Gm 4L80e transmission not shifting

Reply

  #1  
Old 03-29-12, 01:22 PM
Member
Thread Starter
Join Date: Mar 2012
Posts: 2
Gm 4L80e transmission not shifting

Hello, I'm a mechanic in the marine corp. we've had an m1115 hmmwv in our shop for about 2 months now with transmission problems and haven't been able to figure it out. The transmission is a Gm 4L80e. It won't shift out of first and I've done everything from replacing the TPS to testing the TISS and TOSS. If anyone has an idea of what the problem might be please reply with an answer. Thank you for your help
 
Sponsored Links
  #2  
Old 03-29-12, 04:42 PM
Member
Join Date: Mar 2009
Location: Texas
Posts: 1,051
I am no a mechanic, but if the input and output speed sensors tested OK, what about the vehicle speed sensors also. Bad wheel sensors or worn harness wiring will also cause the same symptoms. Are you able to scan for any DTC's ?
 
  #3  
Old 03-30-12, 05:37 AM
the_tow_guy's Avatar
Group Moderator
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,541
Morning Devil Dog; you at Lejeune or Cherry Point?

Left a message for my tranny guy to call me when he gets a minute, so I'll see what he says. You're working on "my" tranny; have the same one in my wrecker (although an older generation). I've had a variety of issues over the years (600,000+ on the truck and its third tranny), but I don't think I've ever had it simply refuse to shift out of 1st gear. I would have started with what you've already covered, too; or rather they are what I would have expected my tranny guy to look at.

Tow Guy
Cherry Point, '85-'99
 
  #4  
Old 03-30-12, 08:32 AM
Member
Thread Starter
Join Date: Mar 2012
Posts: 2
Im at lejuene. I've checked the speed sensor it checks out. Thought the wiring harness was bad so I replaced it works fine now. The only thing I havent checked are the shift solenoids because its not my echelon of maintenance. that belongs to 3rd. Only problem is I cant send the truck to 3rd without figuring out whats wrong with the transmission. So im stuck in a circle.
 
  #5  
Old 03-30-12, 10:50 AM
the_tow_guy's Avatar
Group Moderator
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,541
Okay, got a call back from my tranny guru (Army doggie long time ago). He said he can't recall ever having an issue like you describe. Aside from the input and output sensors [side note: one thing he said he does sometimes is swap the sensors since they're the same; if whatever problem you are having changes in any way it points towards one or the other of the sensors whether or not they check good otherwise]. Only suggestion he had - and admitted is was way past "long shot" - is to check alternator output. he said once or twice he had a problem tyhat defied troubleshooting and turned out to be bad diode in alternator causing overcharging. Only shifting issues I've ever had were usually head-snapping shifts, which is bad input sensor I think, and for a while I had an issue where I would suddenly loose high gear. That was an external (chassis) wiring issue. Didn't think to ask about shift solenoids, but I think the default limp mode is second gear. You're sure it's stuck in first gear and not second, right? Sorry i couldn't be more help. Semnper Fi.

tg
 
  #6  
Old 03-30-12, 04:12 PM
Member
Join Date: Mar 2009
Location: Texas
Posts: 1,051
Only just wondering still is if the tranny unit also relies on vehicle speed sensors as well. Is the speedo functioning from a cable or electronic display, and if electronic was it working properly all the time even though the tranny wasn't shifting? I had this case on a GM car product where the cuprit sensor involved was not the tranny input or output sensor as I imagined it would be, but one from a wheel speed sensor ( yes the same ones that are used for ABS, T/C, and speedo) and it also caused the tranny to stay in first gear despite the other sensors working properly. In my case the sensor harness was worn through. Who knew that a wheel sensor would impact the tranny? I am guessing that the computer compares all speed sensor signals as part of the logic, and if one input is missing such as the wheel speed sensor it won't signal an action to the tranny. Just another potential problem to eliminate.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'