Vehicle will not set readiness monitors?!?!

Matt Technician Gastonia, North Carolina Posted   Latest   Edited  
Case Study
2012 Chevrolet Colorado WT 2.9L (9) 4-spd (4L60-E)—1GCCSBF9XC8161051

2012 Chevrolet Colorado With 114,000 miles

Customer came in for inability to complete readiness monitors for state inspection. (Not Ready Rejection) 

Pulled the drive cycle chart to find out how this vehicle should set readiness monitors. 

Test drove vehicle to complete the readiness monitors. On test drive it was noticed that a lack of power above 40% throttle was present. 

Called the shop and let advisor know about running issue, as I was 15 minutes away from the shop. So my advisor called the customer, the customer said the vehicle has no running issues and he hadn't noticed any loss of power. I pulled the PID list that included fuel rail pressure sensor, throttle blade angle, MAF and calculated load. This test showed above ~40% throttle the fuel pressure sensor would drop and raise with a delta of approximately 20 PSI deviation. I wish I would have saved this data however I didn’t. I somewhat ignored the issue knowing the check engine light would come on eventually for some kind of mixture or driveability fault. After 45 minutes of driving all monitors ran except EVAP and CAT. No biggie because I can set either monitor after an 8hr soak. However, when I turned the vehicle off and started it back up a few minutes later, all the readiness monitors reset and showed as INCOMPLETE.

Another series of tests were run which included a battery test, voltage drop testing of battery cables, even as far as voltage dropping ground and power feeds to the ECM. The only thing I know that can cause this to happen is a power loss of a module, or a code clear command. All tests passed to my satisfaction. See Figure 2

A new Engine Control Module (ECM) was ordered, because it was decided that something internal to ECM seemed to be the problem. This is where a lot of other issues surfaced. When attempting to program the new ECM , GM SPS kept setting error code M6413 OFF-BOARD PROGRAMMING DETECTED. So I kept trying to program because I knew the ECM was in the car and on the network. I made some phone calls to my peers to try and figure out why I was unable to program a brand new ECM on a car line I typically have no issues programming. Several guys (Matt Skundrich ,Earl Davis ,& Chad Ball) as well as GM tech suport; said that SPS needs to see 3 modules to start programing. Okay, easy enough the network might be down for some reason. Code scan performed with new ECM installed. I could talk to 12 modules. HMMMMM maybe its not a network issue. I scoped CAN bus and the Class 2 networks , signals looked proper on scope. So I called back into GM, we did some troubleshooting which included unhooking a few known problem modules (Ex. On-Star, radio.....). I tried a different J2534 device, all with no success in resolving the problem. So I called it quits and developed a plan of action moving forward. See Figure 3

On my way home, running thru the car wash with my personal vehicle I called another friend ( Justin Morgan) to have our normal end of day discussion about each others battles. When I proceeded to tell him the details of this vehicle Justin responed by asking what the terminating resistance measurement was on the CAN bus circuit. IT HIT ME LIKE A TON OF BRICKS! I hadn't checked it, I hadn’t even thought about it. I should have checked it but just overlooked it during the heat of battle. So I head back to the shop to run some more testing. During my drive back to the shop Earl rings in and Justin 3 way calls us. When I get back to the shop I powered the vehicle down and proceeded to check resistance of the CAN bus circuit with a result of 40 ohms! AS WE ALL KNOW, SPECIFIED RESISTANCE IS 60 OHMS . See Figure 4 Further investigation of the wiring diagram showed that the ECM is a terminating resistor and the second resistor is external, plugged into the harness at the rear of the truck by the Fuel Pump Control Module (FPCM). I disconnected the external resistor and recheck CAN resistance. I now have 60 ohms...?!!?? How is that possible? We decide to divided the car and find the extra resistance because even though total resistance went down, a short between lines was unlikely.

So with a little math we determine that the vehicle has an extra 120 ohms of resistance. … = 40ohms in parallel) . During our wiring diagram studying, we found the 3rd module on CAN bus, The FUEL PUMP CONTROL MODULE. I missed it previously , so I assumed GM was looking on class 2 for the 3rd module but it is in fact on the CAN bus. As soon as I looked through the spare tire I saw the sign of the "Devil", a DORMAN symbol. The truck has a Dorman FUEL PUMP CONTROL MODULE. See Figure 6 I dropped the spare tire and ran some test on the FPCM , one test was checking resistance of the CAN circuit again. My meter displayed 120 ohms. However the wiring diagram shows an external terminating resistor and the other terminating resistor within the ECM which was disconnected from the car. See figure 7.

