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Diving into diesel diagnostics

A tale of an ailing F-250 as told by a gas guy. Can you relate?
Friday, December 1, 2017 - 09:00
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I hate working on diesels. A few years ago while working as a diagnostic tech for a mom and pop repair shop I was introduced to a landscaping company’s fleet of not-so-well cared for 6.0 Powerstroke diesel trucks. I had little to no prior diesel experience. These trucks were all banged up and had at least 100k+ miles. Still till this day when I see a white Super Duty on the back of a flatbed I flashback to the horror of seeing one of these trucks being brought in on the hook knowing that my afternoon would be spent scrubbing diesel soot out of my armpits wondering why they couldn’t make that coolant bottle hose a half an inch longer so that it could be easily moved out of the way.

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t this point in my career it has been a while since I have seen any diesel work and frankly, I wasn’t mad after I saw this particular truck get towed in and the work order passed to another tech. But as my luck goes the original tech had a “no communication” issue when he hooked up his scan tool, so it was passed on to me. I normally cringe at the thought of diesel work let alone the term “Powerstroke” but I had recently purchased a truck with a 7.3 engine and I was excited to learn the differences between the 6.0 and the 7.3 as they have completely different reputations.

Initial investigation

I was told that this particular truck was running great and then abruptly died on the road. It would crank but not start and was towed in. I went out to the parking space where the flatbed dumped it and at first had no communication with the PCM or the hybrid electronic cluster. I ignored the hybrid electronic cluster because the cluster in this truck is analog and not listed as a module in the bus diagram. I disconnected the DLC connector, cycled the key a few times, and plugged the DLC connector back in and — voila — communication resumed! The previous technician had been using a different scan tool and it was still a crank no start even with communication so I knew I had some work to do.

The PCM had the following codes stored:

P1280 ICP out of range low

P0237 Turbo boost sensor A circuit low input

P0340 CMP sensor circuit malfunction

P1670 Electronic feedback signal not detected

I started out checking high-pressure oil. The injectors need about 800 PSI to start the engine and this one was reading over 2000 PSI on the scan tool. The fuel injectors in this engine are controlled by an Injector Driver Module (IDM). When energized by the IDM, high-pressure oil from the high-pressure oil pump is sent to the top of an amplifier piston inside the injector that pushes fuel on the bottom of the piston out of the tip of the injector and directly into the combustion chamber at nearly 18,000 PSI. Without oil, this system cannot inject fuel and will not run. Oil level is an often overlooked cause of a “crank — no start” concern by technicians like me who mainly diagnose gas engine no starts. The oil level in this truck was right where it should be.

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Confused about the intermittent no communication I started checking the 5v references KOEO (Key On Engine Off). Modules can fail and lose communication by having one of the 5v reference circuits shorted to ground by a failing sensor. This is common in 4.0 liter Jeeps and my strategy is to disconnect the easiest sensor to access that is on the shared 5v circuit and check for 5v at the connector. If it does have 5v then I check to see if the module is communicating. If it is now communicating chances are I just unplugged the shorted sensor. If there is no 5v reference at the connector I will then go access the wiring diagram, find every sensor on that shared 5v circuit and disconnect each one by one to see if the 5v reference voltage returns. If it does not then I start checking powers and grounds at the module. This can be tricky as the 4.0l Jeep engine has 2 separate 5v reference circuits. If I pull one connector and find 5v but still no communication I will plug it back in and check another sensor on that same 5v circuit to make sure that the first sensor I unplugged is not the one causing the short.

Even though I had communication I decided to quickly check a few sensors and saw a clean 5v at more than one sensor. I figured if it was an intermittent issue with a sensor I might catch it by checking the PIDs for all of the sensors and noticed that the IAT (Intake Air Temperature) sensor was reading -11°F. I unplugged what I thought was the IAT and the reading did not change. After a little research I learned that this truck has two IAT sensors; one located in the airbox and another located in the intake manifold. The one in the manifold is called a manifold temperature sensor. Since that sensor had no effect on my no start so I decided to ignore that value for the time being.

Next I looked into the cam sensor code but my research indicated that a cam sensor failure would be accompanied by no rpm reading. After clearing the codes none returned during cranking including the cam sensor code so I figured that was a dead end.

Figure 1

Next I did a quick check at the glow plug solenoids. This solenoid looks and functions just like a starter solenoid. It gets a command from the PCM and then sends power to each bank of glow plugs on two wires connected to the output terminal of the solenoid. Each glow plug should draw about 20 amps when cold and it is easy to test them right at the solenoid. There are two wires providing power to the glow plugs, one for each bank, so look for a reading near 80 amps on each. In this instance I had only 15 amps on one bank (Figure 1) and almost nothing on the other. At this point it was getting late and both of the batteries in this truck were spent from cranking this engine so I pulled one and threw it on the charger overnight.

Day 2 of the diesel dilemma

I came in the next morning expecting the amperages to change with a fully charged battery but they didn’t. From there I could assume almost all of the glow plugs are bad. Had I seen more amperage on those circuits I could have traced the circuits down and measured the amperage on each individual glow plug to isolate the good and bad ones. Fortunately for me I own a 7.3 and I recently had most of my glow plugs fail and I was able to still get mine started after an extended crank in colder temperatures (it was about 70 degrees this particular day) so I decided to take a different direction in diagnosing this no start, sure I wasn’t yet at the primary cause.

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