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.
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.
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| 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.
Next I did the injector buzz test thinking I wasn’t getting fuel. The buzz test gave me codes P1293 “High Side Open Bank 1” and P1294 “High Side Open Bank 2”, indicating either all of the injectors or the IDM had failed. At this point I needed to get the truck pushed in the shop as I had still been working on it outside as well as get the batteries fully charged. Once in the shop with fully charged batteries I was back to no communication. One important thing to note is that most PCM failures on this engine will cause the “Wait To Start” light to not come on but that never happened in this case. Another common issue with these is an aftermarket chip installed leading to a PCM failure. The PCM is firewall mounted and easily visible from under the dash. I removed the cover over it and checked for any aftermarket chips that would be plugged in the back where there is a factory access cover and there were none.
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| Figure 2 |
With that truck fresh in myI started thinking about any other variables that could cause communication. I try to approach difficult diagnostics by making the main goal to eliminate any possible variables in the testing (things like a broken wire at the connector). I have even at times made lists to help me think through it. So in this case I’m running through all of the things that could cause this truck not to communicate. Since the first tech had no communication with a different scanner I could assume that it wasn’t a scanner issue and it was unlikely that it had anything to do with spread pins at the DLC because that wouldn’t cause a no start. Prior to this I had never actually scoped communication lines so I figured I would give it a shot. I backprobed the DLC and found the bus – stuck at 5v and the bus + stuck at 0v (Figure 2).
On to something
If you look back at the first set of codes I had I noted a code P1670. I went back and looked this code up and it is a communication fault between the PCM and the IDM. Even if I had known this when I pulled the code the first time I probably would have ignored it because communication codes often set due to low batteries and there really is no telling how long that code was stored in memory. It also never reset after being cleared but it is important to mention at this point.
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| Figure 3 |
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| Figure 4 |
Assuming the IDM was causing the bus to be stuck this way I decided to remove it from the bus and see if I could communicate with the PCM again. It is located inside of the driver’s fender well (Figure 3) and is accessible by removing the inner fender liner. With the IDM disconnected I still had 5v and 0v on the same bus wires. I have seen modules lock up but even after disconnecting and reconnecting the PCM and cycling the key I still had no communication. I figured since I had 5v on one bus wire that it was unlikely a circuit issue. It was also unlikely for a rat to chew through the one inch of a 10-foot long wire that I didn’t test so out came the sealed beam headlight as I checked all of the powers and grounds at the PCM (Figure 4). It is easy to forget to check the ignition switch inputs at the PCM during this test as a module will stay “asleep” until it is told to “wake up”. Another thing to keep in mind when module testing is how the modules are told to “wake up”. Some are commanded by other modules and some use power from the ignition switch. This PCM uses the ignition switch feed to “wake up” which tested good along with the powers and grounds so the module should have been functioning. The only other possible cause of this failure that I could come up with would be an issue in the bus wires. I quickly eliminated that as a concern by checking for continuity between the PCM and DLC as well as making sure I had no continuity to ground or B+. At this point I had eliminated any variables I could think of. Let’s recap what I’ve learned so far.
- The IDM could not be shorting the BUS as it still won’t communicate unplugged.
- None of the sensors were shorting the 5V reference causing the PCM not to communicate.
- A blown fuse, bad ground or bad ignition switch couldn’t be the cause.
- The BUS wires couldn’t be broken or shorted to power or ground.
At this point I was comfortable recommending replacing and programming the PCM and then rechecking the IDM.
Are we done yet?
The next day I reconnected the IDM, installed the PCM, and performed the programming. The flash loaded perfectly and I had communication but the engine is still a crank no start. I still had the DLC backprobed so I hooked up the scope and saw good clean opposing square waves on the bus lines (Figure 5). I then tried to perform the injector buzz test and all of a sudden I lost communication! I went back to check the scope and the bus is stuck again at 5v and 0v. The next procedure I performed was a strategic banging of my head on the dash board hoping to shake the gremlins from this diesel nightmare… no luck.
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| Figure 5 |
However I did find some luck that our service writer had first ordered an IDM rather than a PCM and we had one sitting on the “go back” shelf. Exhausted by testing I slammed it in still dangling out of the driver’s fender and wouldn’t you know it the truck starts right up.
I mount the IDM and get ready to test drive the truck and the service engine soon light is on. It had 2 codes.
P0603 PCM KAM test error
P0122 Throttle position sensor circuit.
I clear them assuming the P0603 is from the reprogramming. The P0122 immediately resets. I check Identifix and find the simple testing for the APP on this truck. The APP should have 0.5V at rest and ramp up to 4V at WOT. Testing at the APP connector everything looks good but the scanner PIDs show the voltage starting at almost 0.0 V and ramping up to a just below 0.4V at WOT. I did not think that this was a scanner issue similar to what I had seen with the IAT sensor.
My next step would have been to test the signal voltage at the PCM but since this is the first time I have seen this code and the original PCM was still on my toolbox I decided to swap the old PCM in and see if it will communicate again to check the APP readings. Once back in the truck the engine fires right up with no codes. I check the APP sweep on the scanner and it matched what I saw on the connector.
I still had the new IDM hanging out of the wheel well at this point and when I removed the old one and turned it upside down water came dribbling out of it (Figure 6)! It hadn’t rained recently and we could not identify any area rain water drained into the unit. The customer is certain that the truck had never been in a flood or submerged. The unit is supposed to be RTV sealed but the RTV sealer at the top of the unit had failed.
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| Figure 6 |
We explained to the customer that the PCM had been “locking up” but we believed that it was due to the IDM failure. We explained that we were able to get the truck to run on the old PCM but the customer decided to replace it anyway so we ordered another unit, programmed it and it ran great with no codes. The customer did not want to follow up with the glow plugs and I was more than happy to see him drive this one off the lot.