• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

4l80E shifts out of OD and 4th upgrade but will not shift back on downgrade

Rex Crick

New Member
Messages
1
Likes
0
Location
59218
Thread starter #1
1999 gmc sierra 2500 4x4 6.0l with 247,500 miles. Transmission tends to have this problem when trans temp reaches 170+. Cruising at 75-80 on cruise control will cause trans to downshift normally upgrade, usually to 3rd, but will not upshift on downgrade. Stopping truck, turning off ignition, restarting after a few minutes, and starting up slowly will cause trans to shift properly until next upgrade. If cruise is not used and rpm and speed are allowed to bleed off going upgrade, the trans will remain locked up. Am willing to replace trans, but want to make certain it is trans. Thanks
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,192
Likes
2,680
Location
ZEPHYRHILLS FL
Staff #3
Scan it for codes 1st. Common issues on those trucks is a dirty maf as all shifting is maf based in them. Most any shifting concern should set a code though. It could be so many different things it's impossible to guess.
 

MrMarty51

Well-Known Member
Messages
1,407
Likes
1,405
Location
Miles City, Montana
#4
The terminals at the back of the engine on My truck were all corroded. I cleaned them and that ended the problems with the boost system on My truck. Might be the same on Yours too.
Also, the windshield washer had stopped working, the wipers would go but not the pump system. I pulled the tnk and cleaned those connectors at the pump and they now are working as they should.
Be worth a try and dont cost a dime.
I used a dental file to get into the female ends of the terminals, nothing else was small enough.
Might check with a dentist and see if they have some of those. If not, then I would thingk theyd be available from ebay or amazon.
 
Top