2004 Silverado 1500 4x4 Intermittent Transmission Slipping and Related Symptoms

[chevrolet1616]

Bear with me, this is a long post, but I've been troubleshooting a bizarre intermittent transmission slipping issue and potentially a related electrical issue. Any thoughts/theories would really help here. Just to note, the two shops, Shop A and Transmission Shop B have a reasonable reputation in my area.

 

Vehicle: 2004 Silverado LS 1500 4x4 Ext Cab Short Bed, 111,000 miles (daily driver until 2016), 3,000 miles/year since then. Owned since new, this truck has not lived a super hard life.

 

Symptoms:

1)      Initially had intermittent slipping in 1^st gear, either in DRIVE or manual gear selection in middle of June

2)     Continued to slip intermittently after transmission fluid/filter replaced July 6^th

3)     Check engine light (code P0758 ONLY) appeared late August, would not go into 4^th in DRIVE and slipping in 1^st gear again (in both DRIVE or manually selected). 3^rd gear on highway works fine. Manual selection of 2nd and 3rd seems to be okay to get around.

4)     Solenoids A and B replaced by Transmission Shop B on Sept 3^rd

5)     No obvious power drains EXCEPT on the IPC/DIC circuit which was about 200 milliamps

6)     Previous battery draw issue over the previous 2 years. Usually would kill a battery between 12 and 48 hours. Very intermittent issue.

7)      Recent wet day battery draw issue reappeared.

 

Actions taken so far:

1)      Fluid and filter replaced by Shop A on July 6^th

          a.       Worked for one week, slipping issue reappeared intermittently throughout August

2)      Solenoids A and B replaced by Transmission Shop B on Sept 3^rd and solenoids tested with independent power source, functioning properly.

          a.       No change, Check Engine Light still illuminated with code P0758

3)      Checked all fused circuits on Sept 4^th

          a.       One drain on the IPC/DIC circuit of about 200 milliamps

 

Working theories (in levels of severity): 

1)      Possible gauge assembly short due to the IPC/DIC circuit drain possibly related to a transmission control circuit? Seen this pop up a few times on other forums, but I am doubtful, then again, this was early 00's GM where fog light fuses did not match with the fog light circuits...

2)      Short on the transmission harness

3)      Short between the PCM/ECU unit in the engine bay and the transmission harness

4)      Bad PCM/ECU unit <-- This is my top theory considering the connectors are oriented sideways and have dirt/debris all over, combined with a wet day may result in a small short causing both the transmission issue AND the battery draw.