For the past few weeks my Blazer has been doing something weird with the transmission. When riding right around 40mph not really accelerating just staying at that speed my tranny wants to act like its slipping or doesn't want to pick a gear. I tried doing it while leaving it in 3rd and it still does it. The rpm's will go up and down a few 100 rpms and if you let off or accelerate it will stop. Is this a sensor maybe or something in the transmission?

 

Sounds like your torque converter is going in and out of lock up. Around 40mph is the speed where lockup occurs and the torque converter will lock in both 3rd and 4th.

The next time this occurs, tap the brake lightly (just enough to turn on the lights, but not slow you down). This will cause the torque converter to unlock.

 

Alright I can try that. If that stops the problem does it mean I need a new torque converter or something else?

 

It could just be something you have to deal with at that speed. You could try running a bit slower than 40 or a bit higher (say 45). 40MPH is just a bad speed for these torque converters.

Also, you could try measuring the output from your TPS sensor. If the TPS sensor is worn at that throttle position, it could cause an erratic reading which the PCM would interpret as you opening/closing the throttle, causing the problem.

 

Alright I will try and look at the TPS sensor and see if I can cure the problem.

 

Have you had a chance to test the TPS yet sifd?