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How the PCM and IDM work

53K views 169 replies 45 participants last post by  F250Hoss 
#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
 
#27 ·
Like Patrick said, in terms of electronics, "high" and "low" simply denote one of the two possible states. You could just as easily call it on, and off. 1 and 0. Energized, de-energized, so on and so forth.

In this case it's a 12v output that sits on the line. If the PCM doesn't act it remains 12v+. The PCM then yanks them to ground. It then releases the grounds and the lines jump back to 12v. For example for FDCS it leaves the wire in the high state for the duration of the pw, then pulls it back to ground perfectly defining a "Square". Up, to the right, and back down. A series of this forms the FDCS signal.

It is a pair of modulated grounds by the PCM that forms this digital signaling.


Now I think the PCM/IDM com link is getting confused with the IDM/Injector circuits.

On the output side, there are two high side wires. One per bank. Now when someone says high side, they mean positive (using conventional current notation). It takes both a positive and a negative for electron flow, so the injector needs two wires to fire. The high side is common to all 4 injectors on each bank. It's the center wire in the UVC. This is your 120+volt wire. The next 2 wires to each side are your injector wires (in order of appearance) for that bank. The next two on each side outside that are the GP wires.

For instance. To fire injector number 1, the IDM goes high on bank one (center wire on your passenger's side UVC) and also grounds the #1 injector wire (second wire to the front from the center wire on your passenger's side UVC). The circuit is complete, and #1 fires.

The leading edge of the FDCS into the IDM determined when this would happen, and the width of that wave determined how long it would last. To fire number 3 you would do the same thing except ground the #3 injector wire instead of the number one. This would be the adjacent wire, just forward of the center wire in the passenger's side UVC.

That is the output side of the IDM. High voltage.


Might as well explain IPR while we're at it.

The IPR duty cycle (I'm rusty on this, it's been a while) is like 440hz? I think it is, so lets just pretend for now that we know that it is.

That means that 440 times per second, (that's what "Hz" means...per second) a square wave of variable duration is sent down the line. Meaning about every 2.3ms a command is given. That's the frequency. The timeframe for this pulse is also 2.3ms long. So at 100% duty cycle, the pulse would go high, and remain high for 2.3ms. As you might have guessed, that would mean that at the time it would normally go back low, it's time for the next pulse (remember it's a constant frequency) so in reality, it never actually would turn back off, and instead would simply remain high. The result would be full power output with no modulation.

In contrast a 0% duty cycle would mean that when it came time for each pulse (440 times every second) nothing would happen. Because it's 0%. It would never turn on. Meaning that the output would be 0 volts. Nothing. 440 times per second you would get nothing.

Now everything in between 0 and 100% DC is a proportion of available on time, vs off time.

For instance. Take a 50% DC. That means it's on 50% of the time and off 50% of the time. So out of our 2.3ms pulse window, it would go high at the start (every 2.3ms it does this, 440 times a second) and stay high for 1.15ms. It would then go low for the remaining 1.15ms of the possible 2.3 before the next pulse. At which point it would again go high for 1.15ms, then go low for the remaining 1.15 again. Over and over again, 440 times each second until the DC was changed.

A 25% DC would be an on time of .575ms and an off time of 1.725ms. 75% DC would be an on time of 1.725ms and an off time of .575ms, exactly the opposite of the 25% DC.

I hope that makes sense.
 
#28 ·
You are correct, I was confusing the com link with the IDM/Injector.

Thanks for the clarification.
 
#29 ·
It cut me off at the end. But here's the rest.




Oh, and I should mention. This type of signal is called Pulse Width Modulation, or PWM. With an appropriately high frequency, a device will never actually detect the fluctuations, and the resultant output might as well be an analog voltage proportionate to the average of the PWM. This is no different than as humans, watching a movie. Even though it's really still images taken one photograph at a time, when played fast enough it looks like a seemless display. Our minds cannot detect the individual frames.
 
#30 ·
What is the part labled CI on your diagram Matt? I understand that is the second rotation of the motor just not what CI means?

