

quinnfiske

There is a TPS. It is on the throttle body. The DME looks at for fuel trim, but I don't know it the timing map looks at it, my guess would be yes.

RobHeinl

My mistake, on the N/A i guess it's a "throttle switch" or moreover- "idle switch". So to make the question more specific, does the DME change spark settings when it knows it's idling?
I really should have done more reading on what exactly the DME is doing.

adrial

All the distributor does is dictate which cylinder is going to get the spark, it does not dictate the timing of the spark around TDC. That is controlled by the DME/KLR.

Mike C.

The DME was pretty advanced when the 944 came on the market in 82. It controlled FI and spark in one integrated unit with no points, vacuum advance, trigger wheel or stuff like that (although one might argue that the TDC sensor is similar in function to a trigger wheel). The DME is based on the ability of a computer to control things in real time based on sensor inputs. Some inputs must be there like the AFM (air flow meter), and both TDC and speed sensors while others, such as the O2 sensor, if not working the DME will supply a default value so the engine will still operate but not optimally.

quinnfiske

Take a look at this. Lots of good information:

http://frwilk.com/944dme/