4L80E won't shift out of 1st

[dmak]

I bought a 1995 gmc 2500 4x4 last fall with a 5.7 liter and 4L80E. It was not running at the time and had been sitting for at least a year as of October 2001. I ended up putting a rebuilt engine in it this spring. I drove it for about 100 miles, and everything was fine. It ran and shifted properly. The only issue for those 100 miles was that the oil pressure gauge, and temp gauge would peg all the way to the right as soon as you started the engine. I periodically checked the coolant temp at the thermostat housing with an infrared thermometer and it was always around 185. Last night I was headed into town, and the transmission stopped shifting. Reverse works fine, but it will not shift out of 1st gear. I tried manually moving the shift lever from 1 to 2, etc and it had no effect. It is full of bright red fluid. I tried to retrieve any codes using the paperclip method, and it didn't display any codes other than the normal "12" code. I would prefer to troubleshoot and repair this myself, but I'm having a hard time finding troubleshooting guides that are written in plain english with pictures for a semi literate individual like myself. I am toying with the idea of just ordering all of the solenoids, speed sensors, a wiring harness, and a new filter from rockauto and throwing them in, as it would probably cost less than even having a shop look at it. Any tips, links, starting points, etc would be much appreciated.

 

[stutaeng]

https://www.google.com/url?sa=t&source=web&rct=j&url=https://shop.ukrtrans.biz/wp-content/uploads/catalogs/4L80E.pdf&ved=2ahUKEwjcu5K_yqj4AhXSkGoFHVNaCWEQFnoECAUQAQ&usg=AOvVaw32eAoQwy6d5MGXvZ5X5hyE

Look at Figure 2 to see what is being applied as it shifts into second gear. And look into the troubleshooting tables below to see if any match your symptoms.

You should check the solenoids for resistance. I think you can do that by hooking up a DVOM at the big case connector, just find yourself a pinout.

But if you haven't done a fluid service on it, that's a good start. The 95 models use the "shallow pan." Get the corresponding filter for it. It has grooves on the bottom.

 

[Schurkey]

Won't shift out of FIRST gear???

Are you SURE it's not locked in SECOND gear? That would be the default for a "limp-home" failure. The computer may be in "limp-home" mode due to the failure of the temp sensor(s).

Connect a scan tool, find out what the computer is trying to tell you.

 

[dmak]

Won't shift out of FIRST gear???

Are you SURE it's not locked in SECOND gear? That would be the default for a "limp-home" failure. The computer may be in "limp-home" mode due to the failure of the temp sensor(s).

Connect a scan tool, find out what the computer is trying to tell you.

I'm not sure. I don't have access to a scan tool unless i go into town, which is a bit of a trip for me. Is there another way to tell if it's in limp mode? Thanks for the help!

 

[stutaeng]

I'm not sure. I don't have access to a scan tool unless i go into town, which is a bit of a trip for me. Is there another way to tell if it's in limp mode? Thanks for the help!

Unplug the big case connector and that will make it default in limp mode.

Starting in second in limp mode will be a significant difference vs starting in first. Or you can use one of those gear ratio calculators that input speed to tell you rpm you should be at for a given trans. ratio. Will need to know axle ratio also.

 

[dmak]

Won't shift out of FIRST gear???

Are you SURE it's not locked in SECOND gear? That would be the default for a "limp-home" failure. The computer may be in "limp-home" mode due to the failure of the temp sensor(s).

Connect a scan tool, find out what the computer is trying to tell you.

You were right on. I unhooked the battery for a few minutes, hooked it back up, and it shifts like a dream. The oil pressure and temp gauges are still acting funny. The oil pressure swings all the way to the right as soon as you start the engine. Sometimes when the engine is idling in gear, it will read around 20psi, but as soon as I touch the throttle, it swings all the way to the right again. The temp gauge stays all the way to the left for a few seconds after the engine starts, then swings all the way to the right and stays there. The temperature sending units are both brand new. The fact that the oil pressure gauges sometimes gives a reasonable reading leads me to believe that it's sending unit is operational as well. Any ideas to track down this problem? Thank you very much for the help so far.

 

[Schurkey]

Bare spots in the wires leading to the sensors, grounding occasionally on the engine or metal brackets???

Does a '95 have air-core gauges, or stepper-motor gauges? Newer vehicles use the stepper-motor style, they're driven by the computer. If the computer is screwy, the gauges can act screwy.

 

[MarylandMike]

Wondering if you had an update. Any trouble after this? I'm having a very similar problem with my 93 K2500. No comms with the ECM and same trans issues. If I disconnect the ECM for a while, it will run and shift perfectly for a drive cycle or two. Then, extended crank, finally starts and has high idle and MIL is back on and only 1st (or maybe 2nd) and reverse. Was thinking ECM or EPROM, but afraid to put the trigger.

 

[evilunclegrimace]

How about checking for codes and seeing if the TPS has failed. I bought a "98 suburban in Virginia and drove it home to Pa manually shifting it a every stop light because I did not bring a scan tool with me. When I got to Pa and hooked the scan tool up the first code that popped was TPS. I replaced the TPS and the truck has shifted fine for the last 2 years.

 

[MarylandMike]

How about checking for codes and seeing if the TPS has failed. I bought a "98 suburban in Virginia and drove it home to Pa manually shifting it a every stop light because I did not bring a scan tool with me. When I got to Pa and hooked the scan tool up the first code that popped was TPS. I replaced the TPS and the truck has shifted fine for the last 2 years.

Thanks for the reply. No codes to be had though. No comms with the ECM either with a scan tool or paper clip. I thought of TPS also. That could effect the idle issue and the trans needs to know throttle position. I tried one (although I admit it was a cheap aftermarket one), no change.

Short story behind this truck. Maybe you guys will see a clue that I'm missing...

I bought it to replace my Dodge that literally rusted in two. Thought it smart to buy a truck that I could pay for rather than getting a loan. I went with this one because I had a 92 Suburban in the past that never gave me a second's trouble. I drove this truck for a month while I was getting it ready for inspection and it ran flawless. I had a guy (whom I trust 100%) replace all the fuel lines for me. This started while he had it. Biggest bummer of this all is I spent all my savings on it. Now I have no savings and still no truck.

One thought though. I did have it at a body shop before him, to have a spot on the frame welded. However, it drove fine from the body shop to his shop.