Know How It Works/Know How It Fails
Before you run off because you don't work on this stuff, please give it a read through as I believe the lesson is valuable to all of us in the field (I know I apply it to all aspects of diagnostics; including the occasional ice cream machine I get called for) though the troubleshooting process and overall point is applicable to all of us I believe.
Quick and dirty case study for a John Deere skid steer, 320E (1T0320EBCGJ304347).
- Low power - throttle won't go above 50%
- DPF clogged/regeneration won't occur
- DTC's flashing on monitor
- ECU … - DOC (diesel oxidation catalyst) Inlet Temperature Extremely High - (The DOC inlet temperature sensor signal voltage to the ECU corresponds to a temperature greater than what is physically possible for the DOC inlet temperature sensor.)
- ECU … - DOC Inlet Temperature Signal Out Of Range High
- ECU … - Exhaust Filter Cleaning Inhibited By Operator Interface
One of our technicians had headed out to take a look and so he gathered all this information. He also pulled what is a handy piece of information to have access to, at least in the off-road equipment world. It's called an Engine Load Profile
This chart lays out engine load on the y-axis and engine rpm on the x-axis. The numbers are accumulated hours run at each intersection. It's invaluable when it comes to diagnosing frequent DPF clogging and other issues. In this case it's secondary information that may or may not end up being useful.
Technician then proceeded to perform a live data recording of the machine, which I'll attach some screenshots of. He chose to pull everything on as he is not real comfortable with diagnosing electrical/emissions faults. Instead he calls me, which is great because I enjoy a challenge, especially a remote one.
One of the cool things about the John Deere Software is that you can monitor all modules simultaneously. You can also save your recording out in the field and have someone else access it instantly remotely (assuming you have internet connection) Here is a screenshot of the initial data recording. Every ECU parameter, as well as some EMU (monitor) data pids. Came out to about 65-70 data PIDs I believe.
Fortunately we already have a good direction as all the codes make sense together. DOC Inlet Temperature Extremely High will cause the monitor to inhibit any exhaust filter cleaning (though service information does not tell you that, leads one to believe the operator is hitting the inhibit switch). That means our focus is now on one failure point, the DOC inlet temp sensor, and it's two failure mode indicators (extremely high temperature and signal voltage out of range high). I proceeded to narrow my data pids down to just the bare essentials.
Right away I noticed that the DOC inlet temperature reading was at 662 F (default max value as I found out) @ 1.7V, yet the outlet temperature was 419 F @ 1V . These thermo-couples run on 5V and generally have a much larger spread between temperature/voltage correlation. Now it was time to let the data talk.
Notice the variation in the inlet temp input voltage but not change in the temperature PID. I knew we had it narrowed down but was it simply a failed sensor or was it wiring related. This particular machine has differently keyed pigtails for the inlet and outlet temperature sensors (they are close enough to each other to swap the connectors easily to see if the problem moves but the newer models have them keyed differently so no mix ups occur) so that route was eliminated.
Then I realized that to have an intermittent short/open there was more likely a fault within the temperature sensor itself. In this case my thought was that the internal element was broken and making intermittent contact with the sensor body causing the rapid fluctuations in voltage with no change in temperature (ECU determined the changes were too rapid to be rational and so substituted maximum temperature value).
I called and had the tech tap lightly on the sensor body with a screwdriver and let me know what the data PID changed to. After two quick taps, it remained at 4.95V. Element had finally broken.
Edit: As Anthony pointed out below in the comments, a verification test would be to unplug the sensor, look for a change in data PID, then using a bulb style test lamp or a resistor pull the signal to ground in order to confirm wiring integrity and module operation. No sense in short cutting it, this would be a quick easy test to confirm what our data tells us.
A new sensor was ordered and installed the next day under warranty, machine was run through service regen successfully and put back into service.
I believe it's a useful case study for anyone as it illustrates the power of data recordings, as well as how having knowledge of a given components operation can often times lead you to a diagnosis (or throw out a diagnosis) based on the likelihood or ability to fail in a given way. This time it allowed me to funnel our direction very quickly with no disassembly and the whole matter was resolved in about 15 minutes.
