4L80e Shifting Problems

[TurboDiverArt]

Hi All,

I’ve been having trouble with my 4L80e transmission in my 1999 Suburban K2500 6.5TD (260,000 miles). The transmission has a B&M TransPak (HD towing configuration, not street/strip) that I installed about 10 years ago when I got the truck. I’m using a Health Diesel modified PCM, Bill set it up knowing I had a shift kit in the transmission. The transmission and truck has been working fine for at least 8 years with this combination for more than 80,000 miles. I change the fluid and filter each year or about 10,000 miles since I tow a trailer most weekends during the summer. Recently I just checked and cleaned all grounds, even installed some extra ones between the engine, transmission, frame and body just to be sure as I know grounds can cause all kinds of electrical gremlins on trucks with a lot of miles.

The problems seem to be related to shifting between first and second gear. There are no codes in the PCM. I’ve changed the fluid and there’s no clutch material in the pan, the typical very light coating. No hard bits in the pan or anything, from a fluid standpoint everything looks good so I don’t think I’m damaging the transmission. It feels like it gets delayed shifting between first and second, but it’s not a flare. If I give it more power it’ll then shift very hard into the next gear. Usually this happens when shifting first to second most times. I usually accelerate lightly between these gears so it’s doesn’t bang into second gear. Once in second everything else is fine. No issue 2-3 or 3-4 up-shifting.

When slowing down, like at a light that just changes from red to green before I have stopped, it’ll occasionally seem like it’s stuck between down-shifting from second to first. Again, if I give it a lot of power it’ll bang very hard into first and then usually bang upshifting into second. Because of this I’ll go very lightly at this point until it completes it’s shift so not to break anything.

Recently when towing my trailer, if I’m hard accelerating in first gear, like up a hill I’ll hear a loud horrid screeching at the top of the gear. I always reduce power when this happens and allow the truck to shift into second gear. I believe the screeching is occurring during that 1-2 shifting where it’s sort of between gears. Again, I’m not finding clutch material in the pan so I don’t think it’s burning up clutches. The pan has always been very clean when changing the fluid.

I’ve always believed the problem is in and out of first gear. Reason being is that it’s slow to shift into first gear from Park/Neutral. Park/Neutral to reverse is normal, it’s only into first gear that is slow to bump in.

Here’s what I have done. Changed fluid and filter (twice actually). There’s no clutch material in the pan, the typical very light coating. No hard bits in the pan or anything. I’m using an AC filter and Redline Hi-Temp Synthetic. With the last fluid change it seems to shift hard only with that fluid (never did before, used it for years) so I add Lucas trans fix to it and it seems to shift better. I’ve replaced the EPC solenoid as well as the A and B solenoid (new gray and green ones), all AC Delco parts. I also changed the electronic pressure manifold. I believe the only electronic things in the valve body I have not changed is the TC lockup (that’s working fine) and the wiring harness which also has the temp sensor in it. The fluid is not burnt up.

I have not yet had the chance to check the line pressure. I need to look up what the correct pressures are and where to take them from. My plan is that once I verify the pressure is good and it’s not a pump issue, I’ll drop the valve body, clean it and replace all those little filter things. Beyond that I’ve run out of ideas.

Any help would be appreciated.

[Robyn]

Could well be the 1-2 shift solenoid.

These are easily swapped out with the pan off.

http://www.ebay.com/itm/1991-PRESENT-4L80E-1-2-2-3-A-B-SHIFT-SOLENOID-AC-DELCO-SET-OF-2-/181532299480?hash=item2a442b10d8:g:xogAAOSwxH1UHGl 7&vxp=mtr

Over time these little critters can get sticky or the internal screen plugs up.

With no codes showing I would bet the solenoid is going south.

Here is a good link to some data of use

http://extremeautomatics.com/assets/pdf/gm4L80Reference.pdf

With no crap in the pan or burnt smells it sounds like a solenoid issue, buttttttttttttt ya never know with things like the screeeeeeching noise.

Usually when hard parts fail there will be considerable junk in the pan, so a set of solenoids is not a bad test.

At the miles showing the solenoids are suspect.

[Robyn]

Here is another chart that shows all the different elements and when they are applied.

http://www.transgo.org/demo/4L80E_GD.pdf



Try this.

When going from N TO D ... you say it lags before engaging ???

Try going quickly from N TO 1 and see if this changes things.

The low sprag/roller clutch may be failing.

These can go south and not cough up much junk.

Looking at the charts indicates that the 1-2 solenoid is used for reverse as well as 1 st gear, and if the solenoid is bad then reverse would also be hinky as well.

A failing sprag/roller clutch would sort of explain the hard shifting possibly.

Give things a go and see what ya find.

If going from P/N to 1 give a quick 1st engagement then likely the low roller/sprag is/has gone away.

The low reverse band holds in manual low so this will give you a test point.

A failing low roller/sprag would account for the noise too.
If the low roller is broken and not working properly it can bind and not want to release thus causing the gawd awful screeching noise.

Sadly the low roller/sprag is the last thing in the box as you crawl in from the front end of it heading back.

Good luck

[TurboDiverArt]

Thanks for the reply Robyn. I have replaced both the A and B (1-2, 3-4) shift solenoid’s as well as electronic pressure control (EPC) solenoid and pressure manifold. I too was sure this was going to fix the problem. Even ponied up the big bucks for ACDelco parts… LOL.

I’m gonna pause on the screeching issue. I just discovered last night that the clutch on my 2-year old A/C compressor is slipping. At idle when it slips I hear a screeching sound that sounds similar to what I may be hearing. I’m gonna pick up a replacement clutch today and swap it out. I suspect the sound will disappear. Just turning the compressor by hand causes the inner and outer clutch plates to grind together and make sounds, it’s not the compressor, definitely the clutch.

Thanks for the TransGo info, it’s very helpful in understanding what’s going on in the VB and clutch’s. It’s normal from P/N to R. It’s slow from P/N to D. I can go from P/N to 1 quickly and directly and there is still the same lag. If I pull the idle to 1000RPM the P/N to D/1 is normal. Hence why I want to check the line pressures.

[Robyn]

OK

Get the line pressure checked out and go from there.

I hate to say it, but I doubt that this is gonna be an "inframe" fix.

At this point there may be servo seal issues involved.

A trip to the bench and a good post mortem is likely in order.

Best plan is to get a tech tool plugged into the rig and check the tranny readings.

With no metal/crap in the pan is a good thing, at least its not eating itself.

The forward clutch seals may be hard and not holding well.

Very hard to tell from this angle.

keep us in the loop.