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Discussion Starter · #1 ·
2018 GMC Denali 2500 L5P 86k miles bone stock not under warranty due to a front collision voiding warranty. Everything properly replaced, headlights, front bumper, fan, grille hood, coolers.

Problem: Truck runs and drives smooth, everything works, except for a message saying service emission system speed limited to 158mph.. weird because thats a high speed.. then it has a check engine light on and the only fault code set is U18A2. "Lost communication with fuel pump driver module"

Swapped out the module which sits above the spare wheel in the rear and the problem did not change..

Took the truck to Chevrolet store had their diesel guy look at it for 3 hours, he concluded that the Connector that plugs into Fuel Driver module has three separate circuits that are supposed to supply 12v to it but only 2 are actually battery voltage while the third one is grounded out. Said he looked at a wiring diagram and found the wire leads from the fuel pump driver module thru a couple of the nox sensors and finally thru the turbocharger vane position sensor and then to the fuse box under the hood where it gets fed by a relay..

He said he unlatched the fuse box and looked underneath and found the wires on connector X104 at location K5 and K6 as you can see in the attached wiring diagram... he said when he applied power to them the circuit tested positive at the fuel pump module and concluded that my fuse box was not the right one and not supplying power to the circuit.

I pick up the truck from them, look at prodemand.com wiring diagram and find the circuit he is referring to. A reductant control module relay 2 is supposed to feed a 10amp SCRPM / SEN ENB fuse and from there its supposed to send 12v into K5 and K6 pins of the connector which go to the nox sensors, fuel pump control module and the turbo vane sensor..

Next I double check every single fuse and relay in my box and find all to be good, I unlatch the box and look underneath it and notice that my fuse box doesnt have any pins to slide into k5 and k6 on the connector below it and close the circuit.. its like the pins are simply not there.. (circled in the photo attached).. I call chevrolet give them the VIN number and they confirm that the part number fuse box I have is indeed the correct one to this truck, I also doubt that it has ever been replaced.

So, why would a truck have a wiring diagram showing these wires and relays, actually have those wires in the connectors below the fuse box, those wires actually run to the back of the truck to the module I am experiencing problems with, however the fuse box doesnt seem to contain these two relays and has no pins to send power to this circuit.. What am I missing here? If the wiring diagram was wrong why are the wires there.. If the fuse box is incorrect why is the part number on it the one that 2 different dealerships say my VIN calls for? (part# 84075774)

FYI, I do not believe this truck has ever had its harness and or fuse box replaced and the front collision did not affect the fuse box or the wiring around it.

If there is anyone on here knowledgeable with GM truck electrical circuits or has any idea how to make sense of this, I will gladly compensate for your time or buy you drinks / dinner.

Thanks in advance.
 

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Did you buy it after the accident. If so, it's possible the person that fixed the truck could have used a fuse box that came out a gas truck.

The reductant module is the one in the def tank.

According to this site putting in that number it fits 2015-2016.

 

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Discussion Starter · #3 ·
Yes i bought it after the accident. However if it was a gas fuse box why is chevrolet telling me its the correct one?
 

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Download the wiring diagrams for your truck here https://www.gmupfitter.com/light-duty-vehicles/light-duty-2018/ Click on your truck description then find the ELECTRICAL MANUAL. Search the PDF file for K111 which is the fuel pump driver for your L5P; the manual also covers other RPO's, so make sure to look for the correct one. From what I see on a brief look, your truck must be getting power to the fuel pump driver because it is on the same circuit as other devices that are being powered. If they weren't being powered, you'd have many other problems.

56 SCRPM/SEN ENB F56UA 10A• K115 Reductant Control Module (L5P)
• B195A Nitrogen Oxides Sensor 1 (L5P)
• M103 Turbocharger Vane Position Actuator (L5P)
• B195B Nitrogen Oxides Sensor 2 (L5P)
• K111 Fuel Pump Driver Control Module (L5P)
• B136 Exhaust Particulate Matter Sensor (L5P)

The code is for the communications to the driver, not for the power/ground to the driver. The manual will have the pinout for the driver and note the color for the wire communications pins. Look for GMLAN. Communications problems can be difficult to troubleshoot without a scope. See attached for possible places to look for the problem.

As for your questions about WHY, I have no idea. I note that the part number you quoted is a superceed part an update from the original part number.
 

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Discussion Starter · #5 ·
Download the wiring diagrams for your truck here https://www.gmupfitter.com/light-duty-vehicles/light-duty-2018/ Click on your truck description then find the ELECTRICAL MANUAL. Search the PDF file for K111 which is the fuel pump driver for your L5P; the manual also covers other RPO's, so make sure to look for the correct one. From what I see on a brief look, your truck must be getting power to the fuel pump driver because it is on the same circuit as other devices that are being powered. If they weren't being powered, you'd have many other problems.

56SCRPM/SEN ENBF56UA10A• K115 Reductant Control Module (L5P)
• B195A Nitrogen Oxides Sensor 1 (L5P)
• M103 Turbocharger Vane Position Actuator (L5P)
• B195B Nitrogen Oxides Sensor 2 (L5P)
• K111 Fuel Pump Driver Control Module (L5P)
• B136 Exhaust Particulate Matter Sensor (L5P)

The code is for the communications to the driver, not for the power/ground to the driver. The manual will have the pinout for the driver and note the color for the wire communications pins. Look for GMLAN. Communications problems can be difficult to troubleshoot without a scope. See attached for possible places to look for the problem.

As for your questions about WHY, I have no idea. I note that the part number you quoted is a superceed part an update from the original part number.
Thank you for your reply Ron.

I also was under the same impression that I would have a lot more trouble codes or symptoms.. However, I do have a problem with my emissions which emissions are controlled by the reductant control module and the nox sensors..

Now here is the main reason why I believe this circuit should have 12v. I used my diagnostic tool and went into active test and tried to move the turbo vane actuator and it says error DTC present.. I clear the code for the fuel pump and still cannot move the actuator.. So I manually applied power to the K6 pin on the fuse box and tried to actuate it again and now it moves and works fine. Also when I go into live data and reductant system I have various malfunctions in the reductant level and temperature data.. further leading me to believe that this module also is needing the 12v as the diagram shows.

Because of this I am convinced that my problem lies with no power at the K6 pin and that I am hunting for an answer why theres no relays or pins to supply it.

Let me know your thoughts.
 

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Discussion Starter · #6 ·
UPDATE

Problem is resolved.. Although my part # 84075774 was supposedly the right one and only superseded by 84303174 the 174 box has the missing pins while the other one does not.

Installed the box ending in 174 and all problems are resolved. Truck has full power and no faults.

Lesson learned.. Even though GM's part catalog lists a superseded or replacement part that doesn't mean they are the same.

Block (Replaces Part Number 84075774) - GM (84303174)
 

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Glad to hear it wound up being a relatively easy fix. Sounds like your accident repair may have been the culprit - could very well have been an innocent mistake upon the same reasoning you saw with your info received from GM directly.

Very happy you kept us posted
 
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