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Discussion Starter #1
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.

Tom
 

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:whs: School me too.
 

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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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Discussion Starter #5
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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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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Cool. Following right along.
 

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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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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?
 

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But you get a CEL, correct? ANd the code says to check for IDM codes. And of course the high is > 100 volts and the low is ground.
 

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Correct.

Now go snip the EF wire. Oops, truck still runs, lol.
Pretty sure I've seen that wire cut before.....;)

And good explanations, it's helping put together all the bits and pieces I've picked up from you Charles.

Dave
 

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But you get a CEL, correct? ANd the code says to check for IDM codes. And of course the high is > 100 volts and the low is ground.

No, these signals are 12v. Only the output is 100+. And it's not a squarewave output.
 

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Okay here is a very crude diagram of what is going on.... who can tell me what RPM this depicts? Also since the IDM can only accept a maximum duty cycle of 80% this is how the 3.6ms was derived....

The injector output doesn't look this clean but gives the general idea of what's happening. This is the firing sequence and not the firing order depicted. I thought this might be easier to understand this way...
 

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High side is the "ON" state, the low side is the "Off" state, or can be inverted depending on how a circuit is designed. Low does not mean ground (although it could in some circuits).

The high and low values are circuit specific and can be anything. If you are talking about the voltages on a PCB itself typically high's and lows are in the +5vdc to -5vdc range.

The refernce to high and low is not specific to the IDM, it's used everywhere in electronics.
 

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Correct!

So from this illustration you have a total time to dedicate to an injector of 4.5ms but since the IDM can only accept a 80% duty cycle we have a maximum commanded injector PW of 3.6ms.....

Somebody do the math for 4000 rpms and show your work for complete credit (sorry guys... got harped on about this in school because i hate showing my work).....
 
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