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2001 Silverado 2500HD 8.1L Allison ALL STOCK
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Discussion Starter #1 (Edited)
I just purchased a 2001 Silverado 2500HD 4x4 with an 8.1L/Allison and 114k miles. A few days ago i replaced the fan clutch. When doing so, I removed the shroud and hastily pushed the TCM out of the way without disconnecting it (yes - very dumb). When the truck started back up, I had a CEL along with code 0700 and 5 other DTC's that i neglected to write down. I foolishly erased all the DTC's just to see if they would come back. Only the P0700 remained.

I pulled the TCM harness connectors and found that the J1_32 pin had been pulled back into the connector. I re-seated the pin, reinstalled the TCM connectors, started the truck and erased the P0700 code. The code did not come back but now I have the following problems:
  • Trans Temp Gauge and the Gear Selection Indicator do not work (the PRND321 are present but the line underneath them had disappeared)
  • Tow/Haul mode does not seem to be functioning
  • The truck now shifts at 4000rpm at WOT instead of 4500. The reverse lights ARE working and I checked all fuses with an ohmmeter - none burnt.
Ive looked at several diagrams and they all indicate that the J1_32 wire terminates at the J1939 backbone. I have no idea what the J1939 backbone is or where it is. What did i break? How do i fix it? Any advice is greatly appreciated. I hesitate to take it to a dealer or shop since most seem to just throw a $900 TCM at is and hope that fixes the problem.
 

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Welcome to DieselPlace
First, start with the basics and individually load test your batteries. If you have a bad battery cell you can get some strange electrical gremlins.
 

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2001 Silverado 2500HD 8.1L Allison ALL STOCK
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6 Posts
Discussion Starter #3
OKDually,
Thank you for the reply. The battery is a 16months old NAPA Gold 625CCA. I do not have access to a load tester so i did the best i could with my multimeter:
  • 12.52V when off
  • 12.25V when off after 3 min of ignition ON and headlights and AC fan ON
  • 14.25 Volt when running
Any thoughts?
 

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OKDually,
Thank you for the reply. The battery is a 16months old NAPA Gold 625CCA. I do not have access to a load tester so i did the best i could with my multimeter:
  • 12.52V when off
  • 12.25V when off after 3 min of ignition ON and headlights and AC fan ON
  • 14.25 Volt when running
Any thoughts?
Unfortunately, a voltage test is different from a load test and won't give you any info into the health of the battery cells.
Remove your battery and take it to your local auto parts store and ask them to perform a load test
 

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2001 Silverado 2500HD 8.1L Allison ALL STOCK
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6 Posts
Discussion Starter #5
OkD,
I will take the battery in this evening to be tested. Meanwhile, I discovered that i was reading the pinout diagram backwards. The C1_16 pin was the one that was pulled out of the harness, not the J1_32. According to the wiring diagram below, the C1_16 goes to the VSS. Assuming the battery is ok, any other suggestions or thoughts?
635856
 

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Great detective work. Glad you found the problem.
 

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2001 Silverado 2500HD 8.1L Allison ALL STOCK
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6 Posts
Discussion Starter #7
Battery passed the load test. Any idea on how to test the VSS since it it the magnetic type?
 

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Battery passed the load test. Any idea on how to test the VSS since it it the magnetic type?
I believe you would need a scanner that can read the live data like a Tech II
 

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2001 Silverado 2500HD 8.1L Allison ALL STOCK
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Discussion Starter #9 (Edited)
If the VSS is bad, the truck speedometer should not work??? (is this correct?) However, my speedometer is functioning properly. I also have a basic handheld scanner that can stream live data but not the raw signal from the VSS. The scanner is showing the speed of the truck correctly. I am going to assume that the VSS is functioning properly.

Since i have no gear indicator or trans temp and both of those signals come from the NSBU, the NSBU may not be grounded properly or is broken. My (limited) research of the NSBU says that there isnt a definitive method for testing it without an expensive scanner, such as a Tech II. Has anyone found a method for testing the NSBU?

I'll test the grounds on the harness and see if they are properly grounded. If they are, there are only two things that can be causing these problems, the NSBU or TCM (or both).
 

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2001 Silverado 2500HD 8.1L Allison ALL STOCK
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Discussion Starter #10
I tested the power and grounds on C1 (pins 1-5). All are functioning. I think it is safe to assume that the TCM is receiving power (12.42V) and is properly grounded. Does anyone know how to test harness C2_pin20? According to my wiring diagram, it is the analog ground for the following:
- Trans-H: ???
- ECTS-A: ???
- Temp-B: ???
- NSBU-7D: NSBU ground
- R-Temp_B: ???
- TPS-A: throttle position
Are these correct for my model, year and engine? Of particular interest is the NSBU ground. Is the ground for the NSBU located inside the TCM?
636012

636011
 
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