(Fixed) 2015 Silverado 1500 Reverse works but forward doesn’t

I’ve been asked to look at a Silverado that doesn’t have power moving forward but the reverse works fine. The mechanic believes it’s an electrical problem. I’m fully prepared to deal with any electrical problem, however, transmission mechanical faults are a big weakness. What PIDS or information, tips, experience would you give before I jump into this?

I think checking the input speed chart vs output speed shaft sensors may be the answer depending on if it goes that direction.

I guess the first step will be figuring out what "doesn't have power moving forward" actually means. Kinda like one of those "It won't crank" calls right now :silly:

Guessing it's got a 6L80 in it so this might help.

atracom.blob.core.windows.net/webinars/gm/6l80_introduction.pdf

no forward gears at all? I wouldn't be worried about input and output speed sensors. I can't think of any electrical faults that would cause no forward gears at all. Get a scan tool on that thing and see what you have in the TCM to start with

ScannerDanner wrote: no forward gears at all? I wouldn't be worried about input and output speed sensors. I can't think of any electrical faults that would cause no forward gears at all. Get a scan tool on that thing and see what you have in the TCM to start with

Electrical faults outside the transmission probably not. An internal fault with shift solenoid # 2 looks like it could cause no forward gears if the truck does have a 6L80. If that is the problem hopefully it will have set a code in the TCM. Or if it hasn't 'scoping current draw into the TCM might show it. Looks like you should be able to see its current ramps shifting back & forth between P & R.

If it's a 6L80, you can always disconnect the main connector at the transmission and recheck for forward movement. That'll default it to 3rd and Reverse. If that works, it doesn't rule out an internal problem, but does point more towards a control problem, IMO.

It is the 6L80, The lot actually rescheduled on me so I don’t even get to look at it till Sunday, However on the phone I had the mechanic remove the fuse for the TCM and it was able to move in a forward gear. I’ll update when I get to take a look at it.

Nice! I'll put my money on an Internal Mode Switch issue.

That PDF that Matt T posted is fantastic.

Tyler wrote: That PDF that Matt T posted is fantastic.

ATRA/Gears magazine have a lot of good stuff on electronically controlled transmissions. They're where I look first when I've got a tranny problem that might be fixable. And I usually end up reading other articles while I've got the site pulled up.

It’s fixed now. The TCM was not providing ground to one of the shift solenoids. It couldn’t even ground a 150ma test light.

Very nice. I know tons of shops that would have thrown two reman transmissions in that before checking the TCM.

Do you recall which solenoid, exactly? Any concern that the solenoid smoked the driver?