:whs: School me too.

 

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....

 

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

 

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

 

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.

 

Cool. Following right along.

 

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

 

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?

 

lets see if i can try its like checks and balances. the computer is always checking for a problem just as much as reading inputs and adjusting outputs. it calculates from ie. cps, icp, app, map, what the pulse width should be along with irp duty cycle etc. so it sends a signal to the idm what it wants, the idm executes that signal then relays back to the pcm saying thats done whats next, if there was a problem say the uvc harness unplugged it whould tell the pcm hey i have a problem and the pcm will default and try to adjust to keep the best runability possible due to the circmstances. most of the inputs are accurate unless there is a problem ie icp unplugged generally the engine would die but instead the pcm will set a code then default to i think 700 psi. all in an attempt to keep the engine running long enough to get help like charles said cut the ef wire the engine still runs. it doesnt have to see feedback but feels more "comfortable" when it does. since there is still icp and rpm etc the computer makes every attempt to keep the driver from being stranded. i may have made it worse but that is my understanding of how it works.

 

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?

 

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

 

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.

 

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...

 

3,333.

 

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).....

 

Why is there a highside and a low side? Sorry if i missed this part.

 

im guessing its because in one revolution of hte motor not all 8 cyls will fire, nore in 2 revolutions?

 

4 stroke engine... so 2 revolutions fire all 8 cylinders.

 

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.

 

nevermind im an idiot lol.

so the high side is the power nad the low side is the ground. the idm basically connects the dots to allow the injector to energize and open.

 

Sweet.

 

Clear as a bell really. I knew that logic class in the Air Force would come in handy.

 

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.

 

You are correct, I was confusing the com link with the IDM/Injector.

Thanks for the clarification.

 

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.

 

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

 

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.

 

Makes sense.