2000 GMC transmission issues

[Neil.96]

Hi, I have a 2000 GMC 3500 that is a commercial production unit so it still has the body styling of the 88-98. We towed real heavy with it one day on the farm and now it only likes to move if my foot is to the firewall. It does it in reverse as well. I believe it is a 4L80e but have not climbed under to confirm as I do not know what I'm looking for. could this be bad shift solenoids? any other ideas?

[Jsdirt]

About all you can do is check the fluid. If low, add more. Note the color - dark and stinky, filled to the correct level and still doesn't move, tranny is smoked.

 

Could try some "mechanic in a can" wonder additive in there if you're out of options, but this can add additional labor if you're going to have it rebuilt at some point. All those wonder additives have a "seal sweller" in them, which do what the name implies - that makes it 20x harder for the rebuilder to get things apart. Seals will grow in their confined spaces and make everything stick. Once you take it apart, it'll never go back in it's place again, since the seal will be larger.

[Doug_Scott]

On youtube look up Precision Transmission.  They post lots of videos and Richard has the knowledge of an excellent brand specific transmission specialist, except he the same way with all popular brands of vehicles.  

Once you verify the fluid is at proper level, and doesn't stink so bad that the wife won't let you in the house (tell her the dog got hit by a skunk, at least that way you can use the shower before she remembers you don't have a dog) take the truck for a short drive, pay attention to the RPMs.  When you try to drive off, if the RPMs don't climb somewhat faster than slow, put the shifter into L (or 1) and see if that makes any difference.  If when driving it seems like it never shifts, and RPMs do not go past 4k at speeds you drive at in the city, you may only have one of the last 2 gears left.  Manually may make a difference, but not enough to keep driving it.

If the RPMs go straight towards red line, but truck would rather just sit there, it is time spend some money.  Merry Christmas.

 

If you have a 4L80E it has a TCM.  It should be storing errors codes if the transmission is electronically challenged.  You don't mention mileage, but at 20 years of age, the truck does not owe you anything.  Watch some of those videos before you start thinking you could just fix it yourself.  Transmissions are not difficult once you have done your first 50 or so.  

[Jsdirt]

Yeah, those guys at Precision Transmission are the some of the best there is in the automotive industry. God-fearing Patriots and family men. Top notch guys with some of the best transmission and differential videos I've ever seen.

 

https://www.youtube.com/channel/UCYAAY7V9ifGS7Q8S35EFVnw

Edited December 25, 2020 by Jsdirt

[CamGTP]

There is no TCM on a 4L80e that old.

 

Those old trucks have what we call the black box computer, it's pretty basic for a Vortec computer and it has the engine/transmission functions built into it. Often times the only code you will see is a P0700 if there is a transmission fault and some codes will reset if the ignition is turned off. The fault needs to happen again and the code will come back.

 

And that truck will have a 4L80e on a heavy duty chassis. Both the 350 and 454 came with those transmissions.