Torque Converter & Dash Issues, + P0740 and P0785 Codes

[TStowell]

Hi so ive got a 1996 C1500 with a L31 and 4L60E and suddenly while driving i lost power I could feel it was something wrong with the torque converter because in park i revved great no bogging. In drive it normally only drops down to around 1000 RPMs like it should but it now it would try to sit below there . I limped the truck home and found that I had a P0785 Shifting Timing Solenoid A code and a P0740 Torque Converter Clutch Circuit Open. Also a couple days before I started experiencing issues with my dash it started acting funny suddenly my speedometer would fly around and on the dash my tach would read zero or jump around I plugged in my Autocal Scanner and data logged and looked back my VSS were reading normall and my rpms stayed steady and where they should be so its definitely a gauge problem. Im looking for stuff to check first I recently did a 0411 Swap and it had been running great for about a month before this so i will check wiring and the actual PCM first. (The fact that its 2 codes at once and dash problems makes me think a wiring or PCM issue). Any other recommendations are greatly appreciated.

 

[NickTransmissions]

Hi so ive got a 1996 C1500 with a L31 and 4L60E and suddenly while driving i lost power I could feel it was something wrong with the torque converter because in park i revved great no bogging. In drive it normally only drops down to around 1000 RPMs like it should but it now it would try to sit below there . I limped the truck home and found that I had a P0785 Shifting Timing Solenoid A code and a P0740 Torque Converter Clutch Circuit Open. Also a couple days before I started experiencing issues with my dash it started acting funny suddenly my speedometer would fly around and on the dash my tach would read zero or jump around I plugged in my Autocal Scanner and data logged and looked back my VSS were reading normall and my rpms stayed steady and where they should be so its definitely a gauge problem. Im looking for stuff to check first I recently did a 0411 Swap and it had been running great for about a month before this so i will check wiring and the actual PCM first. (The fact that its 2 codes at once and dash problems makes me think a wiring or PCM issue). Any other recommendations are greatly appreciated.

Do the checks you suggested above then report back. Once you have eliminated wiring (or at least let us know you have done so), we can perhaps make more suggestions.

It's not entirely unheard of for two solenoids to go bad at the same time, though admittedly it's relatively rare but you may want to continuity test the transmission's wiring harness via the pins on the harness case connector. Also check the connector on the vehicle wiring harness as well just to rule it out as a potential source of your problems. Id then drop the pan to test the 3-2 control solenoid. That solenoid should show 18-24 ohms, same with the two shift solenoids...The EPC shows 4-6 ohms while the PWM solenoids is 9-12 ohms.

P0785 indicates a problem with your 3-2 control solenoid.

 

[TStowell]

I just wanted to respond to maybe help someone out with the same issues. So I checked all the PCM connections and they looked good and I could not find anything and before I pulled the transmission pan off I wanted to make 100% sure that was where the problem was. Luckily I got in contact with a family friend that has worked on tranny's most of his career after helping me we narrowed it down to the transmission getting not enough voltage it was jumping between 3-8V and the line we were checking should have been running at around 12V. We narrowed it down to the ignition wiring harness which provides power to the tranny and the dash as well as some other systems. After replacing that and clearing the codes truck ran great and dash was fixed. We found the problem that killed the original harness was a bad pcm connector that was making random connections so I replaced that as well. Sorry for making things so long but if you have issues like transmission solenoids not firing and the torque converter clutch not locking up as well as flickering gauge cluster elements the ignition harness could be your issue.

 

[NickTransmissions]

I just wanted to respond to maybe help someone out with the same issues. So I checked all the PCM connections and they looked good and I could not find anything and before I pulled the transmission pan off I wanted to make 100% sure that was where the problem was. Luckily I got in contact with a family friend that has worked on tranny's most of his career after helping me we narrowed it down to the transmission getting not enough voltage it was jumping between 3-8V and the line we were checking should have been running at around 12V. We narrowed it down to the ignition wiring harness which provides power to the tranny and the dash as well as some other systems. After replacing that and clearing the codes truck ran great and dash was fixed. We found the problem that killed the original harness was a bad pcm connector that was making random connections so I replaced that as well. Sorry for making things so long but if you have issues like transmission solenoids not firing and the torque converter clutch not locking up as well as flickering gauge cluster elements the ignition harness could be your issue.

When you have mutiple transmission-related DTCs, it's almost always electrical and/or electronic issue(s) adversely impacting the solenoids, switches, etc vs being all those parts gone bad themselves. You were right initially and I had agreed with your suspicions.

Thanks for following up to confirm, share the outcome. Glad you got it fixed.