4l80e stuck in limp mode?

[arbboyd]

Hello all. I recently picked up a '92 Suburban 2500 7.4l for cheap. Has a newer crate engine with around 50k miles and 240k on the body which has almost not rust. I knew going in the engine had an issue, it would stall and idle funny unless you applied throttle until it warmed up. Figured it was a vacuum leak. Went to pull codes using the paperclip method and got nothing, no flashes just a steady light. Got a cable and a laptop with tuner pro rt and it will not communicate with the ecm. Checked fuses and grounds, all good. Asked around and was told ecm's rarely fail so I just stared replacing sensors since they were cheap. Did the tps, iac, egr solenoid, o2, coolant sensor, map, plugs and wires, all new grounding straps. Now it runs decent, still have to apply throttle to get it stared but after that drives fine. Now iv'e discovered that the transmission seems to be stuck in either 2nd or 3rd gear. Shifting through the gears dose nothing. Have reverse though. Was thinking shift solenoids? Ecm causing issues? Still cant pull codes from it.

 

[andy396]

There’s several things that can cause it to go into limp mode. An issue with the ECU should probably be addressed first. After that, my first guess is it’s not getting a signal from one of the sensors it needs. It needs a signal from the TPS and the input shaft sensor down on the transmission. You could try swapping the input and output speed sensors down on the trans and see if that does anything. The fact that you have to give it throttle to start might be a clue as well that would point to the TPS.

 

[arbboyd]

Thought of the input/output sensors. Did the swap and even left them unplugged and no change. Tps is new and tests good. I noticed when is disconnected the vss my speedometer was still functioning. Dose it not work of that sensor?

 

[andy396]

VSS is the one at the output shaft on the trans or the one in the transfer case if it’s a 4x4.

 

[andy396]

You must be registered for see images attach

Here’s some other ideas.

 

[arbboyd]

It's a 4x4. Pulled that and still had speedo. Weird. Thinking more and more it's an ECU issue. The trans is controlled via the ECU yes?

 

[andy396]

Yes

 

[Supercharged111]

If you do not have 12V going to the transmission it will default to 2nd gear only. Check this first. 4L60 will default to 3rd. My dad had this happen and he ended up needing a new internal trans harness. They're cheap and easy to do. I ran my 4L80 unplugged to drive home in 2nd after the swap.

 

[Schurkey]

The FIRST thing to address is why the scan tool--or paper-clip--won't communicate with the ECM.

You can (and have) replaced sensors until the cows come home, and you won't really make headway until the computer can be reasoned-with.

Does the MIL come on when the key is turned "on"?

 

[Supercharged111]

Ahh, yeah, forgot about the comm part. Do that first.