4L80E won't hold OD (RPMs drop when coasting in OD)

[pagrosse]

Hello. I have a 1999 P30 motorhome with a 7.4L and 4L80E that appears to not stay in OD. When fully warmed up in OD range at 55mph I'm at 2500RPM (which is 3rd gear). If I coast down a hill and fully lift off the gas, the RPM drops down to ~1300RPM. As soon as I give it some gas, the RPM goes back up to 2500RPM. I then tried dropping it from OD to 3rd and the RPM bumps up ~150RPM to 2650RPM. Based on this, I'm guessing that when I'm in OD range 4th is not holding and I'm really in 3rd with TCC locked. When I drop to 3rd range, it seems all that changes is the TCC unlocks and I'm still in 3rd.

-All other shifting is fine so I think I can eliminate solenoid A and B
-No transmission codes (PXXXX)
-I have a cheap chinese scanner that I can connect to the transmission computer and I can read all the sensors that the TCM uses. TPS responds appropriately (0%-99% when I press the pedal), brake switch responds appropriately, input and output sensors show RPM, tranny temp sensor is working (now I only scanned it when it was cold so maybe the sensor could have non-linear behavior)
-There is no major fluid leakage out the sensor connector
-Per the previous owner, the fluid was changed less than 5k ago but I question that because it doesn't look not look new pink and more grayish. No burnt smell though.
-Fluid level is difficult to read since the dipstick is like 8ft long and I feel like I might be getting higher than usual readings when it drags on the tube
-Only 43k on the engine/tranny

To me it sounds like it could be 4th gear clutch isn't holding either due to it being worn or not enough pressure? When I coast and the RPM drops, it seems that it goes into 4th but just doesn't stay. Per the manual, when dropping from OD to 3rd, the tranny should drop into 3rd and unlock the TCC.
Another thing I need to try is to confirm its commanding 4th. If not, it could be another sensor (i.e. brake) keeping it from engaging.

 

[ak diesel driver]

Sounds pretty normal to me.

 

[pagrosse]

Sounds pretty normal to me.

I would disagree. What is the point of putting in an OD gear if it only engages when coasting?

 

[Will L.]

I cant remember right what wire gets grounded to command the tcc lockup. (Help here someone).but if you force the lockup you can remover that from question and retry the 4th to 3rd shift.

As to the atf not being pink but grey- if they just dropped the pan, did the filter and replaced only what was in the pan and couldn’t drain tcc, and trans cooler then the percentage of what was lost might not be enough to alter the color. Or yeah, could be he lied. Incase you aren’t aware- the red in the atf is just a dye, same dye used in diesel as offroad tax identification. It was originally added to keep the transmission oil identifiable from regular 10 wt, but from there people just figured if not red it must be junk so they all keep adding it under marketing needs.

Remember with this trans do NOT use the dexron 6. The best fluid for it is transynd, but if not then stick with dexronIII.

What scanner are you using please? Apparently my last one loaned out is not returning and all ai want one for at this point is my 4l80e/tcm.

 

[ak diesel driver]

What your describing about the the vehicle coasting and rpm dropping to idle when you let off the gas is exactly what my last 2 4l80e trucks do. That's with a combined total of about 500k on them

 

[WarWagon]

4th is an overrunning gear that doesn't compression brake or hold the engine RPM up. In "manual selected 3rd" there is a small easy to torch clutch that comes on to compression brake. You can burn the clutch out and not notice as it still goes forward in all gears no issue. Tap the brakes to unlock the TCC to help you count gears.

If there is a bad problem like no 4th you will have a code in the ECM. Not emissions related so it likely won't trip the SES light.

 

[pagrosse]

I cant remember right what wire gets grounded to command the tcc lockup. (Help here someone).but if you force the lockup you can remover that from question and retry the 4th to 3rd shift.

As to the atf not being pink but grey- if they just dropped the pan, did the filter and replaced only what was in the pan and couldn’t drain tcc, and trans cooler then the percentage of what was lost might not be enough to alter the color. Or yeah, could be he lied. Incase you aren’t aware- the red in the atf is just a dye, same dye used in diesel as offroad tax identification. It was originally added to keep the transmission oil identifiable from regular 10 wt, but from there people just figured if not red it must be junk so they all keep adding it under marketing needs.

Remember with this trans do NOT use the dexron 6. The best fluid for it is transynd, but if not then stick with dexronIII.

