4L80E Looses OD and goes to N, sometimes

Keitha · April 14, 2005

Sorry about the double post, but thought this forum is more appropriate.

In my 1991 Suburban 4x4 3/4ton: I just swaped a 383TBI in place of the stock 350 (had tranny filter and fluid changed at same time). Now there are times when the tranny looses 4th (OD) gear and just goes into neutral. It seems to happen at highway speeds going up a hill, the tranny will downs**t to Drive under load (like it should) then never get OD back. As I crest the hill and start going down, the tranny just goes into neutral. It will go into 3rd when I get on the gas, then go back to neutral when I let off.

It will reset out of this condition when I shut the truck off then restart.

Any help would be great.

Thanks,

Keith

GM-Tech · April 14, 2005

When you force a downshift, the 1-2 shift solenoid exausts the signal fluid for the 3-4 shift valve. That valve in turn moves over, exhaust 4th clutch oil and you get your downshift.

To go back up into 4th again, the 1-2 shift solenoid has to turn on, blocking the exhaust of the signal fluid to the 3-4 shift valve. That valve in turn moves back over, allowing 4th gear oil to apply the clutch.

The fact that cycling the key is needed to fix this makes it sound like a 1-2 shift solenoid problem. It'd be nice to know if there were any DTC's set (the light would NOT come on for one of these DTC's).

Keitha · April 14, 2005

You guys rock, and thanks for taking the time to answer this.

If I floor the gas and cause the downshift or put the selector into "D", everything works OK. It only happens if I begin going up a hill and keep my foot at mid throttle (or there abouts) and the tranny downshifts on it's own (I give enough throttle to maintain speed).

I used the paper clip trick to pull codes once and got a 68 if I remember right. I will verify that ASAP.

Thanks,

Keith

Keitha · April 20, 2005

I did confirm that there is a code 68 from my ECM.

GM-Tech · April 20, 2005

I did confirm that there is a code 68 from my ECM.

That code is not terribly helpful in determining what's wrong. 68 is an "overdrive ratio error", which means, the PCM thinks the tranny is in overdrive, but the difference between the input speed sensor and the output speed sensor indicates otherwise.

snoman · April 14, 2005

Sorry about the double post, but thought this forum is more appropriate.

In my 1991 Suburban 4x4 3/4ton: I just swaped a 383TBI in place of the stock 350 (had tranny filter and fluid changed at same time). Now there are times when the tranny looses 4th (OD) gear and just goes into neutral. It seems to happen at highway speeds going up a hill, the tranny will downs**t to Drive under load (like it should) then never get OD back. As I crest the hill and start going down, the tranny just goes into neutral. It will go into 3rd when I get on the gas, then go back to neutral when I let off.

It will reset out of this condition when I shut the truck off then restart.

Any help would be great.

Thanks,

Keith

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Do you not have a aux tranny cooler and is it maybe getting too hot and locking OD out? Those trannies do not cool as well in OD oil flow wise and that might be the problem because you said it happens on a hill. If you are running big rubber with stock gears it will aggrevate this problem too.