I have a question for Charles in why does it take a pair of modulated grounds instead of just a single one?

You guys are doing a good job of explaining this.

Tom
 
#31 ·
The CI or (cylinder identification) signal helps keep everything in sync.... ever wonder why a Powerstroke always seems to take a few revolutions to start no matter how warm it is compared to an IDI? The signals need time and revolutions to figure out when to fire the first injector for the first time.

Also the IDM isn't dumb at all, it is a computer that monitors things like current draw by the injectors and other factors. If for some reason the FDCS "freaks out" then the CI signal would allow for a faster re-sync since it can figure out where it is every time the crank rotates or every 4 injector firings.
 
#34 ·
Two PWM grounds because you have two wires Tom.

CID and FDCS.
And to add to what Charles says is that in digital terms the PCM "sinks" voltage to ground to drive a "0" and if you were to disconnect the FDCS and CI wires to the IDM, the IDM inputs would constantly be at a logic level of "1" since their inputs incorporate a "pull-up" resistor. That doesn't mean anything to most and isn't really important to this discussion.
 
#35 ·
Since the other thread was closed, lets continue this here.

Or better yet, I, and I'm sure many of us, would love to hear any one of you explain FDCS and CID, how they relate, and just EXACTLY what they are doing at 4200rpm with 3.5ms of pw on the line.
Since you posed the question and are sticking to your guns on what you believe is happening, let me throw a monkey wrench into this. Please understand that I'm NOT saying that this IS happening, just asking why it couldn't be (since apparently this type of functionality is used in other fuel injection systems).

WHAT IF...the PCM and the IDM use an algorithm to keep a pre-defined "minimum gap" between cylinders in the FDCS signal, an algorithm that takes RPM into account? What if the a 3ms signal at 1000rpm is really 3ms of PW (1:1 ratio), but at 4000rpm there is a 1.2:1 ratio and a 2.5ms FDCS calculates out to a 3ms injection PW? (just BS numbers I threw together for example purposes)

You asked how it would be possible for the FDCS signal to represent 3.5ms at 4500rpm without being a flatline...the above is a simple mathematical representation of how. I don't know that the algorithm would be as simple as a basic multiplier, I would think that it would take into account RPM and might be more complicated in nature...but if both the PCM and the IDM use the same one and both have a good RPM signal, no reason it couldn't happen.
Sorry. You encroach on the IDM buffer and it won't play anymore. There is an algorithm that controls the PCM's own buffer that is there to maintain the signal robustness. However, it has nothing to do with the physical limitations of the IDM. If you remove this you simply jump from there to the physical choke point of the IDM. You don't need a truck to see that. Just set one on a table and keep feeding it longer and longer pw values until it chokes.'' Has nothing to do with the PCM.

And I'm not "sticking to my guns". This is just how it is. If I told you 2 + 2 = 4 then I would be "sticking to my guns" just the same.
If you test on a bench, without a truck and a PCM, how is the test valid? You don't have any load on the IDM (injectors) and you don't have the PCM side of the equation. You find the limits of the IDM when it's fed a raw PW signal without any kind of safety or algorithm the PCM might have built into the communication protocol. Your "bench" setup is not electrically the same on either the input or output side...while interesting it's not representative of what is happening in an actual truck.

If you can't make the PCM fuel passed ####rpm on the bench, you can't test passed ####rpm on the bench with a PCM...so you have to do with with a stand-along controller. Doing it without the PCM eliminates a significant part of the equation. Jody has found specific changes in the PCM that have allowed him to push the fueling passed ####rpm and accomplish what he did with Kevins truck. He is going to try to get with Kevin and scope what is ACTUALLY happening AT THE INJECTORS (where it really counts). Scoping the FDCS signal doesn't tell us anything unless we figure out for sure that there are no other algorithms in play.
 
