u2106 help please!!!!!

ryanvanl

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Dec 4, 2012
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got a 03 here that wont turn over! I can jump power to crank fuse and get it started that way but not with the key. there is NO dash under the PRNDL with check engine, ABS and parking break (parking break is NOT on) lights on. read codes using my edge insight and found P0380 (been there forever and not concerned) and a U2106 which I believe is the cause of all this. It WILL shift into park, reverse and drive but with a nasty clunck (shifting into 3rd which is the default gear with no power to trans). I took the tranny pan off and removed internal harness to clean and inspect but everything was perfectly fine. replaced the NSBU less than a year ago with the updated tan version so I dont believe thats the problem. Checked all wires on NSBU for continuity/B+/ground and everything was good. DOES ANYONE HAVE ANY IDEAS WHATS UP? :(
 

ryanvanl

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Dec 4, 2012
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anyone have any ideas. it was working fine this morning (started up and ran just fine) until i turned on my fan (hvac) while i was driving and then all hell went down. no more dash under gears again. abs light on. park break on. dash says "service 4wd". turned the truck off and tried to start again but nothing. turn the key on and the heater fan doesnt work. i need help BAD! i have a dyno in 2 days to compete in and the damn thing wont start!!! HELP!!!!!!!!!!
 

jc1843

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Sep 27, 2009
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So Calf
Start by testing the batteries-- clean the cables and check for corrosion.
Follow the cables to the jump box, make sure the cables are tight.
 

ryanvanl

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Dec 4, 2012
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of coarse the cables are tight and clean. so I probed pin 31 of the red connector which is a 12v reference wire that runs down to the big transmission plug and supplys power to all the shift solenoids and got nothing. I jumped 12v straight to the pin 31 wire and everything works just fine. it starts, runs and shifts just fine but now my heater/ac doesn't work correctly. I turn the fan on low and it blows at max speed and when I turn the knob to change where the air comes out, nothing happens. somewhere throughout all my reading I read somewhere that one of the computers gets power from the hvac system somewhere. is this true? someone help!!!!!!!
 

jc1843

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Sep 27, 2009
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So Calf
The AC problem could be a bad resistor. Pull the under dash cover from the passenger side-- the resistor is at the bottom of the heater box. The high resistance can also damage the resistor connector. Both are available from most part stores.
 

ryanvanl

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Dec 4, 2012
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yes the fuses are good. I took a test light to EVERY one and they all lit up on both sides.

will a bad resistor on the heater cause my truck not to start and drive though?
 

gmasko

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Jun 14, 2014
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I know this is old but for the sake of anyone looking for answers in the future... This exact thing happened to me yesterday (also have p0380 oddly enough). SteveO on dieselpageforums pointed to IGN 0 fuse on drivers side panel. Sure enough the fuse was blown not giving the TCM any power. Replaced fuse and all seems good. Starting to think my p0380 code is an electrical short somewhere causing my truck to do all these funny electrical things.
 

timberben

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Jan 8, 2012
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I have am 03 2500hd. Truck has abs and park break light on and code u 2106. The IGN O fuse was blown. When I replaced the fuse the purple wire under the engine compartment fuse block melted. I don’t know if the short is there yet. The purple wire goes to row 9 column 3 if I’m saying that right. Anyone have a similar problem with a short somewhere else or a cause.
 

Ron Nielson

Active member
Oct 11, 2009
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Berryton, KS

Chevrolet
Chevrolet DTC U2106​

Make: Chevrolet Code: U2106
Definition: CAN Message Not Received from TCM
Description: The ignition is ON.
Cause: The modules that communicate on the CAN system are wired parallel to each other until the respective circuits are spliced together for the ECM. An open in the CAN circuits of one module will not affect the other module. A short to ground or short to voltage

Row 9 column 3 of WHAT??
 

timberben

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Jan 8, 2012
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The fuse box under the hood. When you unclip the cover and unclip the fuse block and lift it up the bottom side of it. Right side grey or white block. Row 9 column 3
 

timberben

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Jan 8, 2012
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Ok thanks that helps a bit. I don’t see the explanation of the relays. Where do I find that schematic for the interior fuse panel
 

Ron Nielson

Active member
Oct 11, 2009
447
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Berryton, KS
Use the search function of your PDF reader for 'relay' and look for the result that you want. I don't think they have 'explanations' per se regarding delays. The fusebox inside the cab may be called the 'MBEC' or you could search for the fuse or circuit you need.
 

dndj

Boost!
Apr 13, 2019
302
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Oregon
IGN0 fuse, left inside dash fuse block. M74 is the RPO for the Allison transmission. This is the FSM for the '03.

Since you stated the IGN0 fuse was blown, look for a short circuit somewhere downstream, wires rubbed through, etc. Note that the circuit exits the left instrument panel fuse block on pin F8 then goes back through the firewall (P100) to the fuse block underhood, where it then sends power to the TCM and ECM.


IGN0.jpg
 
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