4l80e Shifting Issues. Need Help! :)

green 788 · November 30, 2008

My truck is a 1996 GMC 3500 dually, 4x4 crew cab. Yeah, it's a bus...

I have an issue with the automatic transmission, the 4L80E.

Problem started with it going into limp mode (sticking in 2nd gear)... when I shut the truck off and restarted, then it would begin shifting again, with one weird exception:

When going from 3rd to overdrive, the tach would rev up to around 4500 rpms and then it would drop into overdrive. And so long as I kept the pedal down (accelerating) it would stay in OD and do okay. But if I had to back off the pedal, it would downshift back into 3rd, then later repeat the whole 4500 rpm over-rev maneuver to go back into OD.

So I replaced the rear speed sensor on the transfer case (that's where it is on the 4x4 models). That didn't fix anything.

Then I read on a Hummer forum that you can unplug the speed sensor (and lose your speedometer, of course) and that would cause the tranny to shift at default points. THIS WORKED GREAT! I actually drove the truck a couple days with the rear speed sensor (on the transfer case) unplugged. I had no speedometer, but the tranny shifted great (albeit a bit firm and sharp). No issues with overdrive or anything... just firm, nice shifts.

But of course I've got to have a speedometer or else I'll get whacked by radar somewhere.

So I put the truck on jack stands and checked out the wiring on the side of the transmission, and didn't see anything obviously wrong. I did obtain a front speed sensor (supposedly there are two of these on the left side of the tranny, not counting the one on the transfer case, but I can only find one sensor actually mounted on the tranny)...

So I unplugged this front sensor and drove the truck just to see what would happen: Now it's shifting great through 1, 2, and 3... then it SLAMS into overdrive way too hard. But it actually does stay in overdrive like it should, once it goes there.

I don't think there's anything really wrong with the transmission internals (although it does have 203,000 miles on it)... but since it shifts so nicely with the rear speed sensor unplugged, I have to think the internals are okay.

I have not yet replaced the front sensor, but will be doing that in the morning.

Oh yeah, I also replaced the throttle position sensor, and that didn't do anything other than leave me 40 dollars poorer.

Anything else I should look at while I'm tinkering around with this thing tomorrow?

Any help appreciated...

It's a 5.7 liter engine, if that makes any difference...

Thanks in advance,

Dan

green 788 · November 30, 2008

bump... :o

green 788 · December 3, 2008

Okay... since this one has obviously left the technical gurus speechless...

Let me update:

The transmission is in for a rebuild. Gonna cost me 1800 dollars, but that's for all new parts, and with heavy duty upgrades and modifications.

The reason the transmission was revving up going into OD was that 4th gear was slipping that much. After the transmission slipped a lot, it would go into limp mode, or stuck in 2nd gear as it's better known.

I spoke to four different transmission shops, gathering advice. I had the 4th shop run a scan on it during a road test. He said right away that 4th gear was pretty much toast.

I asked him why it worked so well with the speed-o unplugged. He said that when you unhook the speed sensor, the pump pressure shoots up to a higher than normal level, and this higher level was allowing the burnt clutches to engage and not slip. He said that if I had driven the truck too much with the speedometer unplugged, the higher pump pressure would have eventually grenaded the case--literally. Apparently 4L80e's have had a rep for more or less blowing up due to inadequate venting of excess pressures when they arise. Two shops told me this same thing, and during a rebuild, they drill some holes in certain areas to remedy this.

So for anyone reading this thread, if you do choose to unplug your speedometer to get the transmission to shift well, be sure to take it in for a check up ASAP. Hopefully it'll just be a simple sensor issue. But in my case, it wasn't that simple or cheap.

But the good news is, my 4L80 will be better than GM built it originally. This one lasted 203K miles. I hope the next one lasts at least that long.

Dan

grampadirt · December 3, 2008

Okay... since this one has obviously left the technical gurus speechless...

Let me update:

The transmission is in for a rebuild. Gonna cost me 1800 dollars, but that's for all new parts, and with heavy duty upgrades and modifications.