#36 ·
Dennis the FDCS is the final verdict from the PCM and we know what the maximum duty cycle the IDM will accept before choking (with any controller generating FDCS). Even if you could command more than 80 percent of total available pulsewidth (100% is a straight line) from the IDM it wouldn't work because the idm starts sputtering and cutting out.
 
#37 ·
But what I'm suggesting is that the PCM never sends a straight line to the IDM. I'm suggesting a possible way for there to be a gap between cylinders on the FDCS signal and have it still represent more PW than the width of the actual signal.

There ARE protections in the PCM that prevent exceeding a certain RPM. Jody watched the PW on the scantool get pulled back to almost nothing...so the PCM WAS pulling the PW prior to the safety point being reached. After changing the necessary parameters (you'd be surprised as how many there are that relate to this) in the programming, the popping went away and the PW didn't dip on the scantool. How do you explain that?
 
#38 ·
so that would be a "hidden" part of the protocol in the IDM that multiplies FDCS pulsewidth by some RPM factor and deliveres a bigger pulsewidth to the injectors than what FDCS dictates? That would be cool if it happened that way, and lets say it does for the sake of this post......

Say the typical injector has a .5ms lag time and you are trying to spray in the bowl at 4000 rpms with stock pistons and spray angle. How many crank degrees are you spraying fuel when you are commanding 3ms of pulsewidth? That would also mean that SOI would need to hit the bowl 24 degrees BTDC.
 
#39 ·
I'm suggesting a possible way for there to be a gap between cylinders on the FDCS signal and have it still represent more PW than the width of the actual signal.


Am I the only one that feels dumber after reading that?


FDCS is one wire according my diagrams. I only wish we could see more than a straight line at 4200 RPMs. BBEEEEEEEEEEEEEEEEEEEEPPPPP!
 
#40 ·
I'm NOT saying that there is additional data going across the wire between the PCM and IDM, I'm suggesting that since both the PCM and IDM know what the RPM is, they can both make the same adjustment in how the FDCS signal is created and interpreted at the same "trigger RPM" (if such a thing exists). Again, this is all speculation.

Lets not take this down the road of what is happening in the cylinder yet, not until we know what is actually happening at the injector electrically with Jodys new programming. He's working with Kevin to get that info. Lets solve one puzzle at a time.
 
#41 ·
Am I the only one that feels dumber after reading that?


FDCS is one wire according my diagrams. I only wish we could see more than a straight line at 4200 RPMs. BBEEEEEEEEEEEEEEEEEEEEPPPPP!
You took that one line out of context. If after a certain RPM there is a multiplier in effect, you could have an FDCS signal that is 2.5ms wide but after being run through the algorithm represents 3ms. All that is needed is for both devices to know what the RPM is and when to apply the multiplier.
 
#43 ·
Am I the only one that feels dumber after reading that?


FDCS is one wire according my diagrams. I only wish we could see more than a straight line at 4200 RPMs. BBEEEEEEEEEEEEEEEEEEEEPPPPP!
Are you saying there is no way fuel is injected at or past 4200?
 
#44 ·
a flat line would not represent no fuel, it would piss of the idm. If you go read the explanations earlier in this thread, which is why it was created, youll see how the injector firings are communicated from the pcm to the idm. the signal is carried down the fdcs wire to the idm with an cyl identifier, to tell the idm which injector to fire, and then hte pw to fire it at. The pw is told to the idm by the length of hte pulse. Now say as the rpms go up, and suppose your pw stays the same, these pulses will be getting closer to eachother as they will be coming down the line closer to eachother. At some point, the pulses will be back to back and appear as a straight line, no pulses. At this point, how is the idm to determine the pulsewidth of any injector, as it will think the pcm is just saying to turn an injector on and never back off, as there are no breaks in the pulse.
 
#45 ·
If Dennis' theory is plausable, what would determine pulsewidth then?

