DME TDC sensor thoughts...

[Schocki]

Guys,
Just a short question about the DME timing. Most 944's have to speed sensors (TDC Cyl. #1 and speed sensor). Just found out that my S2 has only one, the opening for the speed sensor is not used. That means that the one sensor senses TDC and engine RPM correct?
The question that popped up in my mind is the following: How does the DME know if it's a power stroke or an intake stroke after passing TDC. Every 4 stroke engine requires 2 complete revolutions to complete a full cycle?
Any clarifications welcome
Michael

[Schocki]

Yes, the computer knows exactly if #1 is on a power or intake stroke. I emailed this question also to Rennlist and got the correct answer. There is a hall sensor mounted on the cam/distributor housing on top of the engine. This sensor tells the computer if it has to inject fuel or not. I do agree that the ignition decision will be made by the position of the rotor. The correct TDC position is than determined by the TDC sensor at the flywheel (accuracy) and the DME gives the injection pulse signal. It would make NO sense that the DME injects unwanted fuel towards the intake valve in a power stroke. This is NOT a continous injection system it is a pulsed system! I got numerous replies from the Rennlist list but only one from the Pelicanparts list. My intend was not to outsmart anybody but to share the most correct explaination also with this list.
Michael

[ChrisBennet]

Originally posted by Schocki
Yes, the computer knows exactly if #1 is on a power or intake stroke. I emailed this question also to Rennlist and got the correct answer. There is a hall sensor mounted on the cam/distributor housing on top of the engine. This sensor tells the computer if it has to inject fuel or not. I do agree that the ignition decision will be made by the position of the rotor. The correct TDC position is than determined by the TDC sensor at the flywheel (accuracy) and the DME gives the injection pulse signal. It would make NO sense that the DME injects unwanted fuel towards the intake valve in a power stroke. This is NOT a continous injection system it is a pulsed system! I got numerous replies from the Rennlist list but only one from the Pelicanparts list. My intend was not to outsmart anybody but to share the most correct explaination also with this list.
Michael

Michael,
It is a common misconception that the injector is somehow timed to squirt based on where in the cycle the engine is. Intuitively, one would think that the injector would squirt when the valve is open or a little before maybe. With the exception of sequential injection, this is not true. (It was a revelation to me too.)
In reality, the injector is responsible for maintaining a little cloud of fuel vapor (at the correct mixture) waiting outside the valve. When the valve opens, the mixture/cloud gets sucked in.
The ratio of gas to air in the mixture is maintained by opening and closing the injector a specified amount of time. There is just not enough time to only squirt when the valve is open. At full throttle the injector may be open as much as 85% of the time.
hope this was helpful,
Chris

[RIchard944]

Schocki,

Dude, are you new? Man, Wilk is like the DME expert on the RennList. You just told he knows ****.

Richard

[Britwrench]

Chris and FR are absolutely right...
Most injectors on Porsche motors are fired as a bank of either two or four. They are not in any way timed. The Motronic system is more advanced than CIS but the injectors still just spray into the intake manifold.

[drew1]

If they don't bank fire, why are they wire as 2 banks?


drew1

[Britwrench]

Thanks for more info on the DME operation, FR. I didn't know about the internal wiring of the circuit board.

Actually, when I stated bank firing, what you wrote is what I meant. the 944 fires all four. The 928s4 has two sets of four;
hence the safety feature of monitoring cylinder head temperatures and switching one bank of four off in case of ignition failure to protect the catalyst.

[SoCal Driver]

Way back when, I went to the Culver City VW training center and was "instructed" on the new electronic injection system. One of the instructors was very serious about this "new fangled" injection and related that the injectors had a serious problem opening and closing above 6,000 rpm so the need for a rotor with a rev limiter. Was also told that the injectors fire twice per cycle.

Am I to assume that the S's have two injectors per cylinder? If so was an attempt to increase the revs or just that this provided more metered fuel rather than letting the injectors run wide open?

If the AFM is "oversized" why change it out to an Audi unit that does not use the flap? Must be some restriction there.

Somewhere in the Porsche repair and maintenance info I've collected there is a referance to the system running richer when the O2 sensor is disconnected. Not?

One idea I've had was to use two injectors per cylinder to increase the revs. Would take a DME redesign.

[SoCal Driver]

Had to dig for these. Been years since I've read them.

