Scopes Help Pinpoint Problem Of Engine Not Running RightPosted 10/15/2004
By Jeff Bach
It's a 2000 3500 series Chevy truck with a 6.5 turbo diesel. "Well, as soon as Jeff gets through playing, we're going to put a pump on it," replies Gary as he's walking up from the other direction.
I hate it when they do that. With my folder full of pin-out charts for the power control module (PCM) in my hands, I'm steadily filling in notes on which signal memory on what scope is connected to which circuit at the PCM. When I get my notes made, I look up at him over my Santa Claus reading glasses from my nest of scope leads and power adapters hooked to my three scopes, a laptop and my Tech II - that are all connected to help me decide what looks out of kilter. "You probably tell the story endings of movies too," I say.
I've been steadily collecting signals and waveforms on this truck for a while - trying to acquire enough data to make a comprehensive, intelligent and confident decision on what to tell this guy he needs to buy to fix his truck. I have a hard time treating these 6.5 turbo diesels with a new pump every time one comes in - like it's a cover charge to get in the door. That's a stiff cover. Yet it seems that the pump is usually what ends up fixing most of them - although I have had some success just replacing the pump-mounted driver (PMD).
This truck belongs to a local salvage yard and is well beyond the 120K warranty period for the pump. It had a problem with a rough idle and surge condition that worsened as it warmed up. I haven't had the best of luck troubleshooting these diesels using the factory information to which I'm privy. I've done a considerable amount of research in an attempt to educate myself on the logic sequence that happens between the injection pump and the PCM to make this thing run. Figure 1 is a current waveform of the fuel solenoid working normally. The engine is warm at idle.
When energized, the fuel solenoid pulls a plunger in to close a valve to hold the fuel inside the high pressure section of the pump during the fuel delivery stroke. When the solenoid is de-energized the valve is allowed to open and dumps the remaining fuel in the high-pressure chamber, ending the injection pulse. While de-energized, the valve remains open to allow the fuel to fill the chamber again during the pump's intake stroke. For the PCM to control the fuel pulse, it needs to know exactly when the solenoid seats and closes the valve. This is accomplished by the PMD, which sends the closure signal to the PCM. By monitoring the voltage level of the solenoid current through a series of resistors connected to the driver circuit for the solenoid, the PMD sees the current rise when the solenoid is energized and watches for it to drop, indicating that the plunger is moving through the coil. The moment the level starts to rise again, the closure signal is sent, indicating that the valve is closed. This pulse in Figure 1 is picture perfect. Notice the clean fall and rise of the current waveform, creating what I like to call "the Seagull Effect." The closure signal occurs right between the wing (see Figure 2).
This signal lasts the duration of the pulse and goes high again as soon as the solenoid current falls off. Take a look at Figure 3 and notice the irregularity of the current in the area where the PMD is trying to see the current and determine when the valve is closed.
Figure 3 shows what the fuel solenoid current waveform looked like while surging at idle.
There appears to be two areas that could indicate where the closure signal should go. You can see in the image on the right of the screen (the shaded area) where the fuel pulse was bouncing back and forth. Figure 4 validates that the PCM was sending the fuel inject signal that way.
The PCM needs to know exactly when the valve seats to be able to time the injection pulse. It controls the timing using a stepper motor mounted on the side of the pump and connected to the high-pressure pump's cam ring. This solenoid's current waveform indicates to me that there is an issue with the solenoid or the valve binding. Either way, it doesn't look like something that can be resolved without replacing the pump.
Figure 5 shows the missing closure signal stacked against the crankshaft signal.
Now I'm convinced the problem is internal to the pump. "You were right, it looks like it needs a pump," I say to Gary - who just looks back and shakes his head.
With the new pump installed and timed, I finish collecting my waveforms.
Figure 6 shows a rock-steady 2000-rpm solenoid current and closure signal.
Figure 7 shows the fuel inject signal from the PCM.
Figure 8 is the cam signal with the fuel solenoid current.
Notice that the PCM times the inject signal so that the closure signal coincides with the cam signal. "The scope says it's fixed," I say to the guys. I vowed to myself that I would never replace another one of these pumps without doing my best to diagnose it first. When I find myself calling a certain engine a "piece of junk" or worse, I can usually relate it to being stung trying to fix one more than once. Such is the case with General Motors' 6.5 liter turbo diesel. I seem to have a tendency to do the insane thing and try the same thing again, expecting different results. At least I did until I got one of those magnetic signs to stick on my toolbox that says, "Make new mistakes." Now I have the "get back on the horse" mentality that keeps me from shying away from the ones that have thrown me. I've heard a few guys say, with a few choice words, they'd never touch one of those again - talking about certain models or certain engines. And maybe that's the smart thing for some guys to do - just work on the cars they know they can fix because they've had good luck with them in the past. Hearing a mechanic say that, however, puts me in mind of hearing a cowboy say, "I can ride any kind of horse except for the brown ones with the white spots."
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