How the PCM and IDM work - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 170 (permalink) Old 11-04-2008, 02:53 AM Thread Starter
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How the PCM and IDM work

From another thread maybe you guys that have a much better understanding then I do of this can school up the masses on how the pcm and idm work and how they work together? Make me smarter then the PCM is the brain and the IDM is the electric engine to fire the injectors.

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post #2 of 170 (permalink) Old 11-04-2008, 03:04 AM
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Re: How the PCM and IDM work

School me too.

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post #3 of 170 (permalink) Old 11-04-2008, 03:12 AM
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Re: How the PCM and IDM work

Okay, So the IDM is a very smart "dumb" device. It provides a high voltage wire to both banks and a ground wire to each individual injector. The PCM sends the IDM 2 important signals. CI or Cylinder Identification and FDCS (fuel delivery control signal). This tells the injectors when and how long to fire. The signals between the two only have two states High or Low (keeping it simple here). The CI signal remains high for the first four firings and goes low for the second 4 cylinder firings then this all repeats every two crankshaft rotations. The FDCS signal is merely repeated at a higher voltage and given to each injector in the right time. I will have to build an illustration unless someone beats me to it....
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post #4 of 170 (permalink) Old 11-04-2008, 03:31 AM
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Re: How the PCM and IDM work

This is going to be a good thread, if it doesn't get filled with bs.

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post #5 of 170 (permalink) Old 11-04-2008, 03:32 AM Thread Starter
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Re: How the PCM and IDM work

I am looking at my wiring book here and on a quick inspection it looks like there are 3 wires plus a ground that connect the PCM to IDM. Does the IDM send back information to the PCM?

Tom

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post #6 of 170 (permalink) Old 11-04-2008, 03:42 AM
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Re: How the PCM and IDM work

Yes. EF. Electronic feedback. It is a mirror image of FDCS returned with a slight delay.


As Matt already stated, we're talking digital communication. Hence the importance of protocol robustness for stability. Digital, meaning either on, or off. No in between.

CID goes high, then injectors 1, 2, 7, and 3 fire. CID goes low and injectors 4, 5, 6 and 8 fire. CID then goes high and again and the cycle is repeated, over and over again.

And illustration would go a mile where description would only go an inch here.

When an injector "fires" that is simply FDCS going high, the leading edge of which is literally the timing, and the width of the pulse is literally the pulsewidth where the tailing edge is the end of the injection event (electronically). These all together form an endless sequence of squarewaves, one right after another. Each one represents a single injection event.
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post #7 of 170 (permalink) Old 11-04-2008, 03:44 AM
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Re: How the PCM and IDM work

Cool. Following right along.

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post #8 of 170 (permalink) Old 11-04-2008, 03:50 AM
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Re: How the PCM and IDM work

see stuff makes so much more sense when someone takes the time to "dumb it down"

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post #9 of 170 (permalink) Old 11-04-2008, 03:50 AM Thread Starter
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Re: How the PCM and IDM work

OK, so there must be a reason that the IDM needs to send the FDCS (fuel delivery control signal) back to the PCM. That would be one of the first questions I have is why it needs to do that? And taking literally what you wrote the IDM in only sending back the PCM the request that the IDM received not what the IDM actually did?
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post #10 of 170 (permalink) Old 11-04-2008, 03:55 AM
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Re: How the PCM and IDM work

Charles, your explanation grew, thanks. Very clear.

Is the feedback to let the PCM know that everything is OK, don't throw an IDM code?

Early '99 F350 DRW, Excab, LB. Auto. 2WD, 2010 bed, 2005 headlights, 298K, on my way to 1000K.
H2E, 238 Hybrids, Adrenaline oil, Tuned by Tony. DIY Regulated Return.
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