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One apex at a time
 
Dean924s's Avatar
 
Join Date: Dec 2005
Location: MA
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DME question

How does the DME control the fueling of the car? In other words what are the inputs it uses. I know that one of the ways the DME controls the Fuel is by knowing the RPM. BUT how does the DME know the RPM.

Is it by the speed/reference sensors or is it by the tach signal or the ignition signal to the coil?

What I am looking for is a way to give the DME the RPM / motor speed it needs to fuel the car. While elimination the Speed and reference sensors. I am hoping it uses something from the tachometer or the ignition / coil and not the Speed sensor. I am hoping that the speed sensor is only used to create timing and the trigger for the ignition and the fueling is actually controlled by RPM measured off the Ignition or possibly the Tach signal it sends to the dash not directly from the the speed sensor.

Does anyone know?
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Dean

General Car Specs & Mods
Project Megasquirt 924s
Old 11-30-2008, 08:31 PM
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Politically Incorrect
 
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Join Date: Aug 2005
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Reference sensor tells the DME when the engine is at TDC so it can synchronize the spark/injector firing sequence. Speed sensor tells the DME the RPM so it can calculate the next spark/injector firing event. Generally, the timing is advanced as RPM's increase.
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'87 924S
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Old 12-01-2008, 04:00 AM
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It is batch fire ignition so it does not matter where the motor is really especially at speeds over a couple K RPMs. The reference was / is for ignition timing. The only things that affect fuel as I can tell is engine speed and the TPS and the AFM. It is then trimmed by the o2 and the tables in the DME. I don't think the reference has anything to do with it. Further more I am not sure that it actually gets its speed form the speed sensor it may get it from the ignition or the tachometer circuit. that is what I am trying to figure out.

BTW my car is now running on a ford EDIS !!!!

I can kiss those $100 wires and $70 cap and rotors good by!!!!! This just about paid for the swap by the way.

For chuckles!!
http://www.youtube.com/watch?v=ufkbAqUPiAQ
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General Car Specs & Mods
Project Megasquirt 924s
Old 12-01-2008, 11:23 AM
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jpk jpk is offline
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Everything you'd ever want to know about the Bosch Motronic DME can be found here:
http://frwilk.com/944dme/hfm.htm

For what it's worth, the speed sensor is the only source of engine speed info; the DME controls the ignition and provides the tachometer signal based on it. The refrence sensor is key to getting the spark at the right time; the DME has to know where in the cycle it is so it can advance or retard the timing.
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'94 968 Iris Blue
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Old 12-01-2008, 03:58 PM
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A very interesting page. http://home.comcast.net/~russelc/dme/dmetune.htm

It's for a BMW, but it's essentially the same box. I was surprised to find out the AFM signal is also ignored at WOT.
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1987 silver 924S 210k mi! - Rebuilt motor survived 60K so far. Bottom end balanced, Custom mild ported head, Port matched intake, LR 944MAX chip, Thush Welded muffler, Bilstein shocks, MSD Blaster 2 coil, ect ect...Needs 8 more valves!
2003 Hyundai Tiburon GT 2.7 V6 24V DOHC- New toy/backup plan...Borla cat back, Injen intake. Needs headers and a real set of cams.
Old 12-01-2008, 07:56 PM
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I saw that and learned a lot!!! I did some searching around the web last night and found that you can still get the software for free. It is obviously limited / crippled (it can not write to the eprom)

Thee is a lot of good information there. I emaild the guy that posted it. I hope he gets back to me as he looks like he could be a wealth if info.
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Old 12-01-2008, 08:24 PM
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Quote:
Originally Posted by HondaDustR View Post
A very interesting page. http://home.comcast.net/~russelc/dme/dmetune.htm

It's for a BMW, but it's essentially the same box. I was surprised to find out the AFM signal is also ignored at WOT.
it makes sense though. on my standalone for my honda, the ECU ignores the o2 sensor since the feedback loop isn't quick enough to respond. by the time the signal loop response comes back, the engine isn't at the same rpm range anymore. it just goes off the "hopefully" tuned maps that's in memory.
Old 12-02-2008, 07:31 AM
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Yes I knew for sure it ignores the O2 sensor, but it's interesting that it also ignores the air flow meter. It just goes by rpms and just knows how much air the motor is capable of pulling. Only problem with that is if the flow capacity is increased, like from exhaust, head porting, etc it probably won't compensate and would need to be remapped at WOT. It's also very interesting since I've been hunting down a fluctuating power delivery problem forever. It pulls smooth as glass at WOT, so now that finally pinpoints the AFM as the likely (previously unlikely) problem, since everything else would affect it even at WOT if they were screwed up.

I guess time to open the AFM for the 3rd time. Anyone know how to build a MAF piggyback converter by any chance?
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1987 silver 924S 210k mi! - Rebuilt motor survived 60K so far. Bottom end balanced, Custom mild ported head, Port matched intake, LR 944MAX chip, Thush Welded muffler, Bilstein shocks, MSD Blaster 2 coil, ect ect...Needs 8 more valves!
2003 Hyundai Tiburon GT 2.7 V6 24V DOHC- New toy/backup plan...Borla cat back, Injen intake. Needs headers and a real set of cams.

Last edited by HondaDustR; 12-02-2008 at 08:09 AM..
Old 12-02-2008, 08:07 AM
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