Trans sometimes drops to 3rd gear and won't upshift

[GM Collector]

Like the title says, after some driving on the highway if I go over 70 it looses 4th gear. It drops down to third and stays there. I noticed this a while ago but it only did it once so I wasn't worried about it but now it has been doing it a lot. It does not do it every time. In the past few days I put about 1600 miles on my car driving for hours and hours at a time and it is getting annoying. If I stop the car and shut it off it will shift into 4th again. The car has almost 153k on it. It is a 1995 Oldsmobile Eighty Eight. Any ideas? I was thinking drop the pan change the fluid/filter. I know the worst is inevitable but I am 1300 miles away from home on vacation and I have to drive all the way back home!

[Tuffguy610]

Are you sure that its downshifting rather than the torque converter unlocking? My car has a problem with the torque converter where it shudders and bucks when its locked up and eventually it will unlock for the remainder of a road trip. It is caused by a worn TCC valve (torque converter clutch valve). One thing you can do if this is the case is to unplug the pigtail connector that goes into the bellhousing of your transmission. Since you restart your car to get it to work properly, it seems electrical (which is what governs the TCC). If you arent familiar with a torque converter clutch, it is a clutch inside your torque converter that locks up in 3rd and 4th on our transmissions to give us better fuel economy and lower rpms. Over time these tend to go bad on the 4T60-E transmissions. When they go bad, it may seem like your trans is downshifting but really its just the clutch unlocking.

[imidazol97]

Agree that it may be the torque converter unlocking. Is the engine overheating? Or is the trans overheating?

If it does it, you should be able to move the selector from OD to 3 and if it actually is down in 3 you won't feel a downshift.

With that you can decide if it's actually in 3rd or in 4th without the TCC locked up. IF it's in 3rd, with the acceleration steady or slightly accelerating, touch the brake pedal and see if th tach goes up slightly indicating your TCC was locked up and unlocked when the brake pedal was touched.

[GM Collector]

I'm sure it's in third. The RPM's are high (no tach but I can hear it) and I can feel it in the pedal it accelerates faster than it should with light throttle @65+. So what I gather is I either need a TCC valve or a TC.

[threshold]

I had a similar problem & it was a vacuum leak. At first I thought it was just the vacuum modulator hose, as it had some cracks in it but it took a few trips under the hood to replace all the rubber hoses. Even one of the hard plastic hoses had a crack in it.

It also affected my cruise control but the worst thing was my mileage went way down.

[GM Collector]

Thanks. It seems to be intermittent and only when I am above 70 mph. I will check the vacuum lines.