Are you saying the idm would have some preset value in it, so once it sees a "flat line" on the fdcs cable, it would just implement certain pw based purely on rpm?
I'm suggesting that there "could be" a calculation that takes place on both sides (IDM and PCM) once a certain trigger rpm is achieved. Any RPM signal into both the IDM and PCM after say 3000rpm (round number for simplicity sake) causes the FDCS signal to no longer be 1:1 (desired PW : FDCS width). Again for simplicity sake, suppose that after the mythical trigger RPM the ratio becomes 1.2:1 (PW = FDCS x 1.2). FDCS signals never "touch and become flatline", instead a 2.5ms wide FDCS signal is calculated (x 1.2) out to be 3.0ms of desired PW.

Like I said, I'm positive that it would be a more complicated formula, but this the simple math version of how it would be possible.

In theory, if the formula took into account RPM, there could be multiple steps. Maybe there is one ratio for 3000+ and another change at 3500+ and another at 4000+. Maybe it doesn't exist at all...I'm just trying to show that there IS a way that the IDM could get a request for a longer PW than the FDCS signal represents on a scope. Since both know what the RPM is, having a simple program that watches RPM and makes a change at a certain point is totally feasible.
 
#46 ·
Hmm Ok I see what your saying. At what rpm would you not be able to inject any fuel. If you go down to .5ms, how many rpms would it go to before it would flat line?
 
#47 ·
Since both know what the RPM is, having a simple program that watches RPM and makes a change at a certain point is totally feasible.


From the way I understand, the PCM is the only one that "knows" RPM. It uses the CMP signal to generate CID and FDCS that go to IDM. CID is just the signal to the IDM of which injector to fire and the FDCS dictates how long. Does the IDM take CID and derive RPM??



Holy crap....my head hurts. I've got some diagrams at home that will help some I think....they have arrows and stuff to show direction of communication. I'll try to scan them tonight if I'm not sitting on a combine.
 
#48 ·
Let's not turn this thread into $hit with name calling and what not.
Dennis, I was making the point that what good does having megapulsewidth do if you are way past your mechanical window. If you are commanding 3ms of pulsewidth at 4000 rpms and getting 2.5ms of actual injection time from that you are injecting for (360*4000/60/1000=24 degrees per ms). You would be injecting for 60 degrees of crank rotation.... How much more pulsewidth than this is useful anyway?
 
#49 ·
How many degrees of rotation between TDC and the exhaust valve opening?




If no one cares to share the info I will document everything when I throw on the degree wheel during assembly of my engine this next week.
 
#50 ·
From the way I understand, the PCM is the only one that "knows" RPM. It uses the CMP signal to generate CID and FDCS that go to IDM. CID is just the signal to the IDM of which injector to fire and the FDCS dictates how long. Does the IDM take CID and derive RPM??



Holy crap....my head hurts. I've got some diagrams at home that will help some I think....they have arrows and stuff to show direction of communication. I'll try to scan them tonight if I'm not sitting on a combine.
There is enough logic in Dennis's theory and the IDM to accomplish this although I will let the results of the O-scope speak for itself. You could simply put a time multiplier in the IDM's logic based on start of consecutive pulses over time thus calculating rpm iside the IDM to create this logic.
 
#51 ·
From the way I understand, the PCM is the only one that "knows" RPM. It uses the CMP signal to generate CID and FDCS that go to IDM. CID is just the signal to the IDM of which injector to fire and the FDCS dictates how long. Does the IDM take CID and derive RPM??



Holy crap....my head hurts. I've got some diagrams at home that will help some I think....they have arrows and stuff to show direction of communication. I'll try to scan them tonight if I'm not sitting on a combine.
RPM can be determined by either the CID or the FDCS signal. The CID has a pulse frequency of one-half of RPM (it cycles exactly once every other rev), and FDCS has a frequency of four times RPM (4 injection events per rev).

Regardless, the PCM doesn't have to be in the equation to verify this "multiplier" theory. If you use ANY controller that gives a valid FDCS and CID signal, the IDM will interpret it accordingly. It has no idea that the device controlling it is not the factory PCM. The Ecolizer guys proved that there is nothing but a 1:1 ratio across the entire RPM band.
 
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