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.