The reason the transmission was revving up going into OD was that 4th gear was slipping that much. After the transmission slipped a lot, it would go into limp mode, or stuck in 2nd gear as it's better known.

I spoke to four different transmission shops, gathering advice. I had the 4th shop run a scan on it during a road test. He said right away that 4th gear was pretty much toast.

I asked him why it worked so well with the speed-o unplugged. He said that when you unhook the speed sensor, the pump pressure shoots up to a higher than normal level, and this higher level was allowing the burnt clutches to engage and not slip. He said that if I had driven the truck too much with the speedometer unplugged, the higher pump pressure would have eventually grenaded the case--literally. Apparently 4L80e's have had a rep for more or less blowing up due to inadequate venting of excess pressures when they arise. Two shops told me this same thing, and during a rebuild, they drill some holes in certain areas to remedy this.

So for anyone reading this thread, if you do choose to unplug your speedometer to get the transmission to shift well, be sure to take it in for a check up ASAP. Hopefully it'll just be a simple sensor issue. But in my case, it wasn't that simple or cheap.

But the good news is, my 4L80 will be better than GM built it originally. This one lasted 203K miles. I hope the next one lasts at least that long.

Dan

Thanks for the info.My 95 Burb has that tranny too albeit a year older and a different computer system.I currently have 211,000K on mine and is working pretty good.I bought my Burb used so I'm not sure what,if any,upgrades have been done to it.....

green 788 · December 5, 2008

I visited the transmission shop today and they have my unit ready to put back in the truck.

What happened to mine: The seal around the electrical plug socket where the computer connects was allowing transmission oil to leak into that plug. That caused the electrical signals to get messed up, and therefore shifting problems began. The truck was apparently driven a lot after the problem started (I bought this truck used). The slipping and improper shifting then caused the clutches to burn in the tranny.

So keep an eye on this plug... make sure there's no fluid leaking around it. The transmission tech told me that if that seal had not been leaking, this unit would have lasted another 100K miles (which is good news for you and your Suburban, too).

That seal can be fixed without a tear down. So if you see tranny fluid seeping down the side of the case around that plug, have her checked out asap.

My truck, even though it's a 5.7 engine, has the 6 lug torque converter I am told. Supposedly, you had to special order the 6 lug converter when you spec'd the 350, otherwise you got the 4L80 with the 4 lug converter. I was also relived to learn I have the largest tranny cooler GM offers, so I'm good there.

Looking forward to getting her back on the road.

Dan

pennzoil29 · December 6, 2008

Summabitch, finally, somebody with the same issues I'm having. Thanks for the info.

Gary, shrink your sig buddy.

green 788 · December 6, 2008

pennzoil,

if it's not been doing that very long, hopefully all you need to do is check for a tranny oil leak down the side of the case, coming out of the electrical plug socket, and have that cleaned out and the seal replaced.

If it's not the electrical plug, the front and/or rear speed sensor can cause this same problem, as well as a bad throttle position sensor. On 4x4, the rear speed sensor is on the transfer case, on 2wd it's on the back part of the transmission.

If you unplug the rear speed sensor (which operates the speedometer) and the tranny shifts great, you may have one of the above problems... unhooking the rear speed sensor causes the transmission to go to default shift points and pressures settings, I am told.

Dan

pennzoil29 · December 6, 2008

pennzoil,

if it's not been doing that very long, hopefully all you need to do is check for a tranny oil leak down the side of the case, coming out of the electrical plug socket, and have that cleaned out and the seal replaced.

If it's not the electrical plug, the front and/or rear speed sensor can cause this same problem, as well as a bad throttle position sensor. On 4x4, the rear speed sensor is on the transfer case, on 2wd it's on the back part of the transmission.

If you unplug the rear speed sensor (which operates the speedometer) and the tranny shifts great, you may have one of the above problems... unhooking the rear speed sensor causes the transmission to go to default shift points and pressures settings, I am told.

Dan

Thanks Dan. I hate sensors. It's wintertime, and almost Christmas. My chances for a simple fix are like 1 in a million.