My guiding principle in diagnostics is stolen from the great Sherlock Holmes "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
For my part, I enjoyed reading this. The bulk of my work is automotive but I have some customers with off highway equipment as welI. I too enjoy a good diagnostic challenge, and have always gravitated towards that facet of the business above all. Thanks for sharing this and way to use your logic to find the root cause quickly. That's really cool that John Deere's software gives you the ability
Zach, I appreciate the good words. I have been involved in various aspects of repair, from heavy duty with trash trucks and fire trucks down to small engines and chainsaws that you plug a laptop into(working 3 jobs will do that). I've found that principles apply across all these fields and are useful in seemingly unrelated incidents. Most of us would probably be amazed at just how successful we
Couldn't agree with you more Chris. I'm a firm believer that if I can understand how the system works, I can figure out why it doesn't. I've always found working on a wide variety was more interesting, and it frustrates me mightily when important information is omitted or withheld from those of us in the trenches. I've been working on a project for one of my customers over the last several
That's what we are all here for. Help each other learn and grow. Should we expect a write up on it in the heavy duty section? I know we need more discussion in that section and I am certainly not the person to be the heavy duty poster child.
That's a good question. I suppose I could though I'm not sure it's worth anybody's time other than for amusement. It's all been fairly straightforward. The biggest issues I've had were locating parts and correct repair information. I will say that the 4BT Cummins in this sucker fired right up once it had been drained of all the water and had fresh fluids in it. Certainly doesn't hurt that it's a
Hi Chris: I appreciate where your head was at but what if the sensor wasn't accessible? You could flat rate it by disconnecting the sensor wiring and look at your PID. The downside is that an OEM may decide to substitute a voltage value (instead of just the quantity) in a default situation. If you have a bulb style test lamp, you could substitute that for the sensor and check your PID reading…
Anthony, I appreciate you bringing that up as I do feel that I made it seem like this was the most valid way to do it. I have additional checks I would do, but I was not onsite for this one. More of a mental exercise. Knowing the system, I knew that the voltage value is never substituted. I also knew where the sensors lived and so all that made this process the primary path. That's no excuse
Hi Chris: As I mentioned, I understood where your head was at. One of the difficulties with a medium such as this is that others also view what is written. While a response may be written to someone's post, it it often written for the forum. On ATN, many of my responses would be private, so as not to come across as trying to upstage the poster but still get my point across. If it dealt with
Anthony, Could you explain how a bulb style test light can be used in place of a diesel EGT sensor & what data would one expect to see?
Glen, A bulb test light is used as a resistor to pull a circuit to ground, or as a power limiting device to power a circuit, or to limit the voltage on the circuit.
Hi Glen: Albin pretty much explained it. What wasn't said, but he understands, is that the circuit is complete. If you measure the circuit with the component disconnected, you are measuring open circuit voltage (OCV). While there may be some obtuse diagnostic value in that, there usually isn't for your purposes. HTH, Guido
Great write up and diag approach!!! Just because you were working on a John Deere, or even an Oshkosh truck, the principle is all the same. Most training would be better if the make, year and model were omitted from the case study. Its way too easy to start thinking about pattern failures and loos track of the theory of operation.
Hello Chris, Any Heavy Duty / off road input is welcomed here. Thank you
Jaxon, I dont know if you saw it, but I added some info to one of your previous questions about Komatsu a few days back. Hopefully it helps. I hate not knowing how things shook out.
Nice job Chris. Keep the case studies coming. I try to share when I can as well. Hhopefully by the end of this week I will have a couple more up. Just out of curiosity how did you get access to the service advisor software? I hear John Deere makes it very hard to obtain.
Thank you Mike. I work for a John Dealer during the day so I have access to the software. Not always the most challenging job but it pays the bills most days. Automotive diagnostic work takes up my nights and weekends. That's my preferred area, though my OEM tooling for that is severely lacking currently. There is a bit more information on what you can/can't from JD in another thread I had
People unfamiliar with or simply ignorant about how a diagnostic process requires a logical thought pattern, service information and tooling can benefit greatly by the services of a good automotive diagnostic tech. I successfully fixed a neighbor's home HVAC a few months ago after using the skills I've honed in the automotive field. A professional HVAC tech had told the homeowner to replace