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Old 09-14-2017, 05:20 PM   #1
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3126E No Start RESOLVED

Hello Folks,
Getting our FL chassis Discovery ready for a trip to find it started but kicked, bucked and idle was ruff. Hit the accelerator pedal and it ran smooth at 1,500 rpm. So, a definite fuel supply issue, I thought! prefilled a new fuel filter installed and same issue. Attempted to start her back up a third time to not even fire this time. Wonderful!!!!

Noting, this engine has had pampered life, 44,000 miles, having oil changes at 7,500 miles along with fuel filter. Stored inside, washed and waxed annually. My heart sunk.

Being an engineer I put my experience to work searching the internet, calling FCC, calling local CAT dealer to find one answer: "Tow it in". What? seriously, I was a little put back that these "support" resources wouldn't take a moment to listen. I should clarify, FCC did listen to my pain but really had no experience with a 3126 to help. CAT, I couldn't get past the "gate keeper" to speak with a guy that probably has some insight.

SOLVED!! I researched the fuel handling system on the internet and how the system functions regarding several sensors feeding back to the ECM.
Not having a code reader to pull any codes, I studied how the system functions and what could cause this condition. (Let me say that this engine has never even farted. It has run like a dream from day one) That said, it may not be your specific issue but well worth testing without spending a dollar.

Cat uses a IAP sensor (injection activation pressure) to monitor the high pressure oil that opens the injectors. The HEUI pump takes engine oil at 60-65 psi and amplifies it to 1,200 psi at starting to 4,000 psi relative to the engine demand/load. The IAP sensor feeds back to the ECM indicating that the pressure is at desired pressure called by the ECM. The IAP is supplied with 5 vdc and feedbacks a linear voltage to the ECM. The IAP is a fancy name for a pressure transducer. IF and when the IAP fails to feedback to the ECM, Fuel is shut off to the injector supply port.

So, I unplugged the IAP. This "fooled" the ECM to think pressure was low. She started up and idled perfect. (((NOTE: I would NOT advise extended running/driving because the ECM will not provide proportioning fuel to the system under high load conditions))) However, this proved the sensor was defective. Went to my local cat dealer, picked up a new IAP sensor and oring.
Took ten minutes to install. Our baby is back to purring. $165.00 w/tax.

Location: The IAP is located on the front left side of the engine and screws into the head just to the right of the HEUI pump. Has a 10" wire harness with a triangular 3 pin connector. Google "3126 sensor locations" for some clear pictures if in doubt.

I hope this helps many that may some day experience this issue.

Travel Safe.
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Old 09-14-2017, 05:33 PM   #2
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GREAT informative post! Thank you George. Rook
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Old 09-14-2017, 05:43 PM   #3
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3126E No Start RESOLVED

I don't have a Cat, but am the Engineering type (Electronics, WAN, LAN). While some probably glazed over at your detailed explanation, I found it quite an interesting read. Good details, troubleshooting, diagnosis, and solution. Good job!
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Old 09-14-2017, 05:57 PM   #4
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Good job on the trouble shooting and repair! You wouldn't believe how many "techs" dive in and just start changing out parts when a rig comes into the shop! I used to just about pull my hair out at times. I would check the logged codes and tell them where to start looking and the next thing I knew, they were coming back from the parts counter with compleatly wrong parts.
Sorry about the rant, am I ever glad to be retired!
Happy Glamping.
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Old 09-14-2017, 06:50 PM   #5
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If you posted your problem originally ; I'm sorry I missed it .
Because I have on file these pdfs that although for a 3126B , probably would have helped you with your problem.
I hope every one reading that has either version B or E , downloads them and has them handy ; JUST in CASE, they are faced with the same problem.
Attached Thumbnails
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File Type: pdf 3126B Heui Engines.pdf (168.7 KB, 153 views)
File Type: pdf 3126B no start.pdf (130.3 KB, 181 views)
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Old 09-14-2017, 08:09 PM   #6
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Thanks Skip. Nice graphics.
I always thought these engines used high pressure fuel to the injectors. I learn something new every day!!!!
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Old 09-14-2017, 09:24 PM   #7
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Quote:
Originally Posted by discovery39s View Post
Thanks Skip. Nice graphics.
I always thought these engines used high pressure fuel to the injectors. I learn something new every day!!!!
I spent 14 years before retirement , doing Dodge/Cummins repairs .
When I bought the coach with the Cat ; I thought " It can't be that different ". Epic wrong!
Over the past 8 years , I've collected a lot of info on this Cat.

BTW: If your coach has the hydraulic pump mounted to the back of the air compressor , the oil pressure sensor is is re-located with a hose "T'd" off the side of the engine at the location shown and mounted up on the intake manifold.
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Old 10-09-2019, 02:39 PM   #8
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Not at all too techie! After trying everything I could think of or find elsewhere on the web, disconnecting the IAP allowed the 3126 to start immediately, but run just a little ragged.

Reconnecting the IAP afterwards, while running, the engine smoothed out and continued to operate normally. Allowing it to run for 20 minutes, it started right back up. 2 hours, $0. Lucky me. Problem Solved.

BIG THANKS out to discovery39s for the solution, and to skip426 for the critical diagram!

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Old 10-10-2019, 06:33 AM   #9
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When removing the IAP sensor, it goes into LIMP MODE.. You can drive a long way in Limp Mode. RPM is limited to 1700 RPM.
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