-----------------------------------------------------------------------

Porsche 944 DME-TESTING PLAN
WKD 450 821

Page 1

Two axes listing of possible causes;

High Fuel Consumption -----X----- Ox Sensor

Also; Ignition System, Air Flow Sensor, Fuel Pressure, Temp Sensor II (intake air), Throttle Switch, CO and Idle settings.

----------------------------------------------------------------

This implies that the Lamda is still in the circuit and behaving badly -- lower resistance than it should be. This may give rise to an assumption that disconnecting the Lamda (no resistance) increases the mixture as a default setting. As the DME has to "guess" what it should be it may run richer as running lean could do more damage.

--------------------------------------------------------------

Page 20 - Test Point 16

Ox Sensor Quick Test

Concect CO tester, warm up engine disconnect OX sensor

Disconnect vacuum hose from fuel regulator and plug.

CO increases.

Reconnect OX sensor

CO decreases to 0.6 plus/minus 0.2%

If not check DME unit.


DME Control Unit Test

Reconnect fuel pressure regulator vacuum hose. Disconnect OX sensor. Connect OX sensor connector (control unit side) to ground.

CO should increase.

-if not, check wire from OX sensor to control unit plug terminal 24 before replacing DME control unit.

-if CO did increase, then OX sensor is defective.

-----------------------------------------------------------------------------

This test/failure mode does not account for a disconnected OX sensor.

So I'm stumped but would have to go with the slight enrichment so as not to lean out the engine and sacrafice the cat.

-----------------------------------------------------------------------------
In reading other info I think find that the L-Jetronic injectors fire twice and the Motronic injectors fire just once. Some of the German to English translations seem to meander about and not directly state this.

As to speed sensors: There are two on my 83 stick, three on the 86 auto I'm assimilating. Besides the standard two, one on the starter ring teeth and one sensing a "pin" (set screw) about 20 degrees off (before?) OT, there is a third sensor (on the 86 auto) looking at two pins. One about 7 degrees and the other about 15 degrees off (toward the single pin).

What for?

[SoCal Driver]

Originally posted by FR Wilk

This is a very odd statement. Is this what is commonly believed on how it works?

The early US DMEs have O2 sensors. The early Euro DMEs are the only true Motronics ever made and do not have O2 sensors. How do Euro DMEs meter fuel? Do they guess?

FR Wilk

Guess?

Let me speculate: The Euro cars have no CAT to damage and no EPA or AQMD to watch over it's little polluting children. Four dollar per gallon socialist taxes tend to keep the car ownership and miles driven per year down. So running a little richer "one Tick over" takes care of it.

As to which is "true" or not: That is a judgemental call. What's "true" in Europe is not here and visa versa.

I don't interject emotions "believed" when I discuss mechanical devices. Tends to drag the illogical part of being human around by it's short hairs.

So why the hostility? Can't you reply to questions with your findings? From what you show on your sites you've done quite a bit of discovery. How about sharing it.

[RIchard944]

I read the material that Socal posted over and over again and i don't see anything about making the engine rich to save the engine. Am I missing something? Richard

[Britwrench]

The only thing to be believed in the Porsche DME testing book is the title...it actually tells you nothing; you didn't have that test voltage? well, try and find out what that means!

Someone, (looking at FR Wilk) should come up with a good Motronic test book which gives the correct answers.

Euro Motronics merely have a different map. Due to the lack of oxygen sensor on the non-catalyst cars there is a bridge plugged into the external wire harness. Where legally allowed (!!) the oxygen sensor and cat can be removed and the bridge piece installed. This allows idle co to be set at the AFM.
On non-cat 928S4, there is a seperate poteniometer by the fuse board to set idle CO. I don't think (but don't know either) that euro cars would run significantly richer, rather the fuel/ignition map is set towards higher octane fuels.

Early Euro 964s do not have cats and the idle CO is set at the AFM. Installing a cat and O2 sensor on these vehicles is fairly straightforward.

[RIchard944]

Early Euro 964s ?

[Britwrench]

Yes, before about 1992 Euro, or rather more specifically, British spec 964s did not have catlysts.

964 = 1989-1993 911

[RIchard944]

I don't know the 911. Do they change the number every 5 years?

I think you are right about FR. He has this new tester thingy and Bosch microfilm on his web. It was not there last month. Richard