What scanner are you using please? Apparently my last one loaned out is not returning and all ai want one for at this point is my 4l80e/tcm.

Thanks for the info. The scanner is Autel Diaglink MD802 on Amazon for like 88$. You get 1 vehicle make for free and additional is 10$ each. Pretty clunky interface and refresh is quite slow, frame every 2sec if you are reading a page of 4 blocks and up to 5sec if you capture all the TCM blocks

 

[THEFERMANATOR]

The rpm's dropping when you coast is normal. These have no real engine braking in 4th. The 3 over run clutches lock the shafts together when it is in 1, 2, or 3 to provide engine braking. You need a scanner to monitor the gear ratio to see if something is slipping or if it is just the tcc not locking. You get lockup in 3rd and OD, so pulling it back to 3rd only unlocks the tcc for a few seconds, and then it will relock the tcc. A scanner will tell you everything, but if you had any slippage of 25 rpm's or greater, it would be setting codes. Try and look through your scanner and find the desired and actual gear and the calculated gear ratio pids. 4th should be a .75:1 ratio. I would be checking your converter as it sounds like it's not locking up.

 

[pagrosse]

4th is an overrunning gear that doesn't compression brake or hold the engine RPM up. In "manual selected 3rd" there is a small easy to torch clutch that comes on to compression brake. You can burn the clutch out and not notice as it still goes forward in all gears no issue. Tap the brakes to unlock the TCC to help you count gears.

If there is a bad problem like no 4th you will have a code in the ECM. Not emissions related so it likely won't trip the SES light.

Per the 4L80E manual I interpret 4th as a compression brake gear, see attached snipet. It also says that manual 3rd range is a coasting gear. If this is the case, then when my RPM drops RPM when coasting while in OD range, I very well could still be in 3rd and never even get to 4th. I'm also pretty sure the TCC is locking up since the RPM increases ~150RPM when I drop from OD range to 3rd which is what the manual states will happen (and eventually re-engage)

 

[pagrosse]

The rpm's dropping when you coast is normal. These have no real engine braking in 4th. The 3 over run clutches lock the shafts together when it is in 1, 2, or 3 to provide engine braking. You need a scanner to monitor the gear ratio to see if something is slipping or if it is just the tcc not locking. You get lockup in 3rd and OD, so pulling it back to 3rd only unlocks the tcc for a few seconds, and then it will relock the tcc. A scanner will tell you everything, but if you had any slippage of 25 rpm's or greater, it would be setting codes. Try and look through your scanner and find the desired and actual gear and the calculated gear ratio pids. 4th should be a .75:1 ratio. I would be checking your converter as it sounds like it's not locking up.

I don't think RPM should drop if in OD range when is meets all the requirements to be in OD since it is not coasting gear per the snipet I posted above. I need to take it back on the road to confirm if it is commanding 4th. That can't happen right now since I'm in the middle of fixing some other issues (front tire rubbed a hole in a hydaulic leveler line).

 

[FellowTraveler]

4th is an overrunning gear that doesn't compression brake or hold the engine RPM up. In "manual selected 3rd" there is a small easy to torch clutch that comes on to compression brake. You can burn the clutch out and not notice as it still goes forward in all gears no issue. Tap the brakes to unlock the TCC to help you count gears.

If there is a bad problem like no 4th you will have a code in the ECM. Not emissions related so it likely won't trip the SES light.

As a side bar; this is why an exhaust brake should not be used on the 4L80e.

 

[WarWagon]

When you let off the throttle the TCC unlocks. Some tuners keep it locked forgetting this is how the 4L80E snaps the input shaft clean off. Regardless this transmission and TC doesn't hold the vehicle back or keep the engine RPM up very well. I have put 50K on em after the overrun clutch smokes off before the weak TCC clutch burns up. Manual 3rd with an overide on the TCC locking up is a damn quick way to smoke that weak clutch with some surging before it's gone. If you want compression braking you need to drop an Allison with a stand alone controller in it.

Again look for codes. IMO the ECM doesn't care about anything but forward problems. Any braking slipping is ignored.

 

[udidwht]

I would disagree. What is the point of putting in an OD gear if it only engages when coasting?

Going downhill in a 10,000lb RV at idle RPM in OD (foot off throttle). Normal. Overdrive in these big RV beasts is somewhat an oxymoron. Even when working perfect they don't spend a lot of time in OD unless the road your on is perfectly flat and no head wind. LOL! At 60mph you should be in the 2600-2800rpm range when in OD. Normal.