After having messed with the FPCM the vehicle will not start now. Earl had stated during our long 3 way phone call that he has seen numerous "odd" issues with Dorman FPCM for these vehicles. We decided I had no choice but to replace the FPCM. As an experiment, I installed an 120 ohm resistor See Figure 8 which brings the network back down to 40 ohms while I programmed the FPCM to see if it would work. It programmed the FPCM perfectly fine. See Figure 9 At this point I have not programmed the new ecm because I now suspected the faulty FPCM COULD be the reason I'm losing memory data after a power cycle. Drive cycle is performed with orginal ECM installed. It runs several monitors leaving 3 incomplete. However, this time with multiple key cycles the ECM is holding the information and monitors are not resetting. This vehicle did not need a ECM after all was said and done. During the drive cycle completion I left the 120 ohm resistor in, thinking the added resistor was somehow causing the loss of memory. My conclusion on the loss of memory is created by defective Dorman programming or hardware . I jokingly said " Dorman couldn’t build this FPCM to pass emission testing So they built in their programming code, that every time the key is cycled from off to on it sends a command to clear the PCM codes" This isn't the case of course. But it did make me feel better after the long battle on this Chevy Colorado.

Final solution was to replace the FPCM, with no ECM replacement, Car is fixed for customer complaint, and also the low power fault that was noted by me on the test drive. 

- Additional files - Scan with original ECM, Scan With new un-programmed ECM, Scope capture of CAN at 60ohms, Scope capture at 40ohms

How could I have prevented calling an ECM?

-Matt …. 

Justin Technical Support Specialist
Fresno, California
Justin Default

Way to stay in the game. Excellent work and I am extremely proud you spent the time after the repair to confirm the changes and effects of different scenarios. Home run and it looks like this will reenforce tons of knowledge you already had.

+3 Default Ð Bounty Awarded
Geoff Diagnostician
Lahaina, Hawaii
Geoff Default

Great stuff Matt. You would think with all the case studies of BAD Dorman electronics products, (here, iATN, and YouTube) that they might "get a clue" and figure out their issues. Aloha from Maui. ps; Think you had a typo, though, in the sentence: "… = 40ohms in series" (In parallel)

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Bob Owner/Technician
East Longmeadow, Massachusetts
Bob Default

I feel your pain on the Dorman issue. I have had a few crazy intermittent failures that were due to newly replaced Dorman electronic components. Watch out for their secondary air injection bypass valves on GM's.

+2 Default Ð Bounty Awarded
Cliff Diagnostician
Santa Maria, California
Cliff Default

Awesome job and way to persevere through a man made problem.... DORMAN= Do Over Man... I know that it is necessary to use the Dorman parts occasionally but it has always baffled me why people will use their modules when OE is readily available. Did the consumer really save money using it in this instance? I too have not had good luck with them,

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Matt Technician
Gastonia, North Carolina
Matt Default

In this instance the DEALER module was 40-50$ less expensive.

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Angel Technician
Boston, Massachusetts
Angel Default

Great case study Matt, thank you for sharing with us. I always thought the same, when it comes to vehicles not passing emissions. Power,grounds, good communication. Great thought process and networking. Did anywhere in service information did it say to expect 40 ohms of resistance? &on the communication diagram, they don’t have the FPCM resistor Drawn in place... unless if I’m missing…

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Matt Technician
Gastonia, North Carolina
Matt Default

40 ohms is wrong. All the service information is correct. The FPCM shouldn't have a 120 ohm resistor.

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Angel Technician
Boston, Massachusetts
Angel Default

Thanks for the clarification. Now it all makes sense. Sorry for the misunderstanding.

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David Educator
Kokomo, Indiana
David Default

Great case study Matt! Thanks for taking the time to share it. Can you clarify that you now have 60 ohms with the replacement FP Module? I ran into an Equinox that would only display 60 ohms with the key off and battery CONNECTED after the bus went to sleep. Battery cable off and I got 120 ohms at DLC pins 6 & 14. This made me assume that one of the modules (I presume the ECM) had an…

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