I'm having a few issues at the moment. I have a 94 Full Size Chevy Blazer. All stock. Ever since I bought the blazer I've noticed that after driving down a highway for a few miles the tach and speedo start flickering and also the transmission is downshifting and sometimes up-shifting back into 4th. Idk why this is happening but its starting to become a issue on my morning commute. The blazer is old and has 258000 miles but I dont think its the transmission itself because this issue only happens while I'm on the highway and it seems perfectly normal any other time. If anyone has any suggestions on what might be the cause of this that would be great!

 

torque converter clutch locking and unlocking? Being mistaken as a shift?

 

That could be a a possibility. Is there any notable differences between the feeling of the trans mis-shifting vs the torque converter? Whenever the truck is not acting up it seem to be running normal with no issues what so ever.

 

Between 45 and 50 my 2001 seems to have a miss. Above and below this it runs perfectly. I've heard about an ignition interrupter that keeps the engine from firing during the shift so it will not slam into gear. Anyone know the fix for this?

 

At a steady speed usually shifts drop RPMs more than TCC engagement does. To see how much comes from TCC just drive a steady 50mph in OD, let off, and tap the brake just enough to turn on the brake light without slowing the vehicle down Observe how much tach goes up. You can use that info to help judge which is happening. Know that the better your vehicle runs, the less on/off TCC engagement you will observe in the 45-50mph range. Later GM 4L60E trannys have PWM and ECCC control to soften the TCC engagement and make things feel smoother. Keep in mind that what you feel may be quite normal if it is TCC - or if it is abnormal, the TCC may be goofy from wear in an internal trans control valve (common 4L60E trans problem). You're right in the years where the transmission controls changed a lot. Maybe get a tranny man to drive it for an opinion.

 

Les, I did a quick test as best I could with traffic. I did as you recommended. The trany kept trying to move in an out of OD or the torque converter acted up. The tach dropped down significantly each time it tried to shift. Not sure what that means yet. I'm going to do another test today somewhere I can get a longer run without traffic. Route A1A has a 45 mph speed limit for 30 miles each direction. I'll check back in later. Thanks.

 

Les, I feel kinda stupid. Last week I did a grease job, oil and filter change. I had checked all the fluids and the trany fluid was o.k. When I backed up the car to go for a test this morning, I found some red fluid on the driveway. Checked the transmission level and it was down. Added one quart and the shift problem went away. Got under the car and found both cooler lines were rusted and one was seeping fluid. Looking for a line repair kit as we speak. Sorry about making "much ado about nothing". Thanks for your help.