4L80E Issue: Random NO OD issue

[TerryD]

We took the Sierra for its first little trip yesterday. Going down, we took roads that we could run between 45 and 60 on mostly and it did great. No issues. I was using the Torque app to watch engine temperatures since my gauge is screwed up. It ran around 194 the whole time and OD was working fine. Coming home we took a different route through National Forrest where I was running 25-45mph most of the way. Well, at one point it opens up a little and you can run about 60. When we hit that stretch, I pushed the shifter into OD and it wouldn't shift up. I pushed it a little and got up to almost 70 on one long straight away and it still just hung in 3rd. It was solid though, didn't even attempt to shift into 4th. Almost like it forgot it had a 4th gear. TCC worked fine also.

Well, this morning I got up early and took it out for a drive. Shifted into OD just fine and I made a little 20 mile trip on the interstate just to verify. It did fine, no issues what so ever.

This is the first time it has happened. The transmission fluid is still nice and pink, doesn't smell burnt or have any black in it. It shifts nice and firm, no slipping or hunting. I'm kind of at a loss over this and was hoping some one out there had had some dealings with this and could shed some light on it for me.

 

[Greg Ducato]

Could have been a little glitch in the manual lever position sensor

 

[TerryD]

Could have been a little glitch in the manual lever position sensor

Thought of that. I stopped, shifted to park then to OD and took off. Didn't help. It has worked fine since. We took it on a trip yesterday to pickup a mower and step side bed and it did fine. I'm kinda lost on this....

 

[TerryD]

Well, the issue reared its head again yesterday. It also threw a P0122 code. It would go into OD, but you had to get up to 63mph and then it would not unlock the TCC nor downshift out of OD.

According to my Torque app, it was seeing the TPS just fine but it wouldn't shift. After sitting all night, it has worked fine today. I went through some of the trouble shooting guide in my GM shop manual for it but I didn't find anything other than the weather pack connector seal being a little out of place. All voltages were correct and the sensor responded just fine.

I'm at a little bit of a loss as to where to go from here on it. It looks like it has a new TPS on it, but I may try another just in case.

 

[1-ton]

Lock-up tourqe converter going bad...maybe? Also check to see if you have gear-oil or ATF in your transfer case. If it is gear-oil, it could conceivably seep into your trans and mix with the ATF, thus causing random problems.

 

[Smokinthehippies]

The t case will have ATF

 

[1-ton]

The t case will have ATF

I am not as familiar with newer drive-trains, is this because the trans feeds fluid to the transfer case, which has no separate fill ability like the older 205 and 208 models?

 

[Smokinthehippies]

Not sure of the reason why. I know they can pass fluid back and forth so I'm assuming that's why it was designed that way. But they do have separate fill port

 

[TerryD]

They should not pass fluid in normal operation. They each have their own fluid reservoirs. The reason for using the ATF in the t-case is that they are pressure lubed with an input driven pump and the heavier fluids like gear oil would cause too much drag.

 

[Keitha]

Make sure the switch for the TCC is being depressed by your brake pedal. There is a switch that depresses when you release your brake. When released, it disengages the TCC - when you press the brake pedle. Check that switch and make sure it is adjusted to be engaged when the brake pedle is realeased.

 

[plyr2k]

I have a bunch of literature on this..... You said your gauges were going wonky... I know in my 95,when the trans fuse blew the gauges wouldn't work.

Check around the drive shafts for wires rubbing grounding themselves out.

If need be, drop the pan and check resistance on solenoids. I have 4 used solenoids that all test out good you can have for the price of shipping.

Believe it or not, a worn ignition switch will cause the transmission to do very strange things.

I would start there. Possibly make sure the 11 pin connector is plugged in tight.

Checking the brake light switch is a good idea. I had a 94 Cummins that should kick out because of that. While driving, just lift the pedal with your left foot and see if it changes.

 

[TerryD]

I'll check the next time it acts up. It hasn't done it but twice now. I talked to a buddy who works at a GM dealer and he was saying it could be the alternator output dropping. I need to drive it till it is hot and see if it will do it again.

 

[Keitha]

Does it seem to "go into neutral" when coasting (going down a hill etc) while off the gass?

 

[TerryD]

If I'm in OD position but the transmission is still in 3rd, yes it does. No engine breaking that way. If I move it to 3rd gear position, the TCC locks up and it will hold back and if I get it to go to 4th it will as well.

 

[Keitha]

Then I'm willing to bet you are getting sliping in you transmission. The elimination of OD is a result of an error code after the slipping. Does it 1st happen under higher loads while in OD - when you depress gas pedel to just maintain speed going up a hill for example (not enough to down-shift to 3rd)?
How many miles on the 4L80e? I hate to say it, but it may be time for a rebuild if it what I think it is.

 

[TerryD]

I'm not sure of the transmission's mileage. It doesn't seem to be slipping. Shifts are fast and firm with no slipping in any gear or any throttle. The loss seems to be after running with the shifter in the 3rd gear position like in town when OD is a waste.

I ran my OBDII blue tooth scanner on it and shift times were all in the 0.4 sec range. Our Suburban's shift times were all in the 0.6 sec range. Less TCC slippage than the Suburban in the truck too.