98LS uncommanded shift into neutral at speed. P1750
 

98LS AT with 219k miles. Runs strong. Sat for 3 years. resurrected about 9 months ago.
This has happened twice now. About 2 weeks apart. First time it happened once. This time a few times.

Shift selector in D. It shifts fine up into second but will not shift to third. When the rpm’s are high (3500) in second if you back off the throttle then the transmission seems to go into neutral. once that happens you can rev the engine but it will not accelerate. Slow down or stop and it goes into first and drives fine till you try to go into 3rd again.

Manually shift into 1st or 2nd and it behaves fine.

I can repeat this over and over again.

FWIW my “D” light does not come on but I think it has been that way for a while.

Have check fluid levels. They are fine. I changed fluid anyways using SPIII. Old fluid was not too dirty.

Other than an engine oil leak somewhere above the starter motor no other symptoms.

The mighty internet suggests:
- Fluids. Have checked.
- TCM. Not sure how to test this. Electrical though for sure...
- Neutral safety switch. Thought that was more for keeping you from starting your car out of gear.
- Shift solenoid. Ok. Maybe but seems like it would drop in/out any time. Not only at high RPM.
- Clutch pack. OK. Seems like you would also get slipping. This is more like in or out of gear

TIA

More info. Did some more driving and got P1750. Solenoid error. Could repeat this by driving up to 25mph. when you would normally shift to 3rd i go to neutral. I can get back to 2nd by manually shifting into second but otherwise i have to stop to get back to 1st gear normally.

Any guidance?

Ensure the throttle position sensor is calibrated and check your detent cable. My detent cable rusted at the cable end and broke off causing similar issues. The detent cable runs alongside the throttle cable but runs down the right side of the engine down towards the transmission. It controls line pressure at different speeds/rpms so without it, the transmission may not shift into a higher gear or lower gear very easily or at all and will require some throttle/rpm play at various speeds to coax it into shifting.

Edit: Much like the throttle cable, the detent cable needs only a little play in it when at idle for the most efficient operation.
Also edit: The solenoid code may indicate a bad solenoid but try the above first.

Source: I have a 98 LS.

I wonder if your 3rd gear clutch is busted. If it is not holding, i.e. slipping, it will behave just like if it is shifting into neutral. The RPMs will just spike, but no power transfer occurs.