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Discussion Starter · #1 ·
I have a 2005 2500hd 6.6L duramax with a 5 speed allison. When starting the truck up, I noticed there was no light under PRND321 showing what gear I was in. When I backed out of the driveway, the truck rocked backwards and I knew something was off. Before I got out of the neighborhood, the ABS and emergency brake lights came on with a "service brake system" message. The truck wouldn't get out of first, but it did eventually get into second. When coming to a red light, downshifting caused the "range shift inhibited" message to come on, and I had to pull over to turn the truck off. When I turned the truck back on, I was able to move again but continued having the same issues.

When I got it back home, I scanned it and I have 9 error codes appear. The codes were P0404, P0401, P0670, P0700, P0802, U0101, U1000, U1024, U1009. The 404 and 401 codes have to do with EGR, which I plan on deleting at some point but I don't think it is the main issue. The 670 is for the glow plug module control circuit, and the 700 and 802 are a general trans code and a malfunction in the TCM. All the U codes have to do with lost communication with the TCM I believe.

I'm thinking it is a faulty TCM, but I wanted to know if anyone has any other ideas of what it might be before I order a $500 TCM. I took it to a transmission shop and they gave it back to me saying they didn't know what was wrong with it. Also, is it complicated to reprogram the new TCM?
 

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I have a 2005 2500hd 6.6L duramax with a 5 speed allison. When starting the truck up, I noticed there was no light under PRND321 showing what gear I was in. When I backed out of the driveway, the truck rocked backwards and I knew something was off. Before I got out of the neighborhood, the ABS and emergency brake lights came on with a "service brake system" message. The truck wouldn't get out of first, but it did eventually get into second. When coming to a red light, downshifting caused the "range shift inhibited" message to come on, and I had to pull over to turn the truck off. When I turned the truck back on, I was able to move again but continued having the same issues.

When I got it back home, I scanned it and I have 9 error codes appear. The codes were P0404, P0401, P0670, P0700, P0802, U0101, U1000, U1024, U1009. The 404 and 401 codes have to do with EGR, which I plan on deleting at some point but I don't think it is the main issue. The 670 is for the glow plug module control circuit, and the 700 and 802 are a general trans code and a malfunction in the TCM. All the U codes have to do with lost communication with the TCM I believe.

I'm thinking it is a faulty TCM, but I wanted to know if anyone has any other ideas of what it might be before I order a $500 TCM. I took it to a transmission shop and they gave it back to me saying they didn't know what was wrong with it. Also, is it complicated to reprogram the new TCM?
Did you run a re-learn on the new transmission? The TCM adjust solenoid timing based how you drive and the transmission characteristics, new transmission means new characteristics. I would also check the wiring harness to the transmission make sure its connected up good and not damaged.
 

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Discussion Starter · #3 ·
I checked the wiring harness connecting the transmission and everything looked good. I have put 10K miles on the truck since I put this transmission in, so I believe it was re-trained already. I also tried to reset the transmission control module but I am still having the same issues.
 

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Personally I would concentrate on the U% codes and get the communications re-established. Then see what's left.
 
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Discussion Starter · #6 ·
So I pulled the TCM out of the truck, and checked the voltages on the wires feeding the TCM. The red connector (C2) didn't have any voltages across it at all. I'm assuming that is because it is outputs from the TCM to the transmission, so I need to plug the TCM back in and check the voltages on the transmission side. Is that correct?

The clear connector (C1) had these voltages across it.

Brown Black Font Wood Astronomical object


The ignition voltage (Pin #3) only had 1.33V on it when running, and dropped down to 0.18V in ACC. Does this seem correct?

All the pins not listed in the chart had 0V across them, and for reference:
pin 7 is TCC BRAKE SW
pin 10 is 4WD LOW SIG
pin 25 is MIL SIGNAL
pin 29 is CAN DATA HI
pin 30 is CLASS 2 DATA
pin 32 is CAN DATA LO
 

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Discussion Starter · #7 ·
I connected the TCM back to the truck, then read the voltages off the 20-pin main transmission connector.

Transmission 20-pin Connector
PinVoltage (While Running)
C
2.28​
D
0.11​
E
0.1​
F
0.1​
G
0.1​
K
0.11​
S
2.28​

Pin C should be a 12V reference
Pin D, E, F, and K all feed the pressure switch manifold
Pin G feeds the TFT sensor
Pin S is the TCC SOL
 
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