4L80E 2nd & 3rd gear only

[rickwils51]

I've got a 16,500lb motorhome with a '95 Chevrolet P32 chassis, 454 TBI and the 4L80E trans with 50K miles (purchased with 38K). The trans has two external coolers, one in the radiator and one in front of the radiator. At the present time it will only run in 2nd and 3rd gears, and the converter will not lock up. I have a code 68, which is "transmission component slipping" and means the slip speed when TCC engaged has been greater than 200 rpm for 2 seconds or more. I have the shop manuals that describe the condition and possible cause, and I have scan tool data that confirms that the 1-2 and 2-3 shift solenoids are getting the correct signals and that the PRNDL positions are all reading correctly to the ECM/TCM.

Regarding the lack of 1st and 4th, I have been driving for a while without 1st gear (no error codes) but only recently lost 4th and TCC lock up (and now have code 68). The first item shown in the manual as possible cause when you only have 2nd & 3rd gears is "1-2 shift solenoid stuck off". This makes sense, and I hope it is true, but I'm wondering why I lost first gear for so long without losing 4th as well, if the sticking solenoid is the cause. So here are some questions:

1. If the solenoid was sticking, could I lose 1st gear while 4th still worked, and then later lose 4th? I.e. partially stuck?

2. Would low operating pressure (due to clogged filter) be a possible cause?

3. Since I don't have a GM Tech I or II scan tool that can vary the force motor amperage manually, I was planning to connect the gauge and drive around while observing the pressure and the force motor amps and see if they follow the specs in the manual. Is there a good road test procedure other than running it through the PRNDL positions?

4. Before I pull the pan, I just want to be prepared. I plan to replace the 1-2, 2-3 and TCC PWM solenoids and the force motor while I'm in there. Do you have other recommendations as long as I have the pan off?

5. I plan to flush the oil (including converter) per your recommended procedure, but since I have two coolers, do I put both return hoses into the 5 gallon bucket? Each cooler has separate tubes from the trans case (4 total).

Any other advice from the experts would be appreciated before I take the next step! Thanks.

[joecar]

1. If the solenoid was sticking, could I lose 1st gear while 4th still worked, and then later lose 4th? I.e. partially stuck?The 1-2 shift solenoid has the 1-2 shift valve behind it; either the solenoid is not exhausting, or the 1-2 shift valve is sticking; the other end of the 1-2 shift valve has 2-3 Signal pressure (present in 3rd) which helps to push the valve to the off (1st/4th) position; so sounds like the 1-2 shift valve may be sticking. Losing 4th may be an additional problem.


2. Would low operating pressure (due to clogged filter) be a possible cause?Maybe, but you may see other problems.


3. Since I don't have a GM Tech I or II scan tool that can vary the force motor amperage manually, I was planning to connect the gauge and drive around while observing the pressure and the force motor amps and see if they follow the specs in the manual. Is there a good road test procedure other than running it through the PRNDL positions?As engine torque increases, line pressure should go up, and it should spike up on each shift (it should range something like ~100-230 psi). The EFILive scantool bidirectional controls should allow you to command the TFM/PCS.


4. Before I pull the pan, I just want to be prepared. I plan to replace the 1-2, 2-3 and TCC PWM solenoids and the force motor while I'm in there. Do you have other recommendations as long as I have the pan off?When you have the 1-2 and 2-3 solenoids off, check that the valves behind them are free (poke them with a small screwdriver, they should move in/out with spring pressure behind them -> BE CAREFUL TO NOT NICK THE BORE).


5. I plan to flush the oil (including converter) per your recommended procedure, but since I have two coolers, do I put both return hoses into the 5 gallon bucket? Each cooler has separate tubes from the trans case (4 total).The coolers are in parallel...?
Are the cooler fittings on the trans case dual port fittings...?
If so, then both return lines have to be diverted in to the bucket (be sure to avoid running the pan dry... you should overfill and keep a close eye on how much fluid enters the bucket... be able to instantly account for how much fluid is in the trans at any given moment.)

[joecar]

Oh, EFILive scantool may not do bidir on your ECM.

[rickwils51]

Joecar-
Thanks for the response. The coolers are indeed parallel and there are two outlet ports and two inlet ports on the right side of the trans. I believe the lower ones are the outlets and the higher ones are the returns.

Regarding the line pressure, I'll be monitoring the force motor amps while I watch the pressure gauge, and I'll see if the numbers are within the table specifications.

When I have the solenoids off and check for shift valve travel, do you know how much travel I should expect? I guess I can measure the travel of the solenoid when it is energized.

Thanks again for the help.

[joecar]

I think it's something like ~1/4 inch or so.

The solenoid is enclosed... you can't see it stroking; it opens/closes an internal path to its